Characterization of a Tropical Estuarine System: The Placencia Lagoon
Technical Report
by
Eugene Ariola
CZMI Water Quality Monitoring Program
Coastal Zone Management Authority and Institute
Princess Margaret Drive, Belize City
Reference:
Ariola, E. A. 2003. Characterization Of A Tropical Estuarine System: The Placencia
Lagoon. Report prepared under the Coastal Zone Management Authority and Institute.
Work in Progress for Public Discussion.
i
Table of Contents
Acknowledgement
iii
List of Figures
iv
List of Tables
iv
List of Plates
iv
Abbreviations and Notations
v
Executive Summary
1
Introduction
2
Objectives
3
Hypotheses
3
Study Area
3
Materials and Methods
5
Characteristics of the terrestrial zone of influence
on the Placencia Lagoon
7
Characteristics of the Belize Barrier Reef Lagoon
(Coastal oceanographic zone of Influence)
10
Importance of the Placencia Lagoonal Estuarine System
11
Major Land use and Activities in the Placencia Lagoon
and surrounding areas
14
Results
17
Discussions
26
Conclusions
29
Recommendations
29
Bibliography
30
Appendices
33
ii
ACKNOWLEDGEMENT
The preparation of this technical report would not have been possible without the
assistance of several persons. Special recognition is due to the members of the survey
team especially Captain Kirk Rodriquez, Hampton Gamboa, Trenton Samuels and
Alexander Garbutt. These gentlemen worked diligently to collect confident data and
made each survey a memorable experience. Thanks to the CZMAI Administrative Staff
especially Melissa Almendarez for providing the logistical support for the field
monitoring surveys.
Thanks are also due to Ian Gillett for providing assistance in resolving the software
glitches that were experienced in processing and analyses of the data presented in this
report.
I am indebted to Natalie from the Belize National Meteorological Service and Rigoberto
Quintana of the Belize Fisheries Department for providing valuable information for this
report.
iii
List of Figures
Figure 1. Geographic location of the Placencia Lagoon
Figure 2. Extent of the terrestrial zone of Influence on the Placencia Lagoon
Figure 3. Villages in the Catchments Surrounding the Placencia Lagoon
Figure 4. Shrimp Mariculture Operations Near the Placencia Lagoon
Figure 5. Bathymetric Map of the Placencia Lagoon
Figure 6. Mean Temperatures for the Placencia Lagoon
Figure 7. Mean pH Measurements for the Placencia Lagoon
Figure 8. Mean Specific Conductivity Measurements for the Placencia Lagoon
Figure 9. Mean Salinity measurements for the Placencia Lagoon
Figure 10a. Mean D.O. mg/L
Figure 10b. Mean D.O. % Saturation
Figure 11. Mean Turbidity Measurements for the Placencia Lagoon
List of Tables
Table 1. Summary of the characteristics of villages in the Placencia Catchment
Table 2a. Shrimp Mariculture Production & Export Data 2001.
Table 2b. Shrimp Mariculture Production & Export Data 2002.
Table 3. Mean Measurments of Several Parameters in the Placencia Lagoon
Table 4. Variation between Surface and Bottom Salinities of the Placencia lagoon
List of Plates
Plate 1. Photographs (a)-(h)
iv
EXECUTIVE SUMMARY
There is growing concern about the current status and fate of the Placencia Lagoon
amidst the changing land use in the adjoining catchments. The destruction of natural
vegetation to construct dwellings, resorts, roads, and aquaculture development is a major
issue in the study area. Another issue that is frequently echoed and poorly substantiated
is the contamination of the lagoon by domestic and industrial effluent.
The first part of this report focuses on the Placencia Lagoon not solely from the
perspective of its immediate borders but rather in the context of the catchments and the
Southern Shelf Lagoon that bear influence on its health. Knowledge about the geological,
hydrological, meteorological and oceanographic characteristics of the zone of influence
on the Placencia Lagoon is essential to understanding its structure and function.
Furthermore, appreciations of the biological and chemical processes that operate therein
are fundamental to determine the lagoons ability to cope with the rising influx of
sediment and effluent loadings. It is in this light that a conscious effort was made to
characterize the lagoon.
The results generated by this research substantiate that the Placencia Lagoon can be
classified as a moderately stratified estuary based on its salinity distribution and
circulation patterns. In addition to this, there is a notably low water exchange rate
between the upper section of the lagoon and the sea. These finding signals how
vulnerable the ecological and environmental conditions of the lagoon are to the impacts
of developmental activities in the area of influence. Without cause for alarm, it is strongly
recommended that future developments be planned to ensure the lowest possible impacts
on the estuarine system.
The second part of this report is an assessment of the water quality in the Placencia
Lagoon. Several water quality surveys were conducted in the lagoon in 2001 and 2003.
Key water quality indicators used to determine the status of the Placencia Lagoon reveal
that it is a healthy estuarine system. This signifies that the conditions therein are
conducive to unimpaired growth and reproduction of estuarine organisms.
It is envisaged that this report will stimulate public discussion about the fate of the
Placencia Lagoon and trigger future monitoring, research and management initiatives in
the area.
1
INTRODUCTION
There are 30 coastal lagoons distributed along the mainland coastline of Belize. Most of
these lagoons remain in pristine condition. This may be attributed to the fact that most of
these lagoons are remote and isolated from direct human impacts. The Placencia Lagoon
is one of the larger coastal lagoons in Belize. It is currently a focal area for human
settlement, tourism, fisheries and aquaculture development.
The rapid expansion of the human settlements in the area augments the perception of loss
of critical habitats associated with destruction of natural vegetation and increase
discharge of untreated domestic wastewater. The constant upgrading of roads that link
one settlement with another is a potential source of sediment to the nearby waterways and
the Placencia Lagoon.
The tourism industry is an integral component of the socio-economic situation in the
communities of the Placencia Peninsula. The current trend to expand tourism
development in the area could increment economic benefits in the short term to the
detriment of the fragile environs of the Placencia Lagoon. Resultant impacts of tourism
development include mangrove destruction and structural modifications such as dredging
and marina development. Other impacts include wastewater discharge, euthrophication
and increase in boat traffic. Evidently, there is the need to for planned tourism
development that is environmentally benign.
Shrimp mariculture development is a viable activity around the Placencia Lagoon. Since
the inception of the mariculture industry, this activity has rendered significant socioeconomic benefits to the local communities. The continued expansion of the shrimp
mariculture operations around the lagoon will destroy natural vegetation and enhance the
salinization of the already poor grade soils. The potential impacts to the coastal lagoon
include increase loadings of nutrients and organic material, euthrophication, sediment
accretion and the spread of pathogens.
There is overwhelming concern about the status of the Placencia Lagoon especially
considering its intrinsic value and the fact that many coastal lagoons around the world are
under threat of damage, severely damaged or already destroyed consequent to
anthropogenic impacts. Thus far there is no comprehensive study that documents the
status and health of the Placencia Lagoon. To this end, a new approach was undertaken to
(1) collate as much information as possible regarding the physical characteristics of the
entire estuarine system and (2) conduct a limited time scale water quality monitoring in
the Lagoon. It is expected that the information presented in this report will be used to
initiate future monitoring and research initiatives to further contribute to informed
management of developmental activities in the Placencia Lagoon and its adjacent
catchments.
2
OBJECTIVES
To identify and document the characteristics of the estuarine system of the Placencia
Lagoon within the context of influences from adjoining catchments and the Belize Barrier
Reef Lagoon.
To elucidate the vulnerability of the Placencia Lagoon to the potential impacts of land use
changes including human settlement and aquaculture development.
To assess the current status of the Placencia Lagoon based on key water quality
indicators.
HYPOTHESIS
The spatial and temporal fluxes of pollutants within the Placencia Lagoon are regimented
largely by the physical characteristics of the lagoon coupled with the attributes of the
terrestrial and coastal oceanographic zones of influence.
Water quality deterioration and habitat degradation in the estuarine system can be
accelerated by improper waste disposal and effluent discharge.
STUDY AREA
The Placencia Lagoon is a semi-enclosed coastal lagoon located in Southern Belize
between 16°30’ and 16°40’ N latitude and 88°20 and 88°25’ W longitude (Fig. 1). The
Placencia Lagoon is 3.4 km wide at its widest extent, 20 km long and covers a surface
area of 30 km2. At mean sea level (MSL), the average depth of the lagoon is about 1.5 m.
The lagoon is physically influenced by the coastal oceanographic conditions of the
Southern Barrier Reef Lagoon and the terrestrial conditions of the surrounding
catchments.
3
4
MATERIALS AND METHODS
The Coastal Zone Management Authority and Institute provided the equipment and
materials that were utilized in this study. These include the following:
(a) a 28 feet skiff equipped with two 115 hp outboard engines, global positioning
system (GPS) and echo sounder.
(b) Two water quality multiparameter sonde [H20 and DataSonde 4a manufactured
by Hydrolab Corporation].
(c) A unidirectional current meter manufactured by Qualimetrics Inc.
(d) Graduated PVC pipe (The measuring Pole)
The objectives of this research were accomplished using the methods and data analyses
described below.
Comprehensive Literature Review
Throughout the duration of this study, the author was fully engaged in reviewing
literature pertinent to the Placencia Lagoon. This step was necessary to collate relevant
information and identify the information gaps. It is believed that the existing information
gaps limit our understanding of how the lagoon functions as a system, how it reacts to
hydrological, meteorological and tidal forces. Furthermore, it is not clear how the lagoon
responds ecologically to anthropogenic inputs of sediment, nutrients and pollutants.
Data Collection
Water Quality
Four water quality surveys were conducted in the Placencia Lagoon on the following
dates:
Survey 1
April 5, 2001
Survey 2
May 31, 2001
Survey 3
March 12, 2003
Survey 4
July 17,2003
Sampling stations were randomly selected in the study area on all surveys. At total of 59
stations were sampled on Survey 1; 65 stations on Survey 2; 53 stations on Survey 3 and
25 stations on Survey 4.
The water quality multiparameter sonde were used to record surface and bottom
measurements of temperature, pH, Specific conductivity, salinity, dissolved Oxygen,
depth and turbidity. Prior to any measurement in the field the sonde was properly
calibrated in the CZMAI Laboratory. The salinity sensor was calibrated using the
synthetic seawater method described in section 1.1H of Parsons et al. (1984). The
accuracy of the salinity sensor is ± 0.1 parts per thousand.
The pH probe was calibrated using a three-point calibration procedure with pH buffers 5,
7 and 10. The accuracy of the pH probe is ± 0.2 units.
5
The turbidity sensor was calibrated with known concentrations of formazine solution.
The accuracy of the sensor was ± 2.0 NTU.
The depth sensor (a pressure transducer) was regularly calibrated in the field to
compensate for changes in water level, wave height and barometric pressure.
Unlike the aforementioned sensors, the temperature sensor is factory calibrated with an
accuracy of ± 0.05 °C hence, there was no need to calibrate this senor in the laboratory or
the field.
Bathymetry
A special field bathymetric survey was conducted on July 18, 2003 in which 202
soundings were recorded in the Placencia Lagoon. The depth measurements were taken
using the pressure transducer installed on the multiparameter sonde and the echo sounder
aboard the CZMAI research vessel (Submarine 2). A graduated PVC pipe was used to
measure the depths in extremely shallow but critical parts of the lagoon. As with the
water quality stations, all bathymetric soundings were geo-referenced using the global
positioning system.
Data Analyses
The data recorded on paper and on the multiparameter sondes were uploaded to
Microsoft Excel worksheets. Using this software, basic mathematical operations were
conducted to prepare several charts and tables. Furthermore, water quality variables were
analyzed with descriptive statistics to determine means, ranges, standard deviation and
variance.
A non-drift krigging interpolation method was used to create contour maps using Surfer
7.0. Maps of this type illustrate the bathymetry, spatial distribution of temperature and
salinity within the study area. Considering that most of the data obtained in this research
are geo-referenced data, the final maps and graphic illustrations were produced using
ArcView GIS 3.1.
6
Characteristics Of The Terrestrial Zone Of Influence On The Placencia Lagoon
Geology
The Placencia Lagoon catchments (Santa Maria, August Creek and Big Creek) are part of
the Central Coastal Plain as described by King et al. (1989). Within this portion of the
coastal plain are relict marine terraces and alluvial fans (Pleistocene sediment) overlain
by more recent floodplains and terraces of the Toledo Flood Plains Land System.
The Placencia Peninsula, which is an appendage of the Santa Maria Creek Catchment, is
a striking geomorphological feature. In fact, the peninsula is the largest sand spit along
the Belize coast. This barrier spit that is comprised of few Holocene beach ridges and is
only a few hundred feet wide, separates the Placencia Lagoon from the Caribbean Sea
(High, 1969).
Based on the geology of the area it is understood that the quaternary alluvium deposited
in the terrestrial zone bearing influence on the Placencia Lagoon, is poorly consolidated
and readily erodible. This could be a major source of sediment influx to the lagoon
especially in light of changing land use, deforestation and desertification.
Hydrology
Three catchments namely the Santa Maria Creek, August Creek and Big Creek border the
lagoon (Figure 2). The Santa Maria Creek watershed has an area of 347 km2. The terrain
is relatively low-lying with a maximum elevation of 500 meters on its westernmost
boundary. Based on a flood risk analysis (Ariola et al., 2000), approximately 12.7 km2 of
the basin is permanently inundated and 32.7 km2 is prone to flooding. Santa Maria and
Hemsley Creeks debouches directly into the Placencia Lagoon. However, Silver Creek
and several small creeks in this watershed flow into wetlands that display connectivity to
the Placencia Lagoon. The vegetation cover within the watershed includes pine ridge
savannah and mangrove swamps in the upper and lower reaches respectively.
The August Creek Catchment has an area of 250 km2. The low relief of the catchment
makes it very vulnerable to annual flooding events. Approximately 16 km2 of the basin is
permanently inundated and while 93.2 km2 is prone to flooding during the usual rain
season. The major waterways in this watershed are Mango Creek and August Creek that
converge at the lower section of the catchment and flow into the Placencia Lagoon. Flour
Camp Creek, Jenkins Creek and several minor waterways also contribute to the net
discharge of Mango Creek. Pine ridge savannahs dominate the higher portions of the
catchment. The lower portion is limited to marshland and mangrove swamps.
The Big Creek Watershed is the smallest of the three catchments adjacent to the
Placencia Lagoon and has an area of 59 km2. Most of the lands in this basin are less than
20 meters in elevation. This basin exhibits low relief and a moderate flood risk potential.
An area of approximately 5.4 km2 is permanently inundated and 15.4 km2 are subject to
7
8
inundation during the rain season. The principal waterway, Big Creek, is fed by
numerous small streams and debouches at the southernmost end of the Placencia Lagoon.
The vegetation cover within this catchment includes marshlands, mangroves, grass
meadows and pine ridge.
It is worth pointing out that none of the watercourses that flow into the Placencia Lagoon
are gauged, hence there are no time series on the discharge and water levels (Hydrology
Service, 2003). However, Grimshaw (2000) purports that the combined low flow for
August and Mango Creeks is 0.7 CMS. Furthermore the low flow for Jenkins and Flour
Camp Creeks was estimated at 0.8 CMS. Silver Creek was estimated to have a low flow
of 1.3 CMS.
The ground water resources map of Belize divides the country into ten regions based on
water availability and quality (Buckalew et al., 1998). The terrestrial zone of influence on
the Placencia Lagoon falls into two of these regions.
(1) The Placencia Peninsula and the western margin of the lagoon are classified as areas
where small to large quantities of brackish to saline water are available. Also, meager to
very small quantities of fresh water are available from quaternary alluvium and coastal
deposits along the coast. Depth to water is 2 to 50 meters.
(2) The wider extent of the terrestrial zone of influence is that part of the Central Coastal
Plain composed of sandy shales, shales, claystones, mudstones, and alluvium. These
deposits bear meager to moderated quantities of freshwater. Depth to water is generally
less than 60 meters.
Although, there is limited information about the ground water distribution in the
terrestrial zone influencing the lagoon, it is envisaged that ground water might have some
effects on the water budget of the Placencia Lagoon.
Meteorology
The Placencia Lagoon and adjacent catchments fall in a tropical climatic zone. Walker
(1973), King et al. (1989, 1992) documented the rainfall distribution for Belize and
indicate that the annual rainfall for the Placencia area varies between 80 inches (2032
mm) and 100 inches (3048 mm). The mean annual temperature for the region is 28°C and
the mean relative humidity is 80%. The predominant winds are out of the Southeast at a
mean speed of 4 knots.
The Belize National Meteorological Service (BNMS) has a countrywide network of
climatological stations. Station 28 located at Rum Point Inn and station 29 located at the
Savannah Forest Station are the stations nearest to the Placencia Lagoon.
9
Characteristics Of The Belize Barrier Reef Lagoon
The Belize Barrier Reef Lagoon is divided into the Northern and Southern Shelf Lagoons
based on differences of physical, chemical, geological and biological characteristics of
the area (Purdy et al., 1975; Purdy, 1974). The immediate coastal oceanographic zone
that exerts influence on the Placencia Lagoon is that portion of the Southern Shelf
Lagoon adjacent to the Placencia Peninsula.
Bathymetry
The bathymetry in the Southern Shelf Lagoon is very irregular due in part to the tectonic
activity that formed the shelf in the first place (Lara, 1993). Subsequent growth of coral
assemblages, the formation of incised valleys, channels and shoals also add to the
complexity of the bathymetry of the southern Shelf. Seaward of the Placencia Peninsula
is the Inner Channel with depths ranging from 10 –20 meters. East of the Inner Channel
is the Victoria Channel with depths of 20- 30 meters.
Sediments
The Southern Shelf is comprised of both biogenic and terrigenous sediments. The
biogenic sediment is derived from the remains of marine organisms such as corals,
coralline algae and mollusk (Purdy et al., 1975; Purdy, 1974).
The terrigenous material includes quartz sand, muds, and clay minerals derived from the
Maya Mountains, the Central and Southern Coastal Plains (Esker, 1998). The fact that the
Southern Shelf Lagoon shows high influence of terrigenous sediment highlights the
influence of terrigenous material on the Placencia Lagoon, which is in closer proximity to
the Central Coastal Plain.
Currents
Thus far there is no comprehensive study of the current patterns near the Placencia
Peninsula or the Placencia Lagoon. The Watershed Reef Interconnectivity Scientific
Study (2001) purports that water within the barrier reef lagoon flows predominantly from
north to south at a rate of 0.05 to 0.15 m/s and rarely exceed 0.3 m/s. The Regional
Monitoring and Environmental Information System component of the MBRS is in the
process of completing a 3-dimensional ocean circulation model of the continental shelf of
the MBRS (Ariola, 2002). In addition to this, a 2-dimensional circulation model is also
being prepared. Undoubtedly, these models will enhance present knowledge about
circulation patterns in Belize.
Tides
Tides of the Caribbean and along the Belize Barrier Reef are microtidal and of mixed
semidiurnal type with a mean range of 15 cm (Kjerve, 1981). PASCO (2002) mentions
that the tidal fluctuations on the windward side of the Peninsula range from 0.40 to 1.5
feet.
10
Salinity
The salinity of the Southern Shelf Lagoon is affected on a seasonal basis by the
precipitation and river discharge in the southern province. The salinity of the Southern
Shelf Lagoon fluctuates range between 33 to 35 parts per thousand (Gibson et al., 1993).
The Importance of the Placencia Lagoon
Like most tropical and subtropical estuarine systems, the Placencia Lagoon has
ecological and environmental significance. The lagoon per se has a diverse amount of
planktonic life that makes it an ideal nursery ground for a myriad of marine and estuarine
organisms. Many of the fish that are caught for food or fun in the higher parts of the
estuary or within the Belize Barrier Reef complex depend on the Placencia Lagoon for
portions of their life cycles. Furthermore, the Placencia Lagoon provides adequate
ecological and environmental conditions for the endangered and threatened West Indian
Manatee (Trichechus manatus). The lower portion of the lagoon is relatively shallow
with extensive beds of seagrass Thalassia testudinum that these creatures feed on. In
close proximity to the lagoon are the mouths of several streams that are adequate source
of freshwater for the manatees to drink. The inner portions of the Placencia Lagoon have
areas that are completely protected from wind and wave activity that offer a safe and
tranquil resting place for adult manatees and their calves.
During the surveys, dolphins were sighted feeding in the middle and lower portions of the
Placencia Lagoon (Plate 1). Fishers from Seine Bight Village confirmed that dolphins
are sighted regularly throughout the year in the lagoon (pers. com., Javier Maritinez).
Morelet’s crocodiles (Crocodylus moreletii) are also a threatened species that have been
sighted in various parts of the Placencia Lagoon system. Platt et al. (1997) found no
evidence of nesting activity in the Placencia Lagoon however, spoil banks in the Maya
Beach area are considered potential nesting habitats. Grimshaw (2000) reported that the
Morelet’s crocodiles use the habitats in the lower reaches of the Placencia Lagoon for
nesting sites.
The vegetation fringing the lagoon supports nesting sites for several marine birds. Quite
frequently pelicans, gulls and bobbies are seen feeding in the lagoon and roosting on the
natural vegetation. In addition to this, the marshlands surrounding the Placencia Lagoon
serve as natural filters for sediment and pollutants. ESTAP (2000) report that the Mango
Creek Special Development Area Development Plan approved in 1997, identified the
need to protect the mangrove wetlands that surround the Placencia Lagoon. The intention
is to ensure that water quality in the lagoon, together with manatee and other wildlife, is
not negatively affected by inappropriate development. Some residents from Placencia and
Seine Bight in conjunction with organizations such as BTIA and Friends of Nature
express concern about the fate of the Placencia Lagoon.
11
In addition to the aforementioned benefits, the lagoon also provides unparalleled
recreational opportunities for residents of the nearby communities and visitors. During
the storm advisories, many shallow draft watercrafts (including catamarans) are moored
in the safe heaven of the Placencia Lagoon.
The aesthetics of the Placencia Lagoon fringed with mangroves and coupled with its high
biological diversity makes it a focal point of tourist related activities for coastal
communities such as Independence, Placencia, Seine Bight and Maya Beach.
Undoubtedly, there are many more benefits that could be used to elucidate the importance
of the Placencia estuarine system. However it is clear that this water body could be
classified as a gem that could not be replaced and hence is priceless.
Plate 1. Photographs:
(a) Profiling the water column with a multiparameter sonde
(b) Validating ground control points for the surveys
(c) Local fisherman seeking the catch of the day
(d) Mangrove fringed channel of the lagoon
(e) & (f) Dolphins feeding in the lagoon
(f) Manatees
(g) Shrimp Mariculture Operation
12
13
Major Land Use and Activities in the Placencia Catchments
Human Settlements
There are eleven villages distributed within the catchments that adjoin the Placencia
Lagoon (figure 3). Even though the villages located towards the westernmost boundary
of the catchments might appear to be isolated from the lagoon, there is connectivity via
the network of streams in the sprawl of the catchment. Therefore some of the activities in
and around these communities such as agriculture, logging, and improper waste disposal
could have direct and indirect impacts on the lagoon.
The villages in closer proximity to the lagoon are generally larger in scale and population
density. The activities in these settlements including destruction of natural vegetation,
improper disposal of sewage and residual water, land reclamation, and road construction
pose more direct ecological and environmental impacts to the waterway. A summary of
the characteristics of the villages in the Placencia catchments is presented in table 1.
Residential Development
In September 2001, the wrath of Hurricane Iris devastated communities of the Placencia
Peninsula and surrounding areas. The National Emergency Management Organization
(2001) reported significant damage to major infrastructure and natural vegetation; and
noted an extraordinary amount of debris scattered on the peninsula. Subsequent to this
event most homes were reconstructed and new areas were also clear and filled to
construct new dwellings.
Currently there are several subdivisions on the Placencia Peninsula. These subdivisions
are primarily for residential areas that would provide the necessary amenities for local
and foreign retired persons.
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15
Table 1. Summary of the Characteristics of villages in the Placencia Catchments (Source:
ESTAP, 2000; Central Statistical Office, 2001)
Villages
Population
Potable Water
Waste
Disposal
facility
Sewage Disposal
Electricity
Santa Rosa
San Roman
Maya Mopan
Georgetown
Cowpen
San Juan
185
351
427
763
399
415
2,881
?
831
458
30
RWS / Pumps
RWS/ Pumps
RWS
RWS
None
None
None
None
None
None
None
None
None
Fair
None
Latrines
Latrines
Latrines
Latrines
Latrines
Latrines
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
Independence
Maya Beach
Seine Bight
Placencia
Riversdale
RWS
RWS
Cisterns
RWS
RWS
Cisterns
Septic tanks /Latrines
Septic tanks /Latrines
Septic tanks /Latrines
Septic tanks / latrines
Latrines
Marinas
Closely linked with the major subdivisions is the need for marinas to dock and safeguard
the boats. Traditionally, boat owners on the Peninsula would dock their vessels in natural
and man-made channels nearest to their homes. The Department of the Environment and
the National Environmental Appraisal Committee are currently reviewing proposals for
the construction of several marinas in the Placencia Lagoon.
Shrimp Mariculture Operations
Coastal lagoons in the world are recognized as suitable sites for aquaculture development
(Contreras, 1993). The western margin of the Placencia Lagoon forms the nucleus of the
Shrimp Mariculture Industry in Belize (Figure 4). Currently there are six shrimp farms
located in this area namely Nova Laguna, Belize Aquaculture Ltd., Royal Mayan, Tex
Mar Ltd., Crustaceans Ltd, and AquaMar. All the farms are functional with the exception
of Nova Laguna. Collectively, these farms contributed 51 % of the total national farmed
shrimp production with a corresponding export value of approximately 23 million dollars
in 2001(Table 2a). It is estimated that during that same year, the farms within the
Placencia Lagoon Catchments employed approximately 200 permanent staff and 230
temporary staff from the nearby communities.
In 2002 the collective production of the mariculture operations in the Placencia area was
38 % of the total national farmed shrimp production valued at 17 million dollars (Table
2b). These farms provided approximately 231 fulltime employment and 247 part time
employment to residents of communities in the vicinity.
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Table 2a. Shrimp Mariculture production and export data 2001
(Data Source: Belize Fisheries Department)
Mariculture Sites
Nova Laguna
Belize Aquaculture
Royal Mayan
Tex Mar
Crustaceans
Aqua Mar
Production
Area (acres)
94
324
158
116
1,000
Production
Heads on lbs/yr
1,689,307
609,535
782,000
239,509
1,753,215
Export
lbs of tails
1,142,812
391,000
512,000
156,000
1,131,790
Table 2b. Shrimp Mariculture production and export data 2002
(Data Source: Belize Fisheries Department)
Mariculture Sites
Nova Laguna
Belize Aquaculture
Royal Mayan
Tex Mar
Crustaceans
Aqua Mar
Production
Area (acres)
180
318
128
195
1,000
Production
Heads on lbs/yr
1,612,903
543,909
251,241
262,467
1,006,884
Export
lbs of tails
369,452
348,102
196,655
170,606
944,768
Tourism Development
Tourism is the base of the economy on the Placencia Peninsula. In order to further
develop this industry there is an urgent need to construct resorts and hotels with the
necessary amenities to host local and international tourists. Some of these amenities
include (but is not limited to) beaches, piers, marinas, and parking lots. In view of this,
there is a notably higher occurrence of land clearing, land reclamation and dredging
activities in the area.
Sport fishing (Fly fishing) and recreational fishing are popular tourist activity in the
Placencia Lagoon. Special tours to observe manatees, birds and crocodiles are also on the
list of things to do in the Placencia Lagoon.
The Department of the Environment has received several applications for Environmental
Clearance to develop Jet Ski rental operations in the Placencia Lagoon. Thus far all
applications of this nature have been looked at unfavorably due to the potential negative
impact on wildlife.
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RESULTS
Bathymetry
The bathymetry of the Placencia Lagoon was computed based on 202 geo-referenced
soundings that were registered within the lagoon. The depth varies from near zero at the
boundaries of the lagoon to approximately 5.8 meters below MSL in the deepest channels
and the mouth of the lagoon. The average depth of the lagoon is 1.5 meters (Figure 5).
The water quality data from each survey was vertically and horizontally averaged to
provide an initial perspective on the water quality of the lagoon. The results of the mean
measurements of temperature, pH, specific conductivity, salinity, dissolved Oxygen and
turbidity are summarized below (Table 3).
Table 3. Mean measurements of several parameters in the Placencia Lagoon
Survey 1
Temp
°C
pH
units
SpCond
mS/cm
Salinity
ppt
D.O.
% Sat
D.O
mg/l
Tubidity
ntu
29.42
8.32
38.86
24.79
122.09
8.02
5.73
29.69
8.08
48.98
32.09
92.40
5.79
4.32
29.30
8.39
41.45
26.66
116.56
7.58
7.89
30.08
8.57
37.65
24.00
112.54
7.55
10.64
(Apr. 5, 01)
Survey 2
(May 31, 01)
Survey 3
(Mar. 12, 03)
Survey 4
(Jul. 17, 03)
Temperature
The temperatures recorded in the lagoon during the four surveys varied between 29.30
and 30.08 (figure 6). Although lagoonal temperatures are controlled principally by solar
radiation intensity, it is usually influenced by the influx of cooler water from the adjacent
catchments and the water exchange with the Belize Barrier Reef Lagoon. The spatial
temperature distribution in the lagoon for the four surveys can be observed in Figures AD in Appendix 1.
19
20
30.20
30.08
Temperature °C
30.00
29.80
29.69
29.60
29.40
29.42
29.30
29.20
29.00
28.80
APR, 01
MAY, 01
MAR, 03
JUL, 03
Survey Dates
Figure 6. Mean temperatures of the Placencia Lagoon during four independent surveys.
pH
pH
The pH determines the acidity or alkalinity of the water by measuring the hydrogen ion
activity. Pure water is described as neutral and has a pH of 7. In the Placencia Lagoon
the mean pH varied from 8.08 to 8.57 (figure 7).
8.70
8.60
8.50
8.40
8.30
8.20
8.10
8.00
7.90
7.80
8.57
8.39
8.32
8.08
APR,
01
MAY,
01
MAR,
03
JUL, 03
Survey Dates
Figure 7. Mean pH in the Placencia Lagoon during four independent surveys.
21
Specific Conductivity
The specific conductivity in the Placencia Lagoon showed some variations on the
surveys. The mean specific conductivities varied between 37.65 and 48.98 miliSiemens
per centimeter (figure 8).
Specific Conductivity mS/cm
60.00
50.00
48.98
41.45
40.00
38.86
37.65
30.00
20.00
10.00
0.00
APR, 01
MAY, 01 MAR, 03
JUL, 03
Survey Dates
Figure 8. Mean Specific Conductivity in the Placencia Lagoon
Salinity
The salinity is derived from the measurements of specific conductivity and is expressed
as parts per thousand (‰). The salinity variation is between 24.00 ‰ and 32.09 ‰.
(figure 9). The spatial distribution of mean surface and bottom salinities for the four
water quality surveys can be observed in figures A-D in Appendix 2.
35.00
32.09
30.00
26.66
Salinity ‰
25.00
24.79
24.00
20.00
15.00
10.00
5.00
0.00
APR, 01
MAY, 01
MAR, 03
JUL, 03
Survey Dates
Figure 9. Mean Salinity in the Placencia Lagoon for four independent surveys
22
The salinity measurements within the Placencia Lagoon were looked at very closely
during the analysis especially since several persons report high salinity gradients and
stratification in the water body. Surveys 1 to 4 indicate that the difference between
surface and bottom salinities were 0.31, 0.62, 0.28 and 2.87 parts per thousand
respectively (Table 4).
Table 4. Variation between Surface and Bottom Salinities of the Placencia Lagoon
MONITORING
SURVEYS
SURFACE SALINITY
Average Min Max Range
BOTTOM SALINITY
Average Min Max
Range
Survey 1
(Apr. 5, 01)
24.63
21.2
35.2
14
24.94
21
35.1
14.1
Survey 2
(May 31, 01)
31.78
18.2
35.5
17.3
32.4
26.7
35.6
8.9
Survey 3
(Mar. 12, 03)
26.52
20.5
34.2
13.7
26.80
20.5
34.3
13.8
Survey 4
(Jul. 17, 03)
22.59
4.9
30
25.1
25.46
13.3
34
20.7
23
Dissolved Oxygen
The dissolved oxygen concentration in the lagoon varied between 5.79 mg/l (92.40 %
saturation) and 8.02 mg/l (122.09 % sat) in the four surveys that were conducted in the
lagoon. Figures 10 a, b illustrate the mean dissolved oxygen levels distributed in the
Placencia Lagoon.
9.00
Dissolved Oxygen mg/L
8.00
8.02
7.58
7.55
7.00
6.00
5.79
5.00
4.00
3.00
2.00
1.00
0.00
APR, 01 MAY, 01 MAR, 03 JUL, 03
Survey Dates
Dissolved Oxygen % sat
Figure 10a. Mean dissolved oxygen concentrations (mg/L) in the Placencia Lagoon
140.0
120.0
100.0
122.1
116.6
112.5
92.4
80.0
60.0
40.0
20.0
0.0
APR, 01 MAY, 01 MAR, 03 JUL, 03
Survey Dates
Figure 10b. Mean dissolved oxygen concentrations (% saturation) in the Placencia
Lagoon
24
Turbidity
The mean turbidity levels of the lagoon were relatively low on all the surveys ranging
from 4.32 to 10.63 NTU. The following is a graphic representation of the turbidity levels
in the lagoon.
12.00
10.64
Turbidity [NTU]
10.00
8.00
6.00
7.89
5.73
4.32
4.00
2.00
0.00
APR, 01
MAY, 01
MAR, 03
JUL, 03
Survey Dates
Figure 11. Mean turbidity measurements for the Placencia Lagoon
25
DISCUSSIONS
The depth variations of the lagoon floor bring to light several depressions. It is highly
probable that these depressions serve as receptacles for organic material that settle and
undergo decomposition within the lagoon during calm conditions. In windy and stormy
conditions the depressions are perturbed and sediment and organic matter are resuspended into the water column.
The water temperature is an important parameter given that it controls the rate of
biological and chemical activity to some extent. Furthermore, several marine and
estuarine organisms synchronize important events such as reproduction and migration
with optimal water temperature. The observed mean water temperatures for two distinct
dry seasons in the lagoon varied between 29.30 and 30.08 °C a difference of less than
1°C. Notwithstanding the fact that the ecological composition of the lagoon was outside
the scope of this study, one could infer that the nektonic and benthic organisms in the
lagoon are adapted to the aforementioned temperature range. The temperature of the
lagoon is dependent on natural conditions such as solar radiation, season, wind action,
storms, tides, level of stratification and river discharge.
Estuarine pH levels generally average from 7.0 to 7.5 in the fresher sections to 8.0 and
8.6 in the more saline areas. Most marine organisms prefer conditions with pH values
ranging from 6.5-8.5 (U.S. EPA, 1993). The average pH conditions observed in the
Placencia estuarine system varied between 8.1 and 8.6. The observed pH are well within
the acceptable range for most marine organisms and healthy estuarine environment. The
slightly alkaline pH of this estuary could be attributed to the natural buffering from the
carbonate and bicarbonate dissolved in the water.
Under natural conditions, processes such as photosynthesis and respiration are known to
influence pH levels. High nutrient loadings from domestic wastewater of untreated
effluent could exacerbate the levels of primary productivity and respiration that could
lead to drastic short-term shifts in pH. This situation can be extremely harmful for many
organisms. Despite the fact that such a condition was not encountered during the
monitoring surveys, it is not at all far fetched for the Placencia Lagoon. The review of all
the anthropogenic activities in the Placencia catchments did not identify any potential
source for long-term acidification of the lagoon.
The salinity is a crucial parameter that helps to define the biological and physical
character of an estuary. The salinity in combination with other critical parameters (e.g.
temperature) controls the types of plants and animals that could inhabit the different
zones of an estuary. The freshwater species may be restricted to the upper reaches of the
estuary while the marine species inhabit the estuarine mouth. In the lower section of the
Placencia Lagoon there is a high density of Thalassia testidinum that declines rapidly
towards the middle portion of the lagoon. Other plant species become dominant in the
middle and upper portions of the lagoon. Euryhaline fish species including snook, jack,
mullet and tarpon are frequently seen in most parts of the lagoon.
26
The salinity maps presented in Appendix 2 display shifts in salinity distribution within
the lagoon. This could be attributed to the effects of evaporation, freshwater influx from
the Placencia catchments and the net water exchange between the entire estuarine system
and the sea. The fact that the salinity is a conservative parameter allows us to make
certain conclusions about the character of the lagoon. For instance, the monitoring
surveys in the peak of two dry seasons (2001 & 2003) indicate that the upper section of
the lagoon maintains a much lower salinity relative to the mouth of the lagoon. In effect
this is a clear indication that the flushing rate of the upper part of the lagoon during the
dry season is very low and is strongly dependent on wind and tidal forces. This is one
natural attribute of the lagoon that highlights its vulnerability to the impacts of human
settlement and aquaculture developments. Hypothetically, if large volumes of wastewater
or untreated effluents were discharged into the upper section of the Placencia Lagoon, its
water quality would rapidly decline and affect the natural ecological balance.
Another characteristic of the lagoon is that during the dry season there are minimal
differences between surface and bottom salinities. Based on this observation, the
Placencia Lagoon is classified as a moderately stratified estuary.
Dissolved Oxygen concentration is one of the best indicators of an estuary’s health. An
estuary with little or no oxygen cannot support healthy levels of animal or plant life. The
aerobic bacteriological decomposition of organic matter requires Oxygen. Quite often
this process depletes the water of oxygen and makes the environment uninhabitable for
other organisms.
Most animals and plants can grow healthy and reproduce when DO levels exceed 5 mg/L.
However, when DO levels falls to 3-5 mg/L living organisms may become stressed. The
results of the surveys indicate that the Placencia Lagoon is a healthy environment
displaying a wide range of dissolved oxygen concentration (between 5.79 and 8.02
mg/L).
Several studies in estuaries have shown that although excess nutrients from human
activities are a major cause of hypoxia and anoxia, these conditions may also occur in
estuaries relatively remote from human impact. However, the severity of low DO and the
length of time that low oxygen conditions persist are less extreme. This is another good
reason for a proactive approach to manage the impact of anthropogenic activities in the
lagoon catchments. Offsetting the balance of the healthy Placencia Lagoon towards an
anoxic and unproductive environment can be avoided.
During the monitoring surveys the turbidity levels in the lagoon were influenced
primarily by the intensity and duration of the wave action. The amount of mixing
ultimately controlled the re-suspension of silt and organic matter into the water column.
The perturbation in the extremely shallow parts of the lagoon was more vigorous as
opposed to deeper parts and this was reflected in the heterogeneous turbidity levels.
Generally, the turbidity levels were low as indicated by the averaged values ranging
between 4.32 and 10.64 NTU. During the final survey (July, 03), some parts of the
27
lagoon exhibited a golden-brown coloration (Plate 1(g)). This was probably a
phytoplankton bloom triggered by some runoff.
High turbidity levels over prolonged periods can diminish the heath and productivity of
an estuarine system. Turbid waters reduce the availability of light to underwater plants
and inhibit growth. Organisms that filter feed in an estuary are usually affected by high
turbidity. In such conditions, suspended particles would accumulate on the gills of the
organisms and impair breathing. The ability of marine birds and some fish to sight and
track their prey is greatly reduced in a turbid estuary.
The turbidity levels of the Placencia Lagoon could be exacerbated by agricultural runoff,
aquaculture effluent, spoil from dredging and land reclamation sites; erosion and boat
traffic.
Considerable attention was given to the impacts of shrimp mariculture operations in the
margin of the Placencia Lagoon. At the time of this study, the upper portion of the lagoon
was not affected by aquaculture effluent. Nova Toledo has been non functional for
several years (Tunich Nah, 2001). On the other hand, Belize Aquaculture Ltd. is a fully
functional super intensive, closed system operation with a zero effluent discharge (Boyd
et al., 2002). Royal Mayan, Tex Mar and Crustaceans Ltd. are three operations that are in
close proximity to each other. Each of these operations meet the settling pond
requirements (10% of the production area) stipulated by the Department of the
Environment. Effluents discharged from these operations are subjected to mangrove
wetlands for nutrient and sediment reduction prior to entry into the lower portions of the
Placencia Lagoon. The fact that the lower portion of the lagoon has a much higher water
exchange rate with the sea (as opposed to the upper portion) is the reason why there is no
significant impact on the ecological and environmental conditions of the lagoon. Aqua
Mar is located at the southern boundary of the lagoon and its effluents are released into
wetlands in the northeastern tip of the Sennis River Catchment. Taking into account the
predominant southerly coastal currents, the impacts of this operation on the Placencia
Lagoon are questionable.
Similarly, attention was also given to the impacts of human settlement on the Placencia
Lagoon. The current growth trends of the communities pose two outstanding threats to
the lagoon: (1) increasing sediment input consequent to destruction of natural vegetation
and (2) increasing nutrient loadings from improper disposal of domestic wastewater.
28
CONCLUSIONS
The characterization of the Placencia Lagoon was essential to put into perspective all the
natural and anthropogenic factors that could affect the health and function of the
estuarine system.
The spatial, vertical and temporal variations in salinity distribution confirms that the
Placencia Lagoon may be classified as a moderately stratified estuary.
The small-scale water exchange in the upper section of the lagoon brings into prominence
the vulnerability of the water body’s natural ecological and environmental conditions to
excessive loadings of nutrients, organic matter and sediment.
The upper section of the lagoon is not impacted by direct effluent discharge from the
adjacent shrimp mariculture operations.
The water quality indicators assessed in this study indicate that during the dry seasons
2001 and 2003 the Placencia Lagoon displayed healthy environmental conditions.
The data and results obtained in this study strongly support the hypotheses postulated.
However, there is the need for more research to quantify impacts in terms of budgets for
conservative and non-conservative parameters.
RECOMMENDATIONS
Cognizant of the invaluable natural ecological services and socioeconomic benefits
provided by the Placencia estuarine system, it is appropriate to make several
recommendations.
Firstly, a reliable environmental quality-monitoring program needs to be implemented
and sustained in the Placencia Lagoon. However, most regulatory authorities are faced
with both financial and human resources constraints hence, it is incumbent on the local
NGOs and residents to be involved in such a program.
Secondly, the local village councils should liaise closely with the Department of the
Environment to ensure that the major developments are compliant with mitigatory
measures and national effluent regulations.
Thirdly, the environmental NGOs should assume the lead role in educational programs to
heighten the residents’ awareness about the importance and also the vulnerability of the
Placencia Lagoon.
Lastly, in view of the low water exchange in the upper portion of the Placencia Lagoon, it
is imperative that future development are planned to ensure the lowest possible impact on
the estuary.
29
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32
APPENDIX
I
Temperature Distribution for four independent surveys of the Placencia Lagoon
33
34
35
36
37
APPENDIX
II
Salinity distribution for four independent surveys of the Placencia Lagoon
38
39
40
41
42
Abbreviations and Notations
BNMS
Belize National Meteorological Service
BTIA
Belize Tourism Industry Association
CSO
Central Statistical Office
CZMAI
Coastal Zone Management Authority and Institute
DoE
Department of the Environment
ESTAP
Environmental & Social Technical Assistance Project
FoN
Friends of Nature
GIS
Geographic Information Systems
GPS
Global Positioning System
MBRS
Mesoamerican Barrier Reef Systems Project
NEMO
National Emergency Management Organization
NGO
Nongovernmental Organizations
NTU
Nephelometric Turbidity Units
RWS
Rudimentary Water System
°C
Degrees Centigrade
CMS
Cubic meter per second
DO
Dissolved Oxygen Concentration
Hp
Horse power
mg/L
Milligrams per liter
mS/cm
MilliSiemens per centimeter
MSL
Mean Sea Level
% sat
Percent Saturation
ppt
Parts per thousand
v