Research Report
Influence of artificial mouth manipulation on the
physicochemical characteristics of Zandvlei
Estuary, Cape Town, South Africa.
Kyle Maurer
21221532
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1. Abstract
Recreational users, home owners and the biological component of Zandvlei Estuary have
conflicting requirements in terms of the physicochemical characteristics of the system.
Artificial mouth manipulation is used in an attempt to satisfy the physicochemical
requirements of the three stakeholders/ components of Zandvlei Estuary in a balanced
manner.
As a result of the importance of the physicochemical properties of the estuary to the three
stakeholders, this study aims to further understand the influence of artificial mouth
manipulation on the physicochemical characteristics of Zandvlei Estuary. By gaining this
understanding artificial mouth manipulation can be used more effectively to satisfy the
physicochemical requirements of the different components of Zandvlei Estuary thereby
ensuring that the estuary operates as healthily as possible.
Sampling was conducted from March through to the end of September with six sampling
days being completed during mouth open conditions and six during mouth closed conditions.
15 sampling stations located throughout the estuary were made use of. At all 15 sampling
stations measurements of water depth, temperature, salinity, conductivity, pH, dissolved
oxygen, total dissolved solids and Secchi depth/ transparency were recorded.
Temperature and dissolved oxygen were found to be statistically significantly higher during
mouth closed state in comparison to mouth open state across the entire estuary, sampling
stations and surface and bottom waters. Salinity and total dissolved solids were statistically
significantly higher during mouth open state in comparison to mouth closed state across
sampling stations and bottom waters. Conductivity was found to be statistically significantly
higher during mouth closed state in comparison to mouth open state across sampling
stations and surface waters. Secchi depth was statistically significantly higher during mouth
open state in comparison to mouth closed state across the entire estuary, sampling stations
and bottom waters. pH displayed statistically significantly higher values during mouth open
state in comparison to mouth closed state across sampling stations and surface waters.
Depth did not display a statistically significant difference between mouth open state and
mouth closed state.
Therefore artificial mouth manipulation was found to have an influence on physicochemical
parameters at Zandvlei Estuary. As a result of the importance of these parameters to
stakeholders, mouth manipulation is a key tool for management. Artificial mouth
manipulation should be used to manage the estuary in a manner that allows the system to
function as naturally as possible without compromising the needs of recreational users and
home owners.
Contents
Page
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Title page
1. Abstract
2. Introduction
3. Research Objectives
4. Materials and Methods
5. Results
5.1 Mouth open state and mouth closed state combined
5.2 Mouth open state and mouth closed state comparison
6. Discussion
6.1 Mouth open state and mouth closed state combined
6.2 Mouth open state and mouth closed state comparison
7. Conclusions
8. Recommendations and Reflections
9. Acknowledgements
10. References cited
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2. Introduction
Estuaries
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The term estuary refers to a body of water which forms the interface between a river and the
sea where the mixing of fresh and saline water occurs (McQuaid, 2013). Estuaries may be
permanently or temporarily open to the sea. 75% of South Africa’s 250 estuaries are
temporarily open/ closed estuaries (Snow and Taljaard, 2007). When estuaries are open,
water levels change as a result of tides and salinities change due to saline water inflow from
the sea and fresh water inflow from the influent rivers (C.A.P.E., 2013). This makes estuaries
highly dynamic systems (McQuaid, 2013).
Estuaries are also generally highly productive and highly valuable in terms of biodiversity.
Estuaries act as nursery areas for juvenile fish, habitat for migrant wading birds and offer
recreational opportunities for people (C.A.P.E., 2013). Their productivity, aesthetic beauty
and the protection they provide means that estuaries are very sensitive and also very
vulnerable to development (C.A.P.E., 2013). Harmful activities in the catchment, estuary
itself or in the sea close to the estuary mouth all have negative impacts on an estuary
according to McQuaid (2013).
In South Africa there are about 250 estuaries, of which almost all have been impacted/
modified as a result of human activity (McQuaid, 2013). The modifications to these estuaries
have had a large influence on deciding their characteristics. In South Africa one of the main
causes of modification by human activity has been a history of miss- management of
estuaries (C.A.P.E., 2013).
Zandvlei Estuary
History
Zandvlei Estuary can be seen on charts from as far back as 1700 (Morant and Grindley,
1982) (Figure 2). In 1673 the Dutch East India Company created a cattle outpost on the
banks of the Zandvlei estuary and this signified the start of the development of the area
(Morant and Grindley, 1982). In 1795 the Dutch lost to the British in the battle of Muizenberg
and South Africa became a British Colony. As a result there was a significant increase in
visitors to Cape Town. According to McQuaid (2013) the popularity of Muizenberg increased
so much that a direct railway line from Johannesburg Park Station to Muizenberg was
constructed. This resulted in further development along the north-western edges of the
estuary and additionally, the estuary became an increasingly popular spot for recreational
activities (McQuaid, 2013). Muizenberg’s popularity as a holiday town dropped off after
World War II. However, the popularity of the area was once again raised due to the
construction of Marina da Gama on Zandvlei Estuary in the 1970s (McQuaid, 2013).
Physical description
Zandvlei Estuary is located on the North West shore of False Bay, 20 kilometres (km) south
of Cape Town (34°05' S: 18°28'E) (Quick and Harding, 1994) (Figure 2). The estuary is a
temporarily open/ closed system which has been classified as eutrophic (McQuaid, 2013).
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Zandvlei Estuary is the largest estuary in False Bay and makes up about 80% of the
estuarine area of False Bay (McQuaid, 2013). The estuary includes a wetland which covers
60 hectares (ha), the main body covering 56 ha, Marina da Gama, 31 ha and an outlet
channel of 9 ha (C.A.P.E., 2013). The main body of the Estuary is 2.6 kilometres (km) long
and 0.5 km wide, at its widest point (Quick and Harding, 1994). Water levels are deepest in
Marina da Gama at 2 meters (m) however the mean water level varies between 0.7-1.3 m
(Morant and Grindley, 1982) (Figure 2).
Zandvlei Estuary’s main influent rivers/ streams include the Westlake Stream, Keysers River
and the Sand River Canal (which includes the Diep River, Langvlei Canal and the Little
Princess Vlei Stream) (C.A.P.E., 2013) (Figure 2). All influent rivers have been affected by
human activity since the 1940’s and this results in low quality, high nutrient water entering
Zandvlei estuary (McQuaid, 2013).
The Zandvlei estuary catchment lies entirely within the borders of the City of Cape Town.
The catchment is made up of an area of approximately 9,200 ha (C.A.P.E., 2013). According
to Thornton et al (1995), the population of the catchment was thought to be as high as
100,000, and is now likely to have increased significantly. Land-use activities in the
catchment vary from industry to housing, agriculture, forestry and conservation (C.A.P.E.,
2013). Rainfall in the catchment occurs predominantly in winter, from May to September and
summers are hot and dry (McQuaid, 2013).
Conservation and management
Despite Zandvlei’s history it remains highly valued for its natural attributes and as an area of
regional importance for recreational activities. Recreational activities include various types of
boating as well as picnicking, birdwatching, hiking/ walking and fishing (C.A.P.E., 2013)
The first form of management came about in 1981 when a village management board was
created and then in 1987 with the formation of a local municipality (McQuaid, 2013). Over
the years, there has been a heightened awareness of the need to maintain the natural health
of Zandvlei estuary in order to maximise the benefits of recreation and conservation
(C.A.P.E., 2013). The first proper recognition of this was when the Cape Town City Council
created the Zandvlei Nature Reserve in 1977. The borders of the nature reserve were
expanded in 2000 from 22ha to 204ha and in 2006 the reserve became the Greater Zandvlei
Estuary Nature Reserve (GZENR) (C.A.P.E., 2013). In 1988 the Zandvlei Trust was created
with the responsibility of conserving the indigenous fauna and flora of the Zandvlei Estuary
(McQuaid, 2013). A variety of projects have been started to promote continuous restoration,
monitoring and education focussed on Zandvlei Estuary (C.A.P.E., 2013).
Artificial mouth manipulation
Reasons for and timing of artificial mouth manipulation
The artificial opening/breaching of an estuary mouth is one of the most significant
hydrodynamic management actions for an estuary to undergo (Whitfield et al, 2012). In
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South Africa, the main reason for artificial breaching is as a result of poor urban planning.
The artificial opening of the mouth is most commonly carried out because of low-lying
developments that are situated within the estuarine floodplain (Whitfield et al, 2012). When
the estuary (influent rivers) floods, the mouth needs to be artificially opened so that flooding
of human infrastructure can be avoided (Whitfield et al, 2012).
The mouth of an estuary might also be artificially breached when the water level is high and
the estuarine water has been polluted as a result of overflow from septic tanks and sewage
systems (Whitfield et al, 2012). The mouth can be breached to flush the estuary of the
polluted water which could cause harmful algal blooms due to the excess nutrients. Artificial
breaching is also undertaken in order to maintain the estuaries functioning as a fish nursery
(Whitfield et al, 2012). This often happens when the estuary mouth has not been open for a
long period of time (Whitfield et al, 2012). The need for opening the estuary mouth may
become necessary if freshwater inflow into the estuary is reduced to the point where the
occurrence of natural breaching is unacceptably low and the estuary exhibits anoxic
conditions, a loss of salinity and or the build-up of excess nutrients (Whitfield et al, 2012).
Reduced freshwater inflow can also cause the need for artificial breaching of the mouth
when evaporation rates are high and results in abnormally high salinities which can be a
danger to the biota of the system (Whitfield et al, 2012). However, artificially breaching an
estuary under these circumstances often results in poor scouring of sediment out of the
estuary (Whitfield et al, 2012).
According to Whitfield et al (2012) the decision to open the mouth should only be made
when there is enough river inflow into the estuary to maintain tidal exchange once the
outflow phase has ended. Barton and Sherwood (2004) stated that receding tides are
regarded as a good time to create as large a mouth opening as possible which will allow
sufficient drainage and tidal flushing. Estuary mouth breaching is generally not conducted on
a rising tide or during stormy sea conditions coinciding with onshore winds (Barton and
Sherwood, 2004). Additionally breaching is generally not conducted when there is no
freshwater inflow into the estuary and if the outlet channel is too shallow or in the wrong
place (Barton and Sherwood, 2004).
Negative effects of artificial mouth manipulation
Artificial breaching may have negative effects on water quality and other ecological
components within an estuarine environment (Donald, 2013). Artificial breaching causes
unnatural flushing conditions in an estuary, which may have negative effects on the ecology
of the ecosystem (Donald, 2013). For example sand bar skimming maintains the sand bar
height at a specific level which prevents the estuary from reaching its natural water level
which in turn has a direct impact on the ecology of the ecosystem (Donald 2013). According
to Whitfield et al (2012) continuous low level artificial opening of the mouth will almost
guarantee a slow shallowing of the estuary as a result of sediment accumulation, particularly
in the lower reaches of the system.
Under natural conditions, the sand bar may reach heights considerably higher than the high
water mark of the estuary (Donald, 2013). This would require natural water levels within the
estuary to be higher than the sand bar height in order for natural breaching to take place. As
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a result of the greater difference in water levels between the ocean and the estuary under
natural conditions, a significant water gradient develops (Donald, 2013). The water gradient
causes high flow velocities throughout the estuary when the sand barrier is breached
(Donald, 2013). The high flow velocities result in high scour rates of sediment and this action
reduces the build-up of sediment at the estuary mouth (Donald, 2013).
When an estuary can flush naturally, during a flood a large amount of freshwater enters the
estuary and this is combined with the exit of estuarine water into the ocean (Donald, 2013).
As a result of higher scour rates in the estuary, a large mouth area is created (Donald,
2013). The estuary mouth stays open for long periods of time and this allows for the
movement of saline water throughout the system (Donald, 2013). Donald (2013) therefore
concluded that an increased flushing potential results in an increased range of salinities
throughout the estuary (Donald, 2013). This has an effect on the diversity of species in the
estuary, as different species survive under different salinities (Donald, 2013).
Artificial mouth manipulation at Zandvlei Estuary
History
Attempts to control the amount of water in Zandvlei Estuary date back to 1866 when the
system was shut off and drained so that it could be used for farming purposes (C.A.P.E.,
2013). When the winter rain started the plan failed and subsequent manipulations to the
system concentrated on keeping water levels constant for recreational activities and avoiding
flooding in Marina da Gama. In the 1950’s the outlet channel was canalised to form a 20
meter long concrete canal and this was followed by the construction of a rubble weir near the
mouth (McQuaid, 2013). The rubble weir serves to protect a sewer line that crosses beneath
the surface of the estuary (C.A.P.E., 2013). Other modifications include concreting the
estuary shores to form steep embankments, the construction of a railway line which
separated the Westlake wetlands from the rest of the estuary, the construction of the Royal
Road Bridge over the outlet channel, the building of the Marina da Gama housing
development and general urbanisation around the estuary and catchment (McQuaid, 2013).
Current mouth manipulation plan
The aims of the current management plan are summarised by C.A.P.E. (2013) which states
that “Zandvlei should function optimally as an estuary with appropriate mouth conditions,
tidal flows and salinity levels and with water levels showing sufficient variation to meet the
needs of biota without compromising socio-economic values.”
Artificial mouth manipulation at Zandvlei Estuary is under the control of the City of Cape
Town (C.A.P.E, 2013). Bodies that have input include Catchment, Stormwater and River
Management, under the Transport, Roads and Stormwater department as well as the Sand
River Catchment Forum, Zandvlei Trust, Zandvlei Nature Reserve management, Scientific
Services, Zandvlei Environmental Monitoring Program (ZIMP) volunteers, city councillors,
scouts, yacht and canoe club members as well as environmental and home owner groups
(C.A.P.E, 2013; Zandvlei Trust, 2006).
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The management plan for Zandvlei Estuary makes use of a rubble weir together with the
manipulation of a sand bar across the estuary mouth to control water levels in the estuary
(C.A.P.E, 2013). The latest mouth manipulation plan was implemented in 2001 after the
previous plan was done away with because it was leading to decreased salinities and
increased sedimentation in the estuary. There was concern about the associated negative
effects on biodiversity and the estuary’s ability to act as a fish nursery (C.A.P.E., 2013). The
changes made to the previous plan included decreasing the height of the weir from 0.9
meters to approximately 0.7 meters (in 2010/2011 it was decreased further to 0.6 meters)
and the manipulation of the sand bar at the estuary mouth was given more emphasis
(C.A.P.E., 2013; McQuaid, 2013).
According to the current management plan during the wet winter months the estuary mouth
(and therefore the sand bar) is kept open to prevent flooding of the houses in Marina da
Gama but also to allow marine migrant fish to move into and out of the system (C.A.P.E.,
2013). During the dry summer months the estuary mouth is kept closed to maintain the
water level for recreational activities (summer is when recreational activities take place most
commonly on the estuary) (C.A.P.E., 2013). The mouth remains closed except for when
there is a high spring tide which happens on five to six occasions every summer. In this case
the mouth is opened to allow the estuary to be flushed by the sea and increase salinity,
improve circulation and allow marine migrant fish to move into and out of the system
(C.A.P.E., 2013). The mouth will also open in summer if water levels remain high for long
periods of time as this could threaten houses in Marina da Gama (C.A.P.E., 2013).
Physicochemical characteristics of estuaries
Temperature: Water temperature is an important parameter as it has an effect on various
chemical and biological processes acting within an estuary (Kaselowski, 2012). Furthermore
the majority of aquatic organisms have a specific temperature range at which optimal
growth, reproduction and general health occur (Mabaso, 2002). An organism is able to
adapt its optimal temperature to subtle temperature changes, however rapid changes may
result in negative effects on the organism (Kaselowski, 2012). Long term temperature
changes can affect the overall distribution and abundance of estuarine organisms
(Kaselowski, 2012).
Salinity: Salinity is a measure of the quantity of dissolved salts in the water (Mabaso, 2002).
According to Kaselowski (2012) salinity is the most important parameter that controls the
habitat preference of the biota of an estuary. The majority of estuarine biota occur within
precise salinity ranges and variations in these ranges will directly affect estuarine organisms
distribution and life history cycles (Kaselowski, 2012).
Conductivity: According to Mabaso (2002) “the conductivity of water refers to its ability to
conduct an electrical current and is measured as the total amount of dissolved anions and
cations in water”.
pH: The pH of water is a measure of the acidity or alkalinity of a solution and is an important
indicator in evaluating water quality (Kaselowski, 2012; Mabaso, 2002). In addition the pH of
water has a very important influence on the survival of estuarine biota (Kaselowski, 2012).
When pH drops below 5 or increases above 9 many species become stressed (Kaselowski,
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2012). According to Mabaso (2002) increased pH creates more suitable conditions for algal
blooms and increased aquatic weed growth and is therefore a concern in estuaries that
experience nutrient enrichment. Furthermore variations in pH levels can change key aspects
of an estuary’s water chemistry which in turn can negatively affect the indigenous biota of an
estuary (Kaselowski, 2012).
Dissolved Oxygen: Dissolved oxygen is a measure of the amount of oxygen present in water
and thus available for respiration (Mabaso, 2002). Dissolved oxygen is an essential
requirement of all aquatic biota and is one of the most accurate indicators of an estuary’s
health (Kaselowski, 2012; Mabaso, 2002). According to Kaselowski (2012) most estuarine
organisms need dissolved oxygen levels of greater than 3 mg/l, however hypoxic (< 3mg/l)
and anoxic conditions (< 0.5mg/l) do occur in estuaries. If dissolved oxygen levels remain
below 3mg/l for an extended period of time, estuarine biota can become negatively affected
which in turn would decrease the productivity and, ultimately, the ecological health of the
estuary (Kaselowski, 2012).
Total Dissolved Solids: Total dissolved solids is the total amount of inorganic salts and
organic matter present in a given volume of water (Mabaso, 2002).
Secchi Depth: Secchi depth reflects water transparency. As a result Secchi depth affects
how deeply light can penetrate the water column which is important for photosynthesis and
oxygen production (Kaselowski, 2012).
Classification of estuaries according to salinity structure
The mixing of salt water from the ocean and freshwater from influent rivers determines the
salinity of an estuary. This mixture is known as brackish water and the salinity can vary
between 0.5 – 35 parts per thousand (ppt) (sometimes higher in areas where evaporation is
high) (Donald, 2013). The salinity of an estuary can change regularly as a result of rainfall,
tides and whether the estuary mouth is opened or closed amongst other factors (Donald,
2013). Estuaries are classified according to salinity structure as follows.
Highly stratified (salt wedge) estuary: In a highly stratified estuary, mixing is poor and a layer
of lower salinity water lies above a denser layer of higher salinity (Barton and Sherwood,
2004). Between these layers is a thin mixing area with an obvious halocline that exhibits
quick salinity changes (Barton and Sherwood, 2004). As a result of the water bodies being
mostly separate from each other, vertical differences may be seen in other water properties
including temperature and dissolved oxygen (Barton and Sherwood, 2004). Stratified
estuaries are located where the ocean tidal range is small (less than 1 m) and there is
inadequate energy to properly mix the two water layers (Barton and Sherwood, 2004). The
more saline layer at the bottom is pushed into a wedge shape as a result of the friction
between the out flowing surface layer and the inflowing bottom water (Barton and Sherwood,
2004). After the closure of the estuary mouth, this stratification can stay constant for long
periods of time (Barton and Sherwood, 2004).
Moderately stratified (partially mixed) estuaries: Moderately stratified estuaries exhibit
increased turbulent mixing because of greater tidal range (Barton and Sherwood, 2004). The
vertical salinity gradients along this type of estuary are lower than those most often found in
a salt wedge estuary (Barton and Sherwood, 2004).
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Well mixed estuaries: Well mixed estuaries show a high degree of turbulence which results
in the absence of a vertical salinity gradient (water is mixed) (Barton and Sherwood, 2004).
In well mixed estuaries, salinity increases in proximity to the mouth (Barton and Sherwood,
2004). Mixing can also take place in closed estuaries with little marine or freshwater inflow,
for example as a result of wind (Barton and Sherwood, 2004). According to Barton and
Sherwood (2004) one issue with classifying estuaries according to salinity structure is that
estuaries do not always exhibit the same type of behaviour either over time or at different
locations along the estuary. At any time an estuary’s mixing behaviour can be changed by
factors including the magnitude of tides or river discharge (Barton and Sherwood, 2004).
Physicochemical characteristics of Zandvlei Estuary
Temperature: Zandvlei Estuary shows similar temperature values throughout including when
moving from the estuary mouth to the estuary head as well as from shallow waters to deep
waters (Morant and Grindley, 1982). The presence of thermal stratification is rare as a result
of the shallow depth of the estuary and the high winds in the area which cause mixing
(Morant and Grindley, 1982). Surface temperatures are significantly different to bottom
temperatures only during calm conditions which occur from May to June (late autumn)
(Morant and Grindley, 1982).
Salinity: Noble and Hemens (1978) stated that Zandvlei is “fresh to saline and shallow with
no vertical salinity stratification” (Morant and Grindley, 1982). In contradiction to this,
Benkenstein (1982) showed that a salt wedge salinity stratification was present at the
“northern end of the main basin”. Furness (1978) stated that salinity stratification is present
in Zandvlei and occurs most often in winter when the estuary mouth has been breached.
Under these conditions fresh water moves out of the estuary and saline water moves in
under the freshwater (Morant and Grindley, 1982).
McQuaid (2013) analysed the salinity records of Zandvlei Estuary for the period between
1989 and 2012. No significant trends were seen. The salinity of Zandvlei Estuary after 2000
was higher than in years before 2000 but this pattern was not significant (McQuaid, 2013).
Statistical analysis showed a significant increase in salinity values for the entire estuary
when making a comparison of data for 1989 and 2011 only (McQuaid, 2013). The highest
salinity value that was recorded in 1989 was at the mouth of the estuary. In 2011 this same
salinity value was now found in the middle of the Estuary, with the value at the mouth being
even higher (McQuaid, 2013). This demonstrates that higher salinity values are being found
higher up the system.
pH: According to Morant and Grindley (1982), Zandvlei Estuary exhibits wide pH ranges
which are due to the inflow of saline water, from the ocean and freshwater, from the influent
rivers. The estuary’s wide pH ranges can also be as a result of photosynthesis. When plants
photosynthesise they remove carbon from the water which can raise pH levels in the water
(Morant and Grindley, 1982). Additionally, the estuary itself generally shows higher alkalinity
than the rivers feeding into it (Morant and Grindley, 1982). These conditions are
representative of an estuary that is being put under stress and this may well contribute to a
reduced number of species in the estuary (Morant and Grindley, 1982).
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Dissolved Oxygen: According to Morant and Grindley (1982) Zandvlei Estuary is eutrophic.
Dissolved oxygen values at the water surface have been found to be similar over the entire
estuary whilst bottom readings of zero, which indicate anoxic conditions, have been seen in
both the main body of the estuary and in the Marina da Gama canals (Morant and Grindley,
1982). The reason for the anoxic bottom conditions in the main body of the estuary was most
likely as a result of large quantities of organic matter building up on the bottom due to the
winter die back of pondweed (Stuckenia pectinatus) and phytoplankton (Morant and
Grindley, 1982). On the other hand, the anoxic bottom conditions in the canals were related
to salinity stratification (explained further in the section “wind” below) according to Morant
and Grindley (1982). The macrophyte, Stuckenia helps oxygenate the bottom waters of the
canals but it needs to be kept under control before it affects recreational activities in the
estuary (Quick and Harding, 1994). Dissolved oxygen readings are distinctively lower after
Stuckenia has been cut in the canals according to Morant and Grindley, 1982.
Transparency: Secchi disk water transparency was shown to range from 0.2 meters to 1.8
meters with an average of 0.7 meters for the entire system (Morant and Grindley, 1982).
Wind: The wind patterns at Zandvlei are a critical physical factor that influences estuary
functioning (Morant and Grindley, 1982). The predominant winds at Zandvlei Estuary are
Southerly winds during summer and Northerly winds during winter (Morant and Grindley,
1982). Due to the windiness of the area (mean wind speed for 1992 was 6 m s -1) and the
alignment of the estuary parallel to prevailing winds, the main body of the estuary is usually
well mixed for most of the year (Harding, 1994). However the canals at Marina da Gama are
positioned east to west so that the houses block the wind and provide shelter to the people
living on the canals and using them for recreation (Morant and Grindley, 1982). As a result of
the calm conditions very little mixing occurs and a halocline forms between denser saline
bottom water and less dense fresh water on top of it (salinity stratification) (Morant and
Grindley, 1982). Anoxic waters quickly become apparent below the halocline and this leads
to the formation of hydrogen sulphide gas, H2S (originates from sea water as it is high in
sulphate) (Morant and Grindley, 1982). When windy conditions and subsequent mixing
occurs, the halocline is disrupted and the H2S gas is released (Morant and Grindley, 1982).
Physicochemical characteristics- management and targets
Salinity: The salinity of the Zandvlei estuary should be at a level that affords fish and bottom
dwelling communities the opportunity of re-establishment in the areas in which they lived
(C.A.P.E., 2013). Fish populations in Zandvlei are quite capable of handling low salinities but
they generally do not cope with abnormally high salinity levels (hypersaline conditions)
(C.A.P.E., 2013). Invertebrates and plants are not as tolerant to changes in salinity (C.A.P.E.,
2013). An example is the invertebrate Sandprawn, Callianassa kraussi, which stops breeding
when salinity levels drop below 25 ppt (van Niekerk et al, 2005). Pondweed, Stuckenia
pectinata is known to be able to handle a salinity range of 5 to 20 ppt and at salinities
between 5 and 10 ppt has an ecological advantage over other macrophytes and
phytoplankton in the system (C.A.P.E., 2013). On the other hand another macrophyte,
Phragmites, will die if it is exposed to salinities greater than 16 ppt for more than twelve
weeks (C.A.P.E., 2013).
Zandvlei Estuary management sets ambient salinity targets for surface and bottom waters in
the outlet channel (extends upstream to a point parallel to the downstream end of the
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marina) and main body of the estuary for both summer and winter (C.A.P.E., 2013). The
current salinity targets are; for the main body of the estuary; in winter salinity must be
between 5 ppt for surface waters and 7ppt for bottom waters and in summer; 10 ppt
throughout the water column (C.A.P.E., 2013). For the outlet channel; in winter salinity must
be between 6 ppt for surface waters and 18 ppt for bottom waters and in summer; between
11 ppt for surface waters and 13 ppt for bottom waters (C.A.P.E., 2013).
Water levels: In its natural, undisturbed state, Zandvlei Estuary’s water levels would have
varied between 0 and 2.5 to 3 meters above mean sea level (AMSL) (C.A.P.E., 2013).
However, currently water levels range from 0.7 to 1.4 m AMSL in order to avoid flooding in
Marina da Gama (houses in danger at 1.4 m) as well as to protect the revetments in the
marina (designed for water levels of 0.7 m) (C.A.P.E., 2013). Additionally recreational
activities in the estuary require a depth of 1 m and the pondweed harvester requires a depth
of 0.8 m to operate (C.A.P.E., 2013).
The targets/goals for the management of water levels in the estuary include; reducing the
height of the rubble weir by 10 to 20 cm in the winter months as a trial, changing the mouth
manipulation protocol so that note is taken of predicted wave and wind conditions as well as
allowing marginally higher water levels (C.A.P.E., 2013).
Dissolved oxygen: The Water Quality Index project recently created guidelines which advise
that dissolved oxygen values ranging from 6 to 8 mg/l (milligrams per litre) are preferred
(C.A.P.E., 2013). It has been proposed by C.A.P.E. (2013) that these values be used as
targets for Zandvlei estuary.
Zandvlei Estuary- overview
Zandvlei Estuary is an important recreational space in South Africa (Quick and Harding,
1994) (Figure 1). The estimated recreational value of the estuary is between one and five
million rand per year according to C.A.P.E. (2013). Recreational activities taking place at
Zandvlei Estuary include canoeing/kayaking, yachting/sailing and windsurfing/kitesurfing with
a number of other activities taking place on the banks of the estuary (C.A.P.E., 2013).
Recreational users require that the water level be maintained at a depth that allows
recreational activities to be practically possible and allows the estuary to be aesthetically
desirable (Quick and Harding, 1994).
In addition to recreational users, home owners of Marina da Gama (a housing development
located on a canal system joined to the main body of the estuary) have a vested interest in
the water levels of Zandvlei Estuary. If water levels rise too high in the estuary, homes in
Marina da Gama will flood. In contrast if water levels drop too low it is aesthetically
undesirable to home owners (C.A.P.E., 2013).
Zandvlei Estuary is highly valuable in terms of biodiversity and conservation (C.A.P.E.,
2013). The estuary has a diversity of fauna and flora including 25 species of fish, more than
150 species of bird, 18 reptile species and 210 plant species (open Green Map, 2014).
Zandvlei is also the only estuary of significance as a fish nursery on the False Bay coastline
which is very important to juvenile marine migrant fish including the critically endangered
white steenbras Lithognathus lithognathus (Quick and Harding, 1994; C.A.P.E., 2013). The
12
estuary forms part of the Greater Zandvlei Estuary Nature Reserve (GZENR), further
emphasising its natural value (C.A.P.E., 2013). However, in order to conserve the
biodiversity of the estuary, it needs to be managed in a way that allows the estuary and the
mouth to function as naturally as possible year round (Quick and Harding, 1994). The natural
functioning of the estuary entails seasonal fluctuations in water levels, high salinities to
support indigenous estuarine species, good circulation to prevent the build-up of polluted,
anoxic water and opportunities for marine migrant fish to move into and out of the system
(C.A.P.E., 2013).
Recreational users of Zandvlei Estuary, home owners in Marina da Gama and the biological
component of the estuary have conflicting requirements in terms of the physicochemical
characteristics of the system. Artificial mouth manipulation is used in an attempt to satisfy the
physicochemical requirements of the three stakeholders/ components of Zandvlei Estuary in
a balanced manner.
As a result of the importance of the physicochemical properties of the estuary to the three
stakeholders, this study aims to further understand the influence of artificial mouth
manipulation on the physicochemical characteristics of Zandvlei Estuary. By gaining this
understanding artificial mouth manipulation can be used more effectively to satisfy the
physicochemical requirements of the different components of Zandvlei Estuary (in a
balanced manner) thereby ensuring that the estuary operates as healthily as possible even
in its current disturbed state.
3. Research Objectives

To quantify water depth, temperature, salinity, conductivity, pH, dissolved oxygen,
total dissolved solids and Secchi depth of the waters of Zandvlei Estuary during
mouth open state and mouth closed state
o To determine if there are statistically significant differences (95% confidence
interval) in the above mentioned parameters between surface waters and
bottom waters, between zones, between the main body of Zandvlei Estuary
and the Marina da Gama canals.

To quantify the influence of artificial mouth manipulation on water depth, temperature,
salinity, conductivity, pH, dissolved oxygen, total dissolved solids and Secchi depth of
the waters of Zandvlei estuary during mouth open state and mouth closed state
o To determine if there are statistically significant differences (95% confidence
interval) between mouth open state and mouth closed state for the above
mentioned parameters across the entire estuary, sampling stations, surface
waters and bottom waters.

To understand whether there are factors other than mouth manipulation influencing
salinity of the waters of Zandvlei Estuary
o To determine if salinity readings demonstrate a statistically significant
correlation (95% confidence interval) with rainfall.
4. Materials and Methods
13
Study site
A detailed description of the study site is given under the headings “Zandvlei Estuary” and
“Physical Description”.
Figure 1: Zandvlei Estuary in relation to the rest of South Africa and False Bay (close up) (source: Google
Earth; last accessed 22/10/15).
Sample collection
Sampling frequency varied from week to week and month to month and took between 3 to 5
hours to complete. Sampling was conducted on twelve occasions from March through to the
end of September. Six sampling days were completed during mouth open conditions and six
during mouth closed conditions. Sampling was not conducted at specific times of the day,
specific tides, specific seasons (although emphasis was placed on sampling when the mouth
was being actively opened or closed, mainly in summer, autumn and spring) nor at a specific
number of days after mouth opening and closing.
Samples were taken throughout the estuary including in the main body of the estuary and in
the canals which form part of the Marina da Gama housing development (Figure 2). In total
15 sampling stations were used (Figure 2). Stations 1 to 9 were located in the main body of
the estuary and stations 10 to 15 in the Marina da Gama canals (Figure 2). Of the six
stations in the canals, stations 10, 12 and 14 were positioned at the mouth of the canals and
stations 11, 13 and 15 at the head of the canals (Figure 2). At all 15 sampling stations
14
measurements of water depth (meters- m), temperature (degrees Celsius- C), salinity (parts
per thousand- ppt), conductivity (micro Siemens per centimetre- µS/cm), pH, dissolved
oxygen or DO (milligrams per litre- mg/L), total dissolved solids or TDS (milligrams per litremg/L) and transparency or Secchi depth (meters- m) were recorded. Samples were taken
through the water column. If the water depth at a particular sampling station was less than or
equal to 0.5 meters (m) only a surface reading was taken. If however the water depth was
greater than 0.5 m but less than 2 m then both a bottom reading and a surface reading was
taken. If the water depth was 2 m or greater a bottom, middle and surface reading was
taken. Surface readings were taken at a depth of 0.1 m.
Figure 2: The study site including sampling stations 1 to 15 (source: Google Earth; last
accessed 22/10/15).
A Secchi disk with a diameter of 0.2 m was used to measure Secchi depth/ water
transparency and water depth at each sampling station. Secchi depth was measured by
lowering a Secchi disk into the water column and waiting for it to disappear from sight. When
the Secchi disk disappeared a depth reading would be taken and the disk would be raised
until it was visible again. At this point a second depth reading would be recorded and the
average of the two depth values would produce the Secchi depth. Water depth was
measured by making use of the depth markers on the Secchi disk cord. A YSI multimeter
(professional plus model) was used to measure temperature, salinity, conductivity, pH,
15
dissolved oxygen and total dissolved solids at each of the sampling stations. A canoe was
needed to access the sampling stations which were located with the use of physical
markers.
Sampling station 2’s depth never exceeded 0.5 m and therefore only surface readings were
taken at this station with the exception being depth which was taken at the bottom of the
water column. Therefore for each of the parameters measured, 29 samples were taken per
parameter on each sampling day for twelve sampling days. This resulted in a total of 348
samples being taken per parameter. Fewer samples were taken for depth (179 samples),
dissolved oxygen (205 samples) and Secchi depth (119 samples); Depth was not recorded
at the surface as the depth at which readings were taken was standardised at 0.1 m into the
water column. Fewer dissolved oxygen readings were utilised in the study due to an
incorrect sampling technique. This resulted in data being removed (143 readings) and only
the data collected using the correct technique was retained. Secchi depth could not be
measured at some sampling stations due to the Secchi disk being visible at the bottom of
the water column and on rare occasions due to pondweed, Stuckenia pectinata (aquatic
macrophyte) covering over the Secchi disk and causing readings to be inaccurate.
Data Analyses
Data was organised in columns so that analyses could be conducted for mouth open state
and mouth closed state combined as well as mouth open state and mouth closed state
comparison. Each column represented one of the eight physicochemical parameters that
had been measured. Columns were also created for mouth state, surface/ bottom waters,
sampling station, zone, main body/ canals, sampling date and sampling time so that the
measured parameters could be analysed across these variables. Sampling stations were
grouped into zones. Sampling station 1 to 4 were grouped as the lower zone (closest to the
mouth of the estuary), station 5 to 7 as the middle zone, station 8 and 9 as the upper zone
(closest to the head of the estuary), station 10, 12 and 14 as the canal mouth zone and
station 11, 13 and 15 as the canal head zone. In order to compare the main body of the
estuary to the canals, station 1 to 9 were grouped together as main body and station 10 to
15 as the canals. Data was organised in Microsoft Excel 2010 and then imported into IBM
SPSS Statistics 23.
Graphs and tables were created using IBM SPSS Statistics 23. The data was then checked
for normality using the Kolmogorov- Smirnov test in combination with reviewing the
skewness and kurtosis values. Normality testing was done for each parameter across mouth
state, surface/ bottom waters, zone, main body/ canals and sampling station. The data was
found to be not normally distributed and therefore non parametric analyses had to be carried
out on the data.
A Mann-Whitney U test was used to determine whether there were statistically significant
differences (95% confidence interval) between surface and bottom waters, between zones
(canal mouth zone and canal head zone), between the main body of the estuary and the
canals, between mouth open and mouth closed state and between mouth open and mouth
closed state across surface and bottom waters. Analyses were carried out on all parameters
measured. A Kruskal- Wallis H test was used to determine whether there were statistically
significant differences (95% confidence interval) between zones (lower, middle and upper
16
zones). Analyses were carried out on all parameters measured. A Wilcoxon Signed Ranks
test was used to determine whether there were statistically significant differences (95%
confidence interval) between mouth open and mouth closed state across sampling stations
as well as between mouth open and mouth closed state across sampling stations and
surface and bottom waters. Analyses were carried out on all parameters measured. The
aforementioned statistical analyses were conducted using IBM SPSS Statistics 23.
Environmental data
Correlation between salinity and rainfall: Rainfall data was sourced from the Zandvlei Trust
internet website, Zandvlei Inventory and Monitoring Program (ZIMP). Citizen scientists
collect rainfall data using a standard rain gauge (a funnel that is approximately 35cm long
and 12.8cm wide at the top) and make it available on the ZIMP website. Five citizen
scientist’s rainfall data was used which came from a number of different locations
surrounding the Zandvlei Estuary. The rainfall data was averaged to get the best
representation of the rainfall that fell in the areas surrounding the estuary. The rainfall from a
particular sampling day was then added to the rainfall that fell both one day and two days
prior to the sampling day. This produced the sum rainfall for a particular sampling day with
the method being repeated for each sampling day. An average salinity per zone was
calculated for each sampling day using the salinity data collected during the study.
Spearman’s Rank Order Correlation was used to determine whether there was a statistically
significant relationship (95% confidence interval) between the mean salinity per zone per
sampling day and the sum rainfall per sampling day. Statistical analyses were conducted
using IBM SPSS Statistics 23.
5. Results
Mouth open state and mouth closed state combined
Descriptive statistics for the entire estuary
A maximum value for temperature of 22.70 ºC was measured at the canal head zone, at the
surface waters during mouth closed state (Table 1). A minimum value of 2.72 ppt (measured
at the upper zone, at the surface waters during mouth open state) and a maximum value of
31.62 ppt (measured at the lower zone, at the bottom waters during mouth open state) were
recorded for salinity (Table 1). A minimum value for dissolved oxygen of 0.09 mg/l was
recorded at the canal head zone, at the bottom waters during mouth open state (Table 1). A
minimum value for Secchi depth of 0.2 m was measured at both the canal mouth and canal
head zones at the bottom waters during mouth closed state (Table 1). Both conductivity and
total dissolved solids displayed large differences (range) between minimum and maximum
values and this contributed to high variance values of 41214532.94 µS/cm and 24979379.18
mg/l respectively (Table 1). Mean salinity for Zandvlei Estuary was found to be 13.55 ppt,
with a standard deviation of 5.10 ppt and a variance of 26.03 ppt (Table 1). Mean depth for
the estuary was 1.24 m, temperature, 17.32 ºC, conductivity, 19007.69 µS/cm, pH, 8.65,
dissolved oxygen, 8.34 mg/l, total dissolved solids, 14464.63 mg/l and Secchi depth, 0.76 m
(Table 1).
17
Table 1: Descriptive statistics for the entire estuary
Descriptive Statistics
Depth
Temperature
Salinity
Conductivity
(m)
(°C)
(ppt)
(µS/cm)
pH
Dissolved
Total
Secchi
Oxygen
Dissolved
Depth
(mg/l)
Solids (mg/l)
(m)
Valid N
179
348
348
348
348
205
348
119
Minimum
.20
11.10
2.72
3935.00
7.09
.09
3282.50
.20
Maximum
1.95
22.70
31.62
39318.00
9.84
16.78
31479.50
1.69
Mean
1.24
17.32
13.55
19007.69
8.65
8.34
14464.63
.76
Median
1.30
17.20
13.20
18924.00
8.70
8.43
14189.50
.65
Standard Deviation
.42
2.11
5.10
6419.85
.61
3.21
4997.94
.41
Standard Error of Mean
.03
.11
.27
344.14
.03
.22
267.92
.04
Variance
.18
4.47
26.03 41214532.94
.37
10.29
24979379.18
.17
Surface waters and bottom waters comparison
Mean pH and dissolved oxygen were found to have statistically significantly higher values at
the surface waters when compared to readings from the bottom waters, whilst mean
temperature was not statistically significantly higher. Mean temperature was 17.37 ºC at the
surface and 17.27 ºC at the bottom (U= 14311, Z= -.86, p= .388), mean pH, 8.76 and 8.54
respectively (U= 12500, Z= -2.79, p= .005) and mean dissolved oxygen, 9.64 mg/l and 6.54
mg/l respectively (U= 2402.5, Z= -6.48, p< .001).
In contrast mean salinity, conductivity and total dissolved solids values were statistically
significantly higher at the bottom waters in comparison to the surface waters. Mean salinity
was 12.40 ppt at the surface and 14.78 ppt at the bottom (U= 11422.5, Z= -3.94, p< .001),
mean conductivity, 17606.56 µS/cm and 20508.89 µS/cm (U= 11503, Z= -3.86, p< .001) and
mean total dissolved solids, 13321.34 mg/l and 15689.59 mg/l respectively (U= 11369.5, Z=
-4, p< .001).
Zone comparison
Main body: An increase from the lower zone through the middle zone to the upper zone was
apparent for both mean depth and pH, with the difference between zones being statistically
significant for mean depth (H(2)= 23.54, p< .001) and pH (H(2)= 52.16, p< .001). A decrease
from the lower zone through the middle zone to the upper zone was apparent for mean
salinity, conductivity and total dissolved solids, with the difference between zones being
statistically significant for mean salinity (H(2)= 86.37, p< .001), conductivity (H(2)= 75.52,
p< .001) and total dissolved solids (H(2)= 84.31, p< .001). Mean dissolved oxygen was
found to be highest at the middle zone, lowest at the lower zone and in between at the upper
zone, with the difference between zones being statistically significant (H(2)= 13.37, p= .001).
Mean Secchi depth was also highest at the middle zone, lowest at the lower zone and in
between at the upper zone but the difference between zones was not statistically significant
(H(2)= 0.6, p= .741). Mean temperature was found to be highest at the upper zone, lowest at
18
the middle zone and in between at the lower zone with the difference between zones being
not statistically significant (H(2)= 2.59, p= .274).
Canals: Mean depth, salinity, conductivity, pH, dissolved oxygen, total dissolved solids and
Secchi depth did not display statistically significantly higher values for canal mouth zone in
comparison to canal head zone. Mean depth was 1.62 m at the canal mouth and 1.53 m at
the canal head (U= 470, Z= -1.89, p= .059), mean salinity, 11.96 ppt and 11.71 ppt (U= 2450,
Z= -.57, p= .57), mean conductivity, 17317.85 µS/cm and 17056.94 µS/cm (U= 2481, Z=
-.44, p= .657), mean pH, 8.93 and 8.84 (U= 2539.5, Z= -.21, p= .834), mean dissolved
oxygen, 8.87 mg/l and 7.66 mg/l (U= 830.5, Z= -1.13, p= .258), mean total dissolved solids,
12954.01 mg/l and 12711.19 mg/l (U= 2444.5, Z= -.59, p= .556) and mean Secchi depth,
0.94 m and 0.75 m respectively (U= 324, Z= -1.68, p= .092). Mean temperature did not
exhibit a statistically significantly higher value for canal head zone in comparison to canal
mouth zone. Mean temperature was 17.99 ºC at the canal mouth and 18.10 ºC at the canal
head (U= 2479.5, Z= -.45, p= .653).
Main body and canal comparison
Mean depth, temperature and pH were recorded to have statistically significantly higher
values at the canals in comparison to the main body of Zandvlei Estuary, whilst mean Secchi
depth was not statistically significantly higher. Mean depth was 1.57 m at the canals and
1.03 m at the main body (U= 834, Z= -8.88, p< .001), mean temperature, 18.04 ºC and
16.81 ºC (U= 8966, Z= -6.19, p< .001), mean pH, 8.88 and 8.49 (U= 8552, Z= -6.64, p< .
001) and mean Secchi depth, 0.85 m and 0.66 m respectively (U= 1438.5, Z= -1.76, p= .
078).
In contrast mean salinity, conductivity and total dissolved solids showed statistically
significantly higher values at the main body of the estuary in comparison to the canals, whilst
mean dissolved oxygen was not statistically significantly higher. Mean salinity was found to
be 11.84 ppt at the canals and 14.76 ppt at the main body (U= 9569.5, Z= -5.54, p< .001),
mean conductivity, 17187.40 µS/cm and 20292.60 µS/cm (U= 10833, Z= -4.17, p< .001),
mean dissolved oxygen, 8.29 mg/l and 8.38 mg/l (U= 4867, Z= -.67, p= .504) and mean total
dissolved solids, 12832.60 mg/l and 15616.65 mg/l respectively (U= 9579.5, Z= -5.53, p< .
001).
Surface/ bottom waters and zone comparison
Zone- main body: Mean salinity, conductivity and total dissolved solids decreased from the
lower zone through the middle zone to the upper zone both at the surface and bottom waters
(Figure 3, Table 2). Mean pH followed the opposite trend increasing from the lower zone
through the middle zone to the upper zone both at the surface and bottom waters (Figure 5,
Table 2). Mean depth, temperature and dissolved oxygen also increased from the lower zone
through the middle zone to the upper zone but only at the bottom waters for depth and
temperature and only at the surface waters for dissolved oxygen (Figure 4, Table 2).
Zone- canals: Mean temperature was higher at the canal head zone in comparison to the
canal mouth zone for both surface and bottom waters (Table 2). For mean salinity,
conductivity, dissolved oxygen and total dissolved solids higher values were witnessed at the
canal head zone (in comparison to the canal mouth) for surface waters and at the canal
mouth zone (in comparison to the canal head) for bottom waters (Figures 3 and 4, Table 2).
19
Mean pH was homogenous over canal mouth zone and canal head zone for surface waters
(Figure 5, Table 2).
Water column- main body: Mean salinity, conductivity and total dissolved solids displayed
higher values at the bottom of the water column in comparison to the surface across all
zones (Figure 3, Table 2). In direct contrast, mean pH and dissolved oxygen had higher
values at the surface of the water column in comparison to the bottom across all zones
(Figures 4 and 5, Table 2). Mean temperature also exhibited higher values at the surface
waters (in comparison to the bottom waters) across all zones with the exception of the
middle zone (Table 2).
Water column- canals: Mean temperature, pH and dissolved oxygen were higher at the
surface waters in comparison to bottom waters for both canal mouth zone and canal head
zone (Figures 4 and 5, Table 2). In contrast mean salinity, conductivity and total dissolved
solids were higher at the bottom waters in comparison to surface waters for both canal
mouth zone and canal head zone (Figure 3, Table 2).
Table 2: Mean surface and bottom readings across zones
Lower
Middle
Upper
Canal (mouth)
Canal (head)
Depth (m)
Bottom
.83
1.18
1.21
1.62
1.53
Temperature (°C)
Surface
16.77
16.57
17.15
18.23
18.25
Bottom
16.64
16.84
17.14
17.75
17.94
Surface
16.49
11.73
8.89
11.12
11.21
Bottom
20.89
15.87
10.83
12.79
12.22
Surface
22567.79
16566.08
12889.79
16331.42
16451.72
Bottom
27730.92
21719.69
15436.67
18304.28
17662.17
Surface
8.25
8.79
8.86
9.04
9.04
Bottom
8.10
8.49
8.73
8.82
8.64
Surface
7.89
9.89
10.04
10.42
10.71
Bottom
6.56
8.02
7.55
7.17
3.97
Total Dissolved Solids (mg/l) Surface
17283.58
12712.10
9801.42
12114.68
12200.85
Bottom
21533.58
16803.39
11799.35
13793.35
13221.53
Bottom
.62
.71
.64
.94
.75
Salinity (ppt)
Conductivity (µS/cm)
pH
Dissolved Oxygen (mg/l)
Secchi Depth (m)
20
21
Mouth open state and mouth closed state comparison
Descriptive statistics for mouth open state and mouth closed state comparison
For both mouth open and closed states 174 samples (Valid N) were taken for all parameters
with the exceptions of depth, dissolved oxygen and Secchi depth for which less samples
were taken (Table 3). Maximum and minimum values for salinity, conductivity, pH and total
dissolved solids were higher and lower respectively (greater range) for mouth open state in
comparison to mouth closed state (Table 3). As a result standard deviation, standard error of
the mean and variance for salinity, conductivity, pH and total dissolved solids were higher for
mouth open state in comparison to mouth closed state (Table 3). Conductivity and total
dissolved solids displayed very high variance for both mouth open and closed states (Table
3).
Mean temperature and dissolved oxygen were found to have statistically significantly higher
values during mouth closed state in comparison to mouth open state, whilst mean
conductivity was not statistically significantly higher (Table 3). Mean temperature was 18.57
ºC during mouth closed state and 16.08 ºC during mouth open state (U= 4770.5, Z= -11.05,
p< .001), mean conductivity, 19185.93 µS/cm and 18829.45 µS/cm (U= 13969, Z= -1.25,
p= .213) and mean dissolved oxygen, 9.21 mg/l and 7.36 mg/l respectively (U= 3372, Z=
-4.4, p< .001).
22
In contrast mean Secchi depth was found to have statistically significantly higher values
during mouth open state in comparison to mouth closed state, whilst mean depth, salinity,
pH and total dissolved solids were not statistically significantly higher (Table 3). Mean depth
was 1.24 m during mouth closed state and 1.25 m during mouth open state (U= 3947.5, Z=
-.166, p= .868), mean salinity, 13.21 ppt and 13.88 ppt (U= 13968.5, Z= -1.25, p= .213),
mean pH, 8.62 and 8.69 (U= 13740.5, Z= -1.49, p= .136), mean total dissolved solids,
14188.92 mg/l and 14740.34 mg/l (U= 14072, Z= -1.14, p= .256) and mean Secchi depth,
0.63 m and 0.91 m respectively (U= 998, Z= -4.04, p< .001).
Table 3: Descriptive Statistics for mouth open state and mouth closed state comparison
Depth
Temperature
Salinity
Conductivity
(m)
(°C)
(ppt)
(µS/cm)
Total
Oxygen
Dissolved
Secchi
(mg/l)
Solids (mg/l)
Depth (m)
Mouth State
Descriptive Statistics
Closed
Valid N
90
174
174
174
174
108
174
65
Minimum
.30
15.20
3.54
5352.00
7.17
.52
4179.50
.20
Maximum
1.95
22.70
25.55
34552.00
9.55
16.78
25954.50
1.55
Mean
1.24
18.57
13.21
19185.93
8.62
9.21
14188.92
.63
Median
1.29
18.50
12.93
19372.50
8.68
9.24
13968.50
.40
Standard Deviation
.43
1.82
3.86
5300.68
.51
3.32
3818.69
.41
Standard Error of Mean
.05
.14
.29
401.84
.04
.32
289.49
.05
Variance
.18
3.31
14.89
28097162.66
.26
11.04
14582419.70
.16
Valid N
89
174
174
174
174
97
174
54
Minimum
.20
11.10
2.72
3935.00
7.09
.09
3282.50
.20
Maximum
1.90
18.60
31.62
39318.00
9.84
13.21
31479.50
1.69
Mean
1.25
16.08
13.88
18829.45
8.69
7.36
14740.34
.91
Median
1.30
16.40
13.29
18694.50
8.75
7.68
14306.50
.88
Standard Deviation
.42
1.59
6.09
7382.83
.69
2.78
5947.08
.36
Standard Error of Mean
.04
.12
.46
559.69
.05
.28
450.85
.05
Variance
.17
2.54
37.09
54506232.32
.48
7.75
35367812.81
.13
Open
pH
Dissolved
Mouth open state and mouth closed state comparison across surface waters and bottom
waters
Mean temperature and dissolved oxygen at both surface and bottom waters displayed
statistically significantly higher values during mouth closed state in comparison to mouth
open state. For surface waters mean temperature was 18.79 ºC during mouth closed state
and 15.94 ºC during mouth open state (U= 878, Z= -9.08, p< .001) and mean dissolved
oxygen, 10.65 mg/l and 8.60 mg/l respectively (U= 936, Z= -4.43, p< .001). For bottom
waters mean temperature was 18.32 ºC and 16.22 ºC (U= 1549.5, Z= -6.28, p< .001) and
mean dissolved oxygen, 7.42 mg/l and 5.44 mg/l respectively (U= 581, Z= -2.88, p= .004).
Statistically significantly higher mean conductivity values were recorded during mouth closed
state for surface waters, whilst mean salinity and total dissolved solids were not statistically
significantly higher. Mean salinity was 12.79 ppt during mouth closed state and 12.00 ppt
23
during mouth open state (U= 3926, Z= -.36, p= .723), mean conductivity 18696.76 µS/cm
and 16516.37 µS/cm (U= 3254, Z= -2.28, p= .023) and mean total dissolved solids,
13759.91 mg/l and 12882.77 mg/l respectively (U= 3890, Z= -.46, p= .647).
Mean Secchi depth was statistically significantly higher at the bottom waters during mouth
open state, whilst mean depth was not statistically significantly higher. Mean depth was 1.24
m during mouth closed state and 1.25 m during mouth open state (U= 3947.5, Z= -.17, p= .
868) and mean Secchi depth, 0.63 m and 0.91 m respectively (U= 998, Z= -4.04, p< .001).
Statistically significantly higher mean salinity and total dissolved solids values were seen
during mouth open state for bottom waters, whilst mean conductivity was not statistically
significantly higher. Mean salinity was 13.67 ppt during mouth closed state and 15.90 ppt
during mouth open state (U= 2840, Z= -2.18, p= .029), mean conductivity 19710.04 µS/cm
and 21307.75 µS/cm (U= 3355, Z= -.55, p= .583) and mean total dissolved solids, 14648.57
mg/l and 16730.61 mg/l respectively (U= 2863.5, Z= -2.11, p= .035). Mean pH was not
statistically significantly higher at both surface and bottom waters during mouth open state in
comparison to mouth closed state. For surface waters mean pH was 8.70 during mouth
closed state and 8.81 during mouth open state (U= 3575.5, Z= -1.36, p= .175). For bottom
waters mean pH was 8.53 and 8.55 respectively (U= 3404.5, Z= -.39, p= .695).
Mouth open state and mouth closed state comparison across sampling stations
Mouth state: Mean temperature (Z= -3.41, p= .001), dissolved oxygen (Z= -3.01, p= .003)
and conductivity (Z= -2.16, p= .031) were found to be statistically significantly higher during
mouth closed state in comparison to mouth open state across sampling stations (Figure 6
and 9, Table 4). Mean pH did not display statistically significantly higher values during mouth
closed state in comparison to mouth open state across sampling stations (Z= -1.19, p= .234)
(Figure 8, Table 4).
Mean salinity (Z= -2.27, p= .023), total dissolved solids (Z= -2.1, p= .036) and Secchi depth
(Z= -2.61, p= .009) displayed statistically significantly higher values during mouth open state
in comparison to mouth closed state across sampling stations (Figure 7 and 10, Table 4).
Mean depth (Z= -1.11, p= .267) and pH (Z= -1.19, p= .234) were found to be not statistically
significantly higher during mouth open state in comparison to mouth closed state across
sampling stations (Figure 8 and 11, Table 4).
Sampling stations- main body: Mean depth was found to increase from sampling station 2 to
6 (lower and middle zones) during mouth open state (Figure 11, Table 4). Mean temperature
increased from station 6 to 9 (middle and upper zones) during both mouth open and closed
states (Figure 6, Table 4). Mean pH increased from station 1 to 8 (lower, middle and upper
zones) during mouth closed state and increased from station 3 to 8 (lower, middle and upper
zones) during mouth open state (Figure 8, Table 4). Mean salinity and total dissolved solids
were recorded to decrease from station 1 to 9 (lower, middle and upper zones) during mouth
closed state and decrease from station 3 to 9 (lower, middle and upper zones) during mouth
open state (Figure 7, Table 4). Mean conductivity decreased from station 2 to 9 (lower,
middle and upper zones) during mouth closed state and decreased from station 3 to 9
(lower, middle and upper zones) during mouth open state (Table 4). Mean Secchi depth was
found to decrease from station 1 to 5 (lower and middle zones) during mouth closed state
and decrease from station 6 to 9 (middle and upper zones) during both mouth open and
24
closed states (Figure 10, Table 4). Dissolved oxygen did not display any obvious trends
across the sampling stations during mouth closed state or mouth open state.
Sampling stations- canals: Temperature, conductivity and dissolved oxygen at the canal
mouth and canal head were higher during mouth closed state in comparison to mouth open
state for station 10, 12, 14 and station 11, 13, 15 respectively. Depth at the canal head was
higher during mouth closed state in comparison to mouth open state for station 13 and 15.
Secchi depth at the canal mouth and canal head was higher during mouth open state in
comparison to mouth closed state for station 10, 12, 14 and station 11, 13, 15 respectively.
Salinity and total dissolved solids at the canal mouth were higher during mouth open state in
comparison to mouth closed state for station 10, 12 and 14. Salinity and total dissolved
solids at the canal head were higher during mouth open state in comparison to mouth closed
state for station 13 and 15. Depth at the canal mouth was higher during mouth open state in
comparison to mouth closed state for station 12 and 14. pH at the canal mouth was higher
during mouth open state in comparison to mouth closed state for station 10 and 12. pH at
the canal head was higher during mouth open state in comparison to mouth closed state for
station 11 and 13.
25
Table 4: Mouth open and closed state comparison across sampling stations (significantly different at p< 0.05 level, Wilcoxon
Signed Ranks test)
Depth (m)
Station
Zone
Closed
Temperature (°C)
Open
Closed
Open
Salinity (ppt)
Closed
Conductivity (µS/cm)
Open
Closed
Open
1
Lower
1.35
1.34
17.99
15.33
19.59
24.46
27176.42
30703.58
2
Lower
.33
.32
19.70
15.93
19.05
16.88
27514.67
22637.67
3
Lower
.85
.78
17.65
15.12
16.74
17.39
23477.08
22765.08
4
Lower
.79
.86
17.82
15.28
15.98
16.51
22528.08
21737.50
5
Middle
.78
.88
17.79
15.28
14.16
15.23
20266.25
20202.17
6
Middle
1.65
1.61
17.72
15.66
13.63
14.49
19419.83
19469.50
7
Middle
1.04
1.09
18.03
15.75
12.36
12.92
17903.58
17596.00
8
Upper
1.17
1.20
18.07
15.91
11.06
11.48
16188.58
15764.50
9
Upper
1.20
1.26
18.38
16.24
8.07
8.83
12117.42
12582.42
10
Canal (mouth)
1.58
1.54
19.33
16.73
12.71
12.91
18881.33
18035.92
11
Canal (head)
1.53
1.48
19.39
16.71
12.84
12.36
19068.33
17386.67
12
Canal (mouth)
1.54
1.63
19.43
16.63
12.13
12.44
18124.92
17411.33
13
Canal (head)
1.47
1.46
19.62
16.86
11.91
12.02
17879.33
16982.75
14
Canal (mouth)
1.70
1.71
19.16
16.66
10.49
11.08
15784.83
15668.75
15
Canal (head)
1.57
1.65
18.99
17.02
10.41
10.75
15622.58
15402.00
Significance
p= .267
p= .001
p= .023
p= .031
Table 4: Mouth open and closed state comparison across sampling stations continued (significantly different at p< 0.05 level,
Wilcoxon Signed Ranks test)
pH
Open
1
Lower
7.87
8.35
5.89
7.42
20372.54
24550.96
1.13
.78
2
Lower
8.02
8.39
7.97
7.26
19860.58
17679.92
.
.
3
Lower
8.19
8.05
7.79
7.06
17704.42
18149.67
.51
.45
4
Lower
8.36
8.28
8.73
7.42
16907.04
17280.21
.48
.60
5
Middle
8.55
8.40
10.14
7.91
15213.96
16082.63
.34
.65
6
Middle
8.64
8.57
8.79
8.21
14629.33
15374.13
.66
1.03
7
Middle
8.87
8.80
11.72
7.77
13371.04
13875.38
.62
.71
8
Upper
8.93
8.93
11.15
7.78
12067.25
12407.96
.60
.80
9
Upper
8.65
8.68
9.24
6.97
8945.50
9780.83
.51
.64
10
Canal (mouth)
8.85
9.03
9.59
7.71
13736.67
13879.67
.88
1.13
11
Canal (head)
8.74
9.01
7.75
6.93
13855.75
13346.67
.63
.96
12
Canal (mouth)
8.74
9.01
8.58
8.35
13153.13
13397.58
.75
1.19
13
Canal (head)
8.85
8.94
9.19
6.86
12939.88
12995.67
.54
1.00
14
Canal (mouth)
8.98
8.95
11.08
7.67
11490.92
12066.13
.68
1.16
15
Canal (head)
8.75
8.75
9.58
5.05
11421.58
11707.58
.61
.90
p= .003
Open
Closed
p= .036
Open
Secchi Depth (m)
Zone
p= .234
Closed
Total Dissolved Solids (mg/l)
Station
Significance
Closed
Dissolved Oxygen (mg/l)
Closed
Open
p= .009
26
27
28
Mouth open state and mouth closed state comparison across sampling stations and surface/
bottom waters
Mean temperature at both the surface waters (Z= -3.41, p= .001) and bottom waters (Z=
-3.3, p= .001) was found to be statistically significantly higher during mouth closed state in
comparison to mouth open state across sampling stations (Table 5). Mean dissolved oxygen
at the surface waters (Z= -3.29, p= .001) and bottom waters (Z= -2.29, p= .022) displayed
statistically significantly higher values during mouth closed state in comparison to mouth
open state across sampling stations (Table 5). Mean salinity (Z= -2.16, p= .031), conductivity
(Z= -3.29, p= .001) and total dissolved solids (Z= -2.39, p= .017) at the surface waters were
statistically significantly higher during mouth closed state in comparison to mouth open state
across sampling stations (Table 5). Mean pH at the bottom waters was not statistically
significantly higher during mouth closed state in comparison to mouth open state across
sampling stations (Z= -.09, p= .925) (Table 5).
Mean salinity (Z= -3.3, p= .001), conductivity (Z= -2.48, p= .013) and total dissolved solids
(Z= -3.3, p= .001) at the bottom waters displayed statistically significantly higher values
during mouth open state in comparison to mouth closed state across sampling stations
(Table 5). Mean pH at the surface waters displayed statistically significantly higher values
during mouth open state in comparison to mouth closed state across sampling stations (Z=
-2.1, p= .035) (table 5).
29
Table 5: Mouth open state and mouth closed state comparison across sampling stations and surface/ bottom waters (significantly
different at p< 0.05 level, Wilcoxon Signed Ranks test)
Depth (m)
Bottom
Station
Zone
Temperature (°C)
Surface
Closed
Open
Closed
Open
Salinity (ppt)
Bottom
Surface
Conductivity (µS/cm)
Bottom
Closed
Open
Closed
Open
Closed
Surface
Open
Bottom
Closed
Open
Closed
Open
1
Lower
1.35
1.34
18.38
15.35
17.60
15.32
19.08
21.18
20.11
27.74
26780.50
26495.17
27572.33
34912.00
2
Lower
.33
.32
19.70
15.93
.
.
19.05
16.88
.
.
27514.67
22637.67
.
.
3
Lower
.85
.78
17.47
14.72
17.83
15.52
16.03
12.82
17.45
21.96
22376.33
17420.50
24577.83
28109.67
4
Lower
.79
.86
17.63
14.98
18.00
15.58
14.70
12.20
17.26
20.82
20775.33
16542.17
24280.83
26932.83
5
Middle
.78
.88
17.82
15.15
17.77
15.40
13.07
11.58
15.26
18.88
18807.67
15853.50
21724.83
24550.83
6
Middle
1.65
1.61
17.70
15.43
17.73
15.88
12.54
11.26
14.71
17.71
17991.17
15529.83
20848.50
23409.17
7
Middle
1.04
1.09
18.00
15.30
18.07
16.20
11.65
10.29
13.08
15.56
16936.00
14278.33
18871.17
20913.67
8
Upper
1.17
1.20
18.25
15.72
17.88
16.10
10.49
9.65
11.63
13.31
15459.33
13452.17
16917.83
18076.83
9
Upper
1.20
1.26
18.60
16.05
18.15
16.43
7.20
8.21
8.95
9.44
10958.67
11689.00
13276.17
13475.83
10
Canal (mouth)
1.58
1.54
19.63
16.77
19.02
16.70
12.36
11.57
13.06
14.25
18525.67
16413.50
19237.00
19658.33
11
Canal (head)
1.53
1.48
19.68
16.65
19.10
16.77
12.52
11.52
13.15
13.21
18759.50
16322.33
19377.17
18451.00
12
Canal (mouth)
1.54
1.63
19.73
16.58
19.12
16.67
11.23
11.20
13.03
13.67
16989.83
15888.33
19260.00
18934.33
13
Canal (head)
1.47
1.46
19.85
16.85
19.38
16.87
11.30
11.45
12.51
12.60
17120.33
16233.50
18638.33
17732.00
14
Canal (mouth)
1.70
1.71
19.97
16.68
18.35
16.63
10.31
10.08
10.66
12.08
15787.00
14384.17
15782.67
16953.33
15
Canal (head)
1.57
1.65
19.50
16.98
18.48
17.05
10.33
10.15
10.49
11.35
15669.33
14605.33
15575.83
16198.67
Significance
p= .267
p= .001
p= .001
p= .031
p= .001
p= .001
p= .013
Table 5: Mouth open state and mouth closed state comparison across sampling stations and surface/bottom waters continued
(significantly different at p< 0.05 level, Wilcoxon Signed Ranks test)
30
pH
Surface
Station
Zone
Dissolved Oxygen (mg/l)
Bottom
Surface
Total Dissolved Solids (mg/l)
Bottom
Surface
Closed
Open
Closed
Open
Closed
Open
Closed
Open
Secchi Depth (m)
Bottom
Closed
Open
Closed
Open
Bottom
Closed
Open
1
Lower
7.93
8.31
7.82
8.38
6.31
6.94
5.32
7.90
19902.92
21136.83
20842.17
27965.08
1.13
.78
2
Lower
8.02
8.39
.
.
7.97
7.26
.
.
19860.58
17679.92
.
.
.
.
3
Lower
8.20
8.23
8.19
7.87
8.55
7.98
6.79
5.23
16971.58
13893.75
18437.25
22405.58
.51
.45
4
Lower
8.46
8.48
8.25
8.09
9.78
8.32
7.32
5.61
15676.83
13146.25
18137.25
21414.17
.48
.60
5
Middle
8.70
8.57
8.40
8.23
10.34
8.55
9.87
6.65
14073.00
12520.08
16354.92
19645.17
.34
.65
6
Middle
8.84
8.78
8.45
8.37
10.87
8.91
6.02
5.40
13556.83
12190.75
15701.83
18557.50
.66
1.03
7
Middle
8.90
8.96
8.83
8.65
11.37
9.33
12.19
4.68
12664.17
11267.75
14077.92
16483.00
.62
.71
8
Upper
8.97
9.00
8.89
8.86
11.87
8.91
10.42
6.27
11492.00
10575.50
12642.50
14240.42
.60
.80
9
Upper
8.73
8.76
8.57
8.61
11.33
8.05
7.16
5.52
8016.50
9121.67
9874.50
10440.00
.51
.64
10
Canal (mouth)
8.89
9.17
8.82
8.89
10.70
9.03
8.49
6.39
13395.42
12539.58
14077.92
15219.75
.88
1.13
11
Canal (head)
8.94
9.29
8.55
8.72
10.64
9.94
3.89
2.91
13539.42
12493.00
14172.08
14200.33
.63
.96
12
Canal (mouth)
8.93
9.08
8.55
8.95
11.89
9.19
5.27
7.22
12242.50
12165.83
14063.75
14629.33
.75
1.19
13
Canal (head)
9.03
9.11
8.68
8.78
12.48
9.45
5.90
3.41
12342.42
12402.00
13537.33
13589.33
.54
1.00
14
Canal (mouth)
9.01
9.13
8.95
8.76
12.47
9.24
9.69
5.58
11314.33
11030.42
11667.50
13101.83
.68
1.16
15
Canal (head)
8.98
8.90
8.52
8.60
13.26
7.70
4.68
2.40
11350.08
11078.17
11493.08
12337.00
.61
.90
Significance
p= .035
p= .925
p= .001
p= .022
p= .017
p= .001
p= .009
Environmental data
Correlation between salinity and rainfall
Mean salinity per sampling day did not correlate significantly with the sum of the rainfall that
fell on the sampling day and two days prior (r(60)= .12, p= .345). When the correlation was
carried out per zone, no significant correlation was achieved for the lower (r(12)= .01, p= .
974), middle (r(12)= .27, p= .389), upper (r(12)= .49, p= .108) (Figure 12), canal mouth
(r(12)= .14, p= .667) or canal head zones (r(12)= .22, p= .484). Spearman correlation
coefficient was a positive number for each zone indicating a positive relationship between
mean salinity per sampling day and the sum rainfall per sampling day. As the correlation
coefficient increased (more positive relationship) from the lower to the upper zone, the p
value decreased (closer to significance). Canal head displayed a higher correlation
coefficient and a lower p value in comparison to canal mouth.
31
6. Discussion
Mouth open state and mouth closed state combined
Descriptive statistics for the entire estuary
Mean depth for the Zandvlei Estuary was found to be 1.24 m, temperature, 17.32 ºC, salinity,
13.55 ppt, pH, 8.65, dissolved oxygen, 8.34 mg/l and Secchi depth, 0.76 m (Table 1).
Harding (1994) sampled various physicochemical parameters across the entire Zandvlei
Estuary between 1978 and 1991 (13 year period). Harding (1994) found that mean depth
was 0.89m, mean pH, 8.6, mean dissolved oxygen, 8.6 mg/l and mean secchi depth, 0.54
m. Quick and Harding (1994) monitored a number of physicochemical parameters in
Zandvlei Estuary during 1992 to 1993 and stated that mean temperature was 18 ºC, mean
salinity, 3 ppt, mean pH, 8.3 and mean sechi depth, 0.4 m (Quick and Harding, 1994).
Furthermore Muhl et al (2004) analysed salinity records for Zandvlei Estuary for the period
32
between 1978 and 2003 (25 year period) and found that mean salinity was 7 ppt. Therefore
with the exceptions of depth, salinity and secchi depth values (which were higher in
comparsison to previous studies) results for the physicochemical parameters measured
were similar between the current study and past studies at Zandvlei Estuary.
Harding (1994) stated that the 1992 mean secchi depth was very low at 0.36 m and
according to Muhl et al (2004) mean salinity levels between 1990 and 1992 were low in
comparison to other years (1978 to 2003). Perhaps this could explain why secchi depth and
salinity values were lower in comparison to the current study. In addition Muhl et al (2004)
stated that since january 2000 salinity peaks have been rising in Zandvlei Estuary. Perhaps
more recently (since 2000) the mouth has been artificially opened more frequently and for
longer time periods and this has caused certain physicochemical parameters such as salinity
to be higher in the current study in comparison to previous studies. According to Snow and
Taljaard (2007) the influx of seawater under mouth open conditions has been documented to
raise salinity in an estuary, particularly at the mouth. Throughout the current study total
dissolved solids was found to track salinity and any reasoning’s or trends related to salinity
could be expected to apply to total dissolved solids.
In the current study minimum and maximum values for temperature were found to be, 11.10
ºC and 22.70 ºC respectively, salinity, 2.72 ppt and 31.62 ppt respectively, conductivity, 3935
µS/cm and 39318 µS/cm respectively, pH, 7.09 and 9.84 respectively and secchi depth, 0.20
m and 1.69 m respectively (Table 1). Harding (1994) found that minimum and maximum
values for temperature were 9 ºC and 25.4 ºC respectively, salinity, <1 ppt and 22 ppt
respectively, conductivity, 410 µS/cm and 29000 µS/cm respectively, pH, 5.8 and 9.9
respectively, dissolved oxygen, 0.09 mg/l and 16.78 mg/l respectively and secchi depth, 0.09
m and 1.2 m respectively. According to Quick and Harding (1994) temperature exhibited a
minimum value of 12 ºC and a maximum value of 22.5 ºC, salinity, <1 ppt and 14 ppt
respectively, pH, 7.1 and 9.00 respectively and secchi depth, 0.01 m and 1.05 m
respectively. Furthermore Muhl et al (2004) found that minimum salinity for the estaury was 0
ppt and maximum salinity, 25 ppt. Therefore salinity, conductivity and secchi depth in the
current study displayed higher minimum and higher maximum values (greater range) in
comparsion to previous studies at Zandvlei Estuary. As mentioned above this could be due
to recent changes in the mouth manipulation strategy to allow the mouth to stay open more
frequently and for longer time periods. A minimum value of 0.09 mg/l for dissolved oxygen
indicated anoxic conditions (<0.5 mg/l) which can negatively affect biota if values remain at
this level for extended periods of time (by looking at the mean dissolved oxygen value this
was most probably not the case) (Kaselowski, 2012).
Surface waters and bottom waters comparison
The current study recorded mean temperature to be 17.37 ºC at the surface waters and
17.27 ºC at the bottom waters (U= 14311, Z= -.86, p= .388). By looking at the similarity in
mean values and the lack of statistical significance one can see that temperature was most
likely relatively homogenous between surface and bottom waters. Morant and Grindley
(1982) analysed physicochemical data from Zandvlei Estuary for the period from 1973 to
1982. Morant and Grindley (1982) found that mean temperature was similar between surface
and bottom waters due to the shallowness of the Zandvlei Estuary and the wind induced
mixing that occurs in the system.
33
Mean salinity was found to be 12.40 ppt at the surface waters and 14.78 ppt at the bottom
waters (U= 11422.5, Z= -3.94, p< .001) and mean conductivity, 17606.56 µS/cm at the
surface waters and 20508.89 µS/cm at the bottom waters (U= 11503, Z= -3.86, p< .001).
Harding (1994) found a comparable result whereby bottom waters exhibited 5 ppt higher
mean salinity and 13- 20% higher mean conductivity in comparison to surface waters for the
entire estuary. Harding (1994) mentioned that differences between surface and bottom
waters were most obvious after the artificial opening of the estuary mouth which resulted in
denser sea water moving into the estuary underneath (at the bottom waters) the outflowing
fresh water (at the surface waters).
The current study found that mean pH was 8.76 at the surface waters and 8.54 at the bottom
waters with the difference being statistically significantly different (U= 12500, Z= -2.79, p= .
005). Morant and Grindley (1982) mentioned that wide pH ranges exist at Zandvlei Estuary.
pH differences in the system can be caused by a number of factors including seawater
intrusion during mouth open state, freshwater inflow from rivers as well as stormwater drains
and the photosynthetic activity of aquatic macrophytes and phytoplankton (Morant and
Grindley, 1982).
The current study found that mean dissolved oxygen was 9.64 mg/l at the surface waters
and 6.54 mg/l at the bottom waters with the difference being statistically significantly different
(U= 2402.5, Z= -6.48, p< .001). The minimum dissolved oxygen value recorded was 0.09
mg/l and was measured at the bottom waters in the canals of the estuary. Morant and
Grindley (1982) stated that very low values for dissolved oxygen (as low as 0 mg/l) have
been recorded at the bottom waters in the Zandvlei Estuary (in comparison to surface
waters). Morant and Grindley (1982) went on to say that low dissolved oxygen values at the
bottom waters are due to large quantities of organic matter collecting on the bottom under
calm weather conditions (and subsequently being broken down by bacteria) for example
during the die back of Stuckenia pectinata and phytoplankton in winter as well as when S.
pectinata has been harvested in the canals. Another reason could be as a result of the
orientation of the canals so that they are protected from the wind. This results in salinity
stratification in the canals which in turn causes anoxic conditions to build up below the
halocline.
Zone comparison
Harding’s (1994) study to understand the physicochemical parameters of Zandvlei Estuary
between 1978 and 1991 made use of 11 sampling sites. The results for sampling station 1
from Harding’s (1994) study were compared to the upper zone in this study, station 2 and 3
were combined and compared to the middle zone, station 4 and 5 were combined and
compared to the lower zone, station 6 and 7 were combined and compared to the canal
mouth zone and station 8 was compared to the canal head zone. Three other stations (A, B,
C) were not used as they were located in the influent rivers.
Main Body: Mean temperatures were lower at the lower zone (16.72 ºC and 17.80 ºC
respectively), middle zone (16.70 ºC and 17.75 ºC respectively) and upper zone (17.15 ºC
and 17.80 ºC respectively) when compared to the study by Harding (1994). In addition,
mean temperature was found to be highest at the upper zone, lowest at the middle zone and
in between at the lower zone, with the difference between zones being not statistically
34
significantly different (H(2)= 2.59, p= .274). No trend was found for mean temperature across
zones in Harding’s (1994) data.
Mean pH was higher at the middle zone (8.64 and 8.60 respectively) and upper zone (8.80
and 8.50) when compared to the study by Harding (1994). In contrast, mean pH was lower at
the lower zone (8.19 and 8.30 respectively) when compared to the study by Harding (1994).
Furthermore, mean pH was found to increase from the lower zone through the middle zone
to the upper zone, with the difference between zones being statistically significant (H(2)=
52.16, p< .001). No trend was found for mean pH across zones in Harding’s (1994) data.
Mean salinity and conductivity were higher at the lower zone (18.38 ppt and 10.00 ppt
respectively, 24780.56 µS/cm and 14645.00 µS/cm respectively), middle zone (13.80 ppt
and 7.00 ppt respectively, 19142.89 µS/cm and 10985.00 µS/cm respectively) and upper
zone (9.86 ppt and 6.00 ppt respectively, 14163.23 µS/cm and 10250.00 µS/cm respectively)
in comparison to the study by Harding (1994). Furthermore, a decrease from the lower zone
through the middle zone to the upper zone was apparent for mean salinity and conductivity,
with the difference between zones being statistically significant for mean salinity (H(2)=
86.37, p< .001) and conductivity (H(2)= 75.52, p< .001). The same trend was found in the
study by Harding (1994) for mean salinity and conductivity across zones. Morant and
Grindley (1982) stated that salinity and conductivity were highest nearest the mouth of the
estuary and decreased moving towards the head of the estuary as a result of sea water
intrusion at the mouth and fresh water intrusion at the head.
Mean dissolved oxygen was higher at the lower zone (7.43 mg/l and 7.05 mg/l respectively),
middle zone (9.20 mg/l and 8.55 mg/l respectively) and upper zone (8.88 mg/l and 8.4 mg/l
respectively) in comparison to the study by Harding (1994). In addition, mean dissolved
oxygen was highest at the middle zone, lowest at the lower zone and in between at the
upper zone, with the difference between zones being statistically significant (H(2)= 13.37, p=
.001). The same result was witnessed in the study by Harding (1994) for mean dissolved
oxygen across zones.
Mean Secchi depth was higher at the middle zone (0.71 m and 0.59 m respectively) and
upper zone (0.64 m and 0.53 m respectively) in comparison to the study by Harding (1994).
In contrast, mean Secchi depth was lower at the lower zone (0.62 m and 0.89 m
respectively) in comparison to the study by Harding (1994). Furthermore, mean Secchi
depth was highest at the middle zone, lowest at the lower zone and in between at the upper
zone but the difference between zones was not statistically significant (H(2)= 0.6, p= .741).
In contrast, both Harding (1994) and Morant and Grindley (1982) found that mean Secchi
depth decreased from the lower zone through the middle zone to the upper zone.
Canals: Mean pH (8.93 and 8.65 respectively, 8.84 and 8.70 respectively), salinity (11.96 ppt
and 7.00 ppt respectively, 11.71 ppt and 7.00 ppt respectively) and conductivity (17317.85
µS/cm and 10840 µS/cm respectively, 17056.94 µS/cm and 11730 µS/cm respectively) were
higher at both the canal mouth zone and canal head zone in comparison to the study by
Harding (1994). Mean dissolved oxygen (8.87 mg/l and 8.55 mg/l respectively, 7.66 mg/l and
8.50 mg/l respectively) and Secchi depth (0.94 m and 0.68 m respectively, 0.75 m and 1.01
m respectively) were higher at the canal mouth zone and lower at the canal head zone in
comparison to the study by Harding (1994). Mean temperature was lower at the canal mouth
35
zone (17.99 ºC and 18.05 ºC respectively) and canal head zone (18.10 ºC and 18.30 ºC
respectively) in comparison to the study by Harding (1994).
Mean salinity (U= 2450, Z= -.57, p= .57), conductivity (U= 2481, Z= -.44, p= .657), pH (U=
2539.5, Z= -.21, p= .834), dissolved oxygen (U= 830.5, Z= -1.13, p= .258) and Secchi depth
(U= 324, Z= -1.68, p= .092) displayed higher values for canal mouth zone in comparison to
canal head zone (all of differences however were not statistically significant). The same
trend was found by Harding (1994) for dissolved oxygen. In contrast, Harding (1994) found
that mean pH, conductivity and Secchi depth exhibited lower values for canal mouth zone in
comparison to canal head zone (salinity was homogenous). Mean temperature (U= 2479.5,
Z= -.45, p= .653) was lower at the canal mouth zone in comparison to the canal head zone
(the difference however was not statistically significant). The same trend was found by
Harding (1994).
Interestingly none of the sampled parameters displayed a statistically significant difference
between canal mouth zone and canal head zone. This could be due to the proximity of the
canal mouth zone to the canal head zone and as a result the two zones are affected equally
by factors such as wind mixing and stratification.
Main body and canal comparison
In order to compare results from this study to the study by Harding (1994) station 1 to 5 in
the study by Harding (1994) were combined and compared to the main body in the current
study and station 6 to 8 were grouped together and compared to the canals in this study.
Mean salinity (14.76 ppt and 8.00 ppt respectively, 11.84 ppt and 7.00 ppt respectively)
conductivity (20292.60 µS/cm and 12302.00 µS/cm respectively, 17187.40 µS/cm and
11136.67 µS/cm respectively), pH (8.49 and 8.46 respectively, 8.88 and 8.67 respectively)
and Secchi depth (0.66 m and 0.65 m respectively, 0.85 m and 0.79 m respectively)
displayed higher values at both the main body of the estuary and the canals when compared
to the study conducted by Harding (1994). Mean dissolved oxygen was higher at the main
body (8.38 mg/l and 7.92 mg/l respectively) and lower at the canals (8.29 mg/l and 8.53 mg/l
respectively) when compared to the study by Harding (1994). Mean temperature was lower
at both the main body (16.81 ºC and 17.78 ºC respectively) and canals (18.04 ºC and 18.13
ºC respectively) when compared to the study by Harding (1994).
Mean salinity (U= 9569.5, Z= -5.54, p< .001), conductivity (U= 10833, Z= -4.17, p< .001) and
dissolved oxygen (U= 4867, Z= -.67, p= .504) displayed higher values for the main body of
the estuary in comparison to the canals (the difference for dissolved oxygen however was
not statistically significant). The same trend was found by Harding (1994) for mean salinity
and conductivity. Mean dissolved oxygen however was lower for the main body in
comparison to the canals in the study by Harding (1994). Mean temperature (U= 8966, Z=
-6.19, p< .001), pH (U= 8552, Z= -6.64, p< .001) and Secchi depth (U= 1438.5, Z= -1.76,
p= .078) were recorded to have lower values for the main body in comparison to the canals
(the difference for Secchi depth however was not statistically significant). The same trend
was found by Harding (1994). Morant and Grindley (1982) reported that the depth of the
canals was greater than the depth of the main body of the estuary. The same result was
found in the current study whereby the mean depth of the main body was 1.03 m and the
mean depth of the canals was 1.57 m (U= 834, Z= -8.88, p< .001).
36
Interestingly looking at the comparisons made with Harding’s (1994) data for “zone
comparison” and “main body and canal comparison” mean salinity and conductivity were
considerably higher in the current study in comparison to Harding’s (1994) study. As
mentioned earlier this could be due to changes in the mouth manipulation strategy to allow
more marine influence on the system by breaching the mouth more often and for longer time
periods.
Mouth open state and mouth closed state comparison
The current study found that mean temperature was statistically significantly higher during
mouth closed state in comparison to mouth open state (18.57 ºC and 16.08 ºC respectively)
(U= 4770.5, Z= -11.05, p< .001) (Table 3). In the study by C.A.P.E. (2013) two probes that
recorded depth, temperature and salinity were positioned in the main body of the Zandvlei
Estuary, one adjacent the yacht club (the same location as station 7 in this study) and the
other near the mouth of the estuary. Data was recorded for the period between September
2012 and January 2013 (C.A.P.E., 2013). Temperature data collected by C.A.P.E. (2013)
demonstrated the same result as the current study whereby mean temperature was higher
during mouth closed state in comparison to mouth open state. C.A.P.E. (2013) stated that
the reason for the lower temperature values during mouth open state (in comparison to
mouth closed state) was as a result of the influx of cold seawater into the system.
In the current study mean salinity displayed a greater range during mouth open state in
comparison to mouth closed state (28.9 ppt and 22.01 ppt respectively). The same result
was found by Riddin and Adams (2008), who studied the influence of mouth state and water
level on macrophytes in the small temporarily open/closed East Kleinemonde Estuary,
Whitfield et al (2008), who studied the influence of mouth state on the ecology of the East
Kleinemonde Estuary and Kaselowski (2012), who studied physicochemical and micro algal
characteristics of the Goukamma Estuary.
Snow and Taljaard (2007) developed a conceptual model for water quality characteristics in
temporarily open/closed estuaries. Snow and Taljaard (2007) compared the model to results
from various temporarily open/closed estuaries including the Diep Estuary (sampled in 1988
and 1989) and the Palmiet Estuary (sampled between 1986 and 2000).
Mean dissolved oxygen in the current study was found to have a statistically significantly
higher value during mouth closed state in comparison to mouth open state (U= 3372, Z=
-4.4, p< .001) which is in direct contrast to the conceptual model for temporarily open/closed
estuaries and the result found by Whitfield et al (2008). However, the Groot Brak Estuary
was often found to display low dissolved oxygen values at the bottom waters of the middle
and upper reaches (deep sections of the estuary) during mouth open state (Snow and
Taljaard, 2007). The explanation given was that the Groot Brak Estuary is a long, large
estuary (in comparison to other temporarily open/closed estuaries) and therefore the
effectiveness of tidal flushing on bottom waters was reduced (Snow and Taljaard, 2007).
Perhaps at Zandvlei Estuary the reduction in tidal flushing is not due to its size but rather
due to the physical modifications that have taken place at its mouth which reduce sea water
intrusion. Additionally, high dissolved oxygen readings during mouth closed state at Zandvlei
Estuary could be as a result of wind mixing the water column and maintaining aerated
conditions as was witnessed in the Diep Estuary (Snow and Taljaard, 2007).
37
Zandvlei Estuary exhibited a statistically significantly higher mean Secchi depth reading
during mouth open state in comparison to mouth closed state (U= 998, Z= -4.04, p< .001).
According to Snow and Taljaard (2007) the “turbidity levels in seawater entering estuaries
along the cool and warm temperate regions of South Africa are relatively low” (indicating
relatively high water transparency/ Secchi depth).
Mouth open state and mouth closed state comparison across surface waters and bottom
waters
Mean temperature was statistically significantly higher at both the surface (U= 878, Z= -9.08,
p< .001) and bottom waters (U= 1549.5, Z= -6.28, p< .001) during mouth closed state in
comparison to mouth open state. This could be as a result of the influx of cold seawater into
the estuary during mouth open conditions (C.A.P.E., 2013: Snow and Taljaard, 2007).
Mean dissolved oxygen was statistically significantly higher at both the surface (U= 936, Z=
-4.43, p< .001) and bottom waters (U= 581, Z= -2.88, p= .004) during mouth closed state in
comparison to mouth open state. Higher dissolved oxygen readings during mouth closed
state for surface and bottom waters could be as a result of wind causing mixing and
therefore aerating the water column (Snow and Taljaard, 2007). Lower dissolved oxygen
readings during mouth open state for surface and bottom waters could be as a consequence
of reduced tidal flushing due to the physical modifications to the mouth of Zandvlei Estuary.
A statistically significantly higher mean salinity value was seen during mouth open state in
comparison to mouth closed state for bottom waters (U= 2840, Z= -2.18, p= .029). This
occurrence could be as a result of the influx of seawater into the system during mouth open
conditions (C.A.P.E., 2013; Snow and Taljaard, 2007).
Mouth open state and mouth closed state comparison across sampling stations
When mouth open state and mouth closed state data was analysed across sampling stations
a number of parameters that were not statistically significantly different for “mouth open state
and mouth closed state comparison” were then found to be statistically significantly different
and these parameters are discussed below.
Mouth state: The current study found that there was no statistically significant difference in
mean depth between mouth open state and mouth closed state across sampling stations (Z=
-1.11, p= .267) (Figure 8, Table 4). In contrast, the research conducted by C.A.P.E. (2013)
demonstrated that the depth of the estuary was higher when the mouth was closed in
comparison to periods when the mouth was open. C.A.P.E. (2013) explained that the reason
for this occurrence was that when the mouth is closed, water levels build up in the estuary
due to the inflow of freshwater from influent rivers.
Mean temperature was found to be statistically significantly higher during mouth closed state
in comparison to mouth open state across sampling stations (Z= -3.41, p= .001) (Figure 6,
Table 4). In temporarily open/closed estuaries temperature is usually related to seasonal
trends in atmospheric temperature as was the case with the Diep Estuary (Snow and
Taljaard, 2007). The current study did not look at trends in physicochemical parameters over
seasons and therefore a more plausible explanation mentioned by Snow and Taljaard (2007)
is that temperature in a temporarily open/closed estuary is affected by the temperature of the
seawater that moves into the estuary under mouth open conditions (as was seen in the
38
Palmiet Estuary). If the temperature of the ocean water is low enough it can lower the
temperature in the estuary. This would explain why at Zandvlei Estuary temperatures are
statistically significantly lower during mouth open state in comparison to mouth closed state.
Mean salinity displayed statistically significantly higher values during mouth open state in
comparison to mouth closed state across sampling stations (Z= -2.27, p= .023) (Figure 7,
Table 4). The same result was found by C.A.P.E. (2013) at Zandvlei Estuary and Kaselowski
(2012) at the Goukamma Estuary. According to C.A.P.E. (2013) the influx of saline water
from the ocean during mouth open conditions can increase salinity in the estuary, in
particular at the estuary mouth. Snow and Taljaard (2007) commented that in temporarily
open/ closed estuaries salinity is also influenced by freshwater inflow at the head (mainly
during winter) as well as evaporation (mainly during summer). Fresh water inflow could have
lowered salinity levels during mouth closed state at Zandvlei Estuary as was seen in the
study by Kaselowski (2012) at the Goukamma Estuary.
No statistically significant difference was found in mean pH between mouth open state and
mouth closed state across sampling stations (Z= -1.19, p= .234) (Figure 11, Table 4). In
agreement with this result, Whitfield et al (2008) stated that the East Kleinemonde Estuary
did not show any considerable variation between mouth open and closed states for pH.
According to Snow and Taljaard (2007) high freshwater inflow lowers pH and high saltwater
inflow increases pH, however pH generally ranges between 7 and 8.5 in temporarily
open/closed estuaries (this result was found in the Diep and Palmiet Estuaries).
Mean temperature (Z= -3.41, p= .001) and dissolved oxygen (Z= -3.01, p= .003) were found
to be statistically significantly higher during mouth closed state in comparison to mouth open
state across sampling stations (Figure 6 and 9, Table 4). Mean Secchi depth displayed
statistically significantly higher values during mouth open state in comparison to mouth
closed state across sampling stations (Z= -2.61, p= .009) (Figure 10, Table 4). As a result the
same reasoning’s used in the section “mouth open state and mouth closed state
comparison” for temperature, dissolved oxygen and Secchi depth can also be applied here.
Sampling stations- main body: Mean depth was found to increase from sampling station 2 to
6 (lower and middle zones) during mouth open state (Figure 11, Table 4). This is as a result
of bathymetry of the estuary which displays shallow depths close to the mouth getting
deeper towards the middle reaches. Dissolved oxygen did not display any obvious trends
across the sampling stations during mouth closed state or mouth open state. In contrast
Kaselowski (2012) stated that dissolved oxygen was negatively correlated with distance from
the mouth during both mouth open and mouth closed state.
Mean temperature at Zandvlei Estuary increased from station 6 to 9 (middle and upper
zones) during both mouth open and closed states (Figure 6, Table 4). Snow and Taljaard
(2007) commented that during mouth open state a longitudinal temperature gradient can
sometimes occur in temporarily open/closed estuaries with the lowest values being
witnessed at the estuary mouth and increasing towards the estuary head (as was seen in the
Palmiet Estuary). Furthermore Kaselowski (2012) found that temperature was positively
correlated with distance from the mouth but only for mouth closed state.
Mean salinity was recorded to decrease from station 1 to 9 (lower, middle and upper zones)
during mouth closed state and decrease from station 3 to 9 (lower, middle and upper zones)
during mouth open state (Figure 7, Table 4). A similar result was found by Kaselowski
39
(2012), whereby salinity was significantly negatively correlated with distance from the mouth
during both mouth open and mouth closed state. Snow and Taljaard (2007) mentioned that
during mouth open state a longitudinal salinity gradient is present in temporarily open/closed
estuaries with the highest values being witnessed at the estuary mouth and decreasing
towards the estuary head (as was witnessed at the Diep Estuary during periods when fresh
water inflow was low). When the mouth is closed salinity is homogenous, however some
longitudinal stratification may be apparent as was the case with the Palmiet Estuary (Snow
and Taljaard, 2007).
Mean pH increased from station 1 to 8 (lower, middle and upper zones) during mouth closed
state and increased from station 3 to 8 (lower, middle and upper zones) during mouth open
state (Figure 8, Table 4). According to Kaselowski (2012) pH was negatively correlated with
distance from mouth at the Goukamma Estuary. Furthermore Snow and Taljaard (2007)
stated that high freshwater inflow lowers pH and high saltwater inflow increases pH. The
findings from both Kaselowski (2012) and Snow and Taljaard (2007) disagree with the
current studies results regarding pH. Perhaps the photosynthetic activity of aquatic
macrophytes and phytoplankton is having an effect on pH in Zandvlei Estuary. According to
Morant and Grindley (1982) when plants photosynthesise they remove carbon from the
water which can raise pH levels in the water.
Mean Secchi depth (transparency) at Zandvlei Estuary was found to decrease from station 1
to 5 (lower and middle zones) during mouth closed state and decrease from station 6 to 9
(middle and upper zones) during both mouth open and closed state (Figure 10, Table 4).
Kaselowski (2012) found that transparency was significantly negatively correlated with
distance from the mouth and therefore decreased from the estuary mouth to the estuary
head. The decrease from station 6 to 9 could be as a result of river water with low
transparency (in comparison to estuarine water) flowing into the system at the head of the
estuary (near station 9).
Mouth open state and mouth closed state comparison across sampling stations and surface/
bottom waters
When mouth open state and mouth closed state data was analysed across sampling stations
and surface/ bottom waters a number of parameters that were not statistically significantly
different for “mouth open state and mouth closed state comparison across surface waters
and bottom waters” were then found to be statistically significantly different and these
parameters are discussed below.
Mean salinity at the surface waters were statistically significantly higher during mouth closed
state in comparison to mouth open state across sampling stations (Z= -2.16, p= .031) (Table
5). According to Snow and Taljaard (2007) salinity in an estuary is affected by seawater
intrusion, fresh water intrusion and evaporation. Under mouth closed conditions salinity
levels would be affected by fresh water intrusion and evaporation. The presence or absence
of any of these factors could have resulted in salinity at the surface waters being statistically
significantly higher during mouth closed state in comparison to mouth open state across
sampling stations.
Mean pH at the surface waters displayed statistically significantly higher values during mouth
open state in comparison to mouth closed state across sampling stations (Z= -2.1, p= .035)
(table 5). According to Snow and Taljaard (2007), high freshwater inflow lowers pH and high
40
saltwater inflow increases pH. Perhaps the inflow of saltwater from the sea into the estuary
during mouth open conditions caused the elevated pH levels witnessed at Zandvlei Estuary
in the current study.
Mean pH at the bottom waters was not statistically significantly higher during mouth closed
state in comparison to mouth open state across sampling stations (Z= -.09, p= .925) (Table
5). The lack of significance would indicate that pH at the bottom waters was similar for both
mouth open and mouth closed states. Whitfield et al (2008) found a similar result whereby
pH did not display any considerable variation between mouth open and mouth closed state
at the East Kleinemonde Estuary. Snow and Taljaard (2007) commented that pH generally
ranges between 7 and 8.5 in temporarily open/closed estuaries whether the mouth is open
or closed.
Mean temperature at both the surface waters (Z= -3.41, p= .001) and bottom waters (Z=
-3.3, p= .001) was found to be statistically significantly higher during mouth closed state in
comparison to mouth open state across sampling stations (Table 5). Mean dissolved oxygen
at the surface waters (Z= -3.29, p= .001) and bottom waters (Z= -2.29, p= .022) displayed
statistically significantly higher values during mouth closed state in comparison to mouth
open state across sampling stations (Table 5). Mean salinity (Z= -3.3, p= .001) at the bottom
waters displayed statistically significantly higher values during mouth open state in
comparison to mouth closed state across sampling stations (Table 5). As a result the same
reasoning’s used in the section “mouth open state and mouth closed state comparison
across surface waters and bottom waters” for temperature (at the surface and bottom
waters), dissolved oxygen (at the surface and bottom waters) and salinity (at the bottom
waters) can also be applied here.
Physicochemical targets for Zandvlei Estuary
Targets were outlined by C.A.P.E. (2013) for salinity and dissolved oxygen at Zandvlei
Estuary. Targets for salinity were separated according to season (summer and winter),
according to zone (main body and outlet channel) and according to surface and bottom
waters. Therefore in order to find out whether the current study’s data met the targets the
lower zone in the current study was compared to the outlet channel in the study by (C.A.P.E.,
2013) and the middle and upper zones in the current study were combined and compared to
the main body in the study by (C.A.P.E., 2013). The current study was not conducted over
specific seasons and therefore the data could not be compared to specific seasonal targets.
The current study met the winter targets of 5 ppt (surface) and 7 ppt (bottom) targets with
values of 10.31 ppt (surface) and 13.35 ppt (bottom) for the main body of the estuary as well
the summer target of 10 ppt (throughout the water column) with a value of 11.83 ppt
(throughout the water column) (Table 2). The current study also met the winter targets of 6
ppt (surface) and 18 ppt (bottom) with values of 16.49 ppt (surface) and 20.89 ppt (bottom)
as well as the summer targets of 11 ppt (surface) and 13 ppt (bottom) with values of 16.49
ppt (surface) and 20.89 ppt (bottom) (Table 2). C.A.P.E. (2013) set a target for dissolved
oxygen of 6 to 8 mg/l for the entire estuary. The current study exceeded the target with a
value of 8.34 mg/l for the entire estuary (Table 1). The mean depth for the entire estuary
was found to be 1.24 m (Table 1). This depth is sufficient for recreation activities to be
practically possible, it allows the pond weed harvester to operate and does not put the
houses of Marina da Gama in danger.
41
Environmental data
Correlation between salinity and rainfall
Muhl et al (2004) made use of rainfall records to calculate the mean rainfall per month.
Monthly salinity was calculated using the mean salinity of the first measure of each month for
four sampling stations in Zandvlei Estuary. Mean salinity was then correlated with mean
monthly rainfall from the previous month to determine the relationship between rainfall and
salinity for the period between April 1978 and March 2003. Muhl et al (2004) found that
salinity values were significantly negatively related to rainfall. This meant that when rainfall
was high (winter) salinity was low and when rainfall was low (summer) salinity was high.
In contrast the current study found that mean salinity per sampling day did not correlate
significantly with the sum of the rainfall that fell on the sampling day and two days prior
(r(60)= .12, p= .345). When the correlation was carried out per zone, no significant
correlation was achieved for the lower, middle, upper, canal mouth or canal head zones.
Interestingly Spearman correlation coefficient was a positive number for each zone
indicating a positive relationship between mean salinity per sampling day and rainfall sum.
This means that when rainfall was high, salinity was high and vice versa. An explanation for
this could be that there are other factors besides rainfall that are affecting salinity values.
One such factor could be mouth state. Currently the mouth of Zandvlei Estuary remains
open during the winter months. Perhaps this is causing high salinity values (due to salt water
intrusion) in winter which is also the time of year when rainfall is expected to be high in the
area.
7. Conclusions
Salinity, conductivity, pH, dissolved oxygen and total dissolved solids were found to be
statistically significantly different between surface and bottom waters. Depth, salinity,
conductivity, pH, dissolved oxygen and total dissolved solids displayed a statistically
significant difference between lower, middle and upper zones. No parameter exhibited a
statistically significant difference between canal mouth and canal head zones. Depth,
temperature, pH, salinity, conductivity and total dissolved solids were found to be statistically
significantly different between the main body of Zandvlei Estuary and the Marina da Gama
canals. When the sampled parameters were compared to previous literature from Zandvlei
Estuary, salinity and conductivity displayed considerably higher values across zones and
across the main body and canals whilst the other parameters were similar.
Temperature and dissolved oxygen were found to be statistically significantly higher during
mouth closed state in comparison to mouth open state across the entire estuary, sampling
stations and surface and bottom waters. Salinity and total dissolved solids were statistically
significantly higher during mouth open state in comparison to mouth closed state across
sampling stations and bottom waters. Conductivity was found to be statistically significantly
higher during mouth closed state in comparison to mouth open state across sampling
42
stations and surface waters. Secchi depth was statistically significantly higher during mouth
open state in comparison to mouth closed state across the entire estuary, sampling stations
and bottom waters. pH displayed statistically significantly higher values during mouth open
state in comparison to mouth closed state across sampling stations and surface waters.
Depth did not display a statistically significant difference between mouth open state and
mouth closed state. In addition mean salinity per sampling day did not correlate significantly
with the sum of the rainfall that fell on the sampling day and two days prior.
Therefore artificial mouth manipulation has an effect on physicochemical parameters at
Zandvlei Estuary. As a result of the importance of these parameters to recreational users of
the estuary, home owners in Marina da Gama and the biological component of Zandvlei
Estuary, artificial mouth manipulation is a key tool for management. Artificial mouth
manipulation should be used to manage the estuary in a manner that allows the system to
function as naturally as possible without compromising the needs of recreational users and
home owners. Furthermore the current study found that physicochemical targets for Zandvlei
Estuary are being met and therefore the current mouth manipulation strategy should be
continued.
8. Recommendations and Reflections
The current study adds to the existing body of knowledge regarding the physicochemical
properties of Zandvlei Estuary by providing new/ current data in this area. As no prior
research had been conducted at Zandvlei Estuary dedicated to understanding the influence
of artificial mouth manipulation on the physicochemical characteristics of the system, the
current research provides valuable baseline data in this regard. This data can be used to
assist the GZENR and the bodies’ manging mouth manipulation to make informed decisions
regarding the artificial mouth manipulation plan for Zandvlei Estuary. If the artificial mouth
manipulation plan is correctly managed then the estuary will be able to function in a way that
benefits all stakeholders equally. Furthermore the results provided add to the existing
knowledge on the influence of artificial mouth manipulation on the physicochemical
properties of temporarily open/closed estuaries in South Africa.
The results gathered and the conclusions drawn by this study give the opportunity for
expansion whereby further research and analyses can be done on the physicochemical
characteristics of Zandvlei Estuary. This could include:



conducting a seasonal study to quantify if physicochemical parameters change over
seasons during mouth open state and mouth closed state;
correlating tidal height as well as the number of days after mouth opening/closing
with mean salinity levels in the estuary. This would explain whether there are other
factors besides mouth manipulation influencing salinity levels;
studying the effects of the physicochemical characteristics of Zandvlei Estuary on the
flora and fauna of the system This could be done for example by correlating salinity
levels to the abundance of a particular species;
43



Conducting an inter correlation between physicochemical parameters during mouth
open state and mouth closed state;
developing salinity contour maps for better data representation; and
studying the physicochemical characteristics of the influent rivers. The inflowing
water will most likely have an influence on the physicochemical characteristics of the
estuary.
The current study found that physicochemical targets set for Zandvlei Estuary are being met
by the current mouth manipulation plan and therefore the plan should be maintained. It is
important to note that ongoing monitoring of physicochemical parameters must continue so
that management (of mouth manipulation) is kept up to date with this data. This will avoid a
situation whereby changes to the system (for example a drop in salinity or dissolved oxygen
which could threaten biota) go unnoticed and have a drastic and irreversible negative effect
on the system. By noticing negative physicochemical changes in the system, the mouth
manipulation plan can be altered by management so as to keep the estuary functioning in a
healthy manner; in a way that takes all stakeholders physicochemical requirements into
consideration.
9. Acknowledgements
44
Thank you to my supervisor, Dr Walker for assisting me throughout the study, to Russell
Maurer and Torey Westgarth- Taylor for helping me with sampling, the Cape Peninsula
University of Technology for giving me permission to use their YSI multimeter and to the
citizen scientists, Timm Hoffman, Lucia Rodrigues, Gerrard Wigram, Bert Bron and John
Fowkes who’s rainfall data I made use of from the Zandvlei Trust website. I greatly
appreciate the advice, assistance and support I have received over the duration of the study.
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45
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