Department of Environmental Engineering
Centre for Water Research, The University of Western Australia
EVALUATION OF REMEDIATION OPTIONS FOR LAKE
MARACAIBO, VENEZUELA
Robert Medley
This thesis is submitted in partial fufillment for the degree of Bachelor of Engineering (Hons.)
from the Department of Environmental Engineering, at the University of Western Australia.
November 2001
Robert Medley
6 Glover Street
Dianella, WA 6156
5th November 2001
The Dean
Faculty of Engineering
The University of Western Australia
Nedlands WA 6009
Dear Sir,
This thesis entitled "Evaluation of Remediation Options for Lake Maracaibo, Venezuela" is
submitted as partial fulfillment of the requirements for the combined degree of Bachelor of
Engineering (Environmental)/ Bachelor of Commerce (Gen. Mngt. & Hum.Res. Mngt.) with
honours.
Yours Sincerely,
Robert Medley
Abstract
Historically Lake Maracaibo has been a focal point for the people of Venezuela, providing
aesthetic, economic and recreational benefits to the residents. Increasing salinisation and
anthropogenic nutrient inputs has culminated in Lake Maracaibo showing signs of accelerated
eutrophication, indicated by intense phytoplankton blooms.
Salinity, along with the methods required to reduce it, has been a central issue in many past
discussions and reports on the future of Lake Maracaibo. Presently there is discussion to close
the navigation channel and re-locate the port facilities to the adjacent Gulf of Venezuela.
An in-depth review and interpretation of past data and studies was undertaken, in combination
with hydrodynamic-ecological modelling in order to advise on the effectiveness of
environmental remediation strategies aimed primarily at reducing salinity and nutrient inputs.
The abundance and composition of the phytoplankton standing crop is one of the most important
indicators of water quality and eutrophication in lakes and reservoirs. Thus, a critical issue that
must be considered when assessing the merit of any proposed remediation strategy is the impact
on primary production and algal species composition. This study was the first to model both
dominant algal species in Lake Maracaibo and accurately represent the observed phytoplankton
dynamics, allowing for the conclusion that closing the navigation channel would lead to a slight
improvement in water quality. However, the Lake would still suffer from intense blooms of toxic
blue-green algae; the main problem being the nutrients coming from the Catatumbo River which
has its catchment in Columbia.
Acknowledgements
Acknowledgements
I would like to acknowledge the following people for their contribution to this thesis –
whether it has been through inspiration, support, encouragement, or friendship.
My Supervisor Prof. Jörg Imberger for creating this project for me, motivating me to work to
my full potential, imparting his knowledge of all things hydrodynamic, and for the
stimulating conversations throughout the year
Dr. David Hamilton for taking an interest in my project, finding time to see me, even though
he was busy with students of his own, and for his knowledge on all things ecological,
willingly shared.
My fellow Maracaibo man, Bernard Laval, who has helped me throughout the year, as we
both grappled with the beast that is Lake Maracaibo. Thanks Bernard, your help has been
invaluable!
Peter Yeates for his help with modelling and providing me with background on the project.
David Horn for taking an interest in my work and providing me with documentation on
previous studies.
Andre_ Gomez-Giraldo for his help with coding the new mixing algorithm.
The final year class – special thanks to Tom Ewing, Alia Abid and Rachel Murphy. You guys
have made a tough year an enjoyable one!
To my mates – thanks for taking me out on the weekends and keeping me sane.
Tash, thanks for everything, your support throughout the year has been enormous.
Mum and Dad – I dedicate this thesis to you…without your help, assistance, guidance and
support throughout the years, none of this would have been possible!
Cheers,
Rob.
Table of Contents
1
MOTIVATION ..........................................................................................................................................................1
2
COMPONENTS OF THE LAKE MARACAIBO SYSTEM ..............................................................................4
3
4
2.1
LAKE MARACAIBO ..................................................................................................................................................4
2.2
STRAIT OF MARACAIBO ..........................................................................................................................................5
2.3
TABLAZO BAY .........................................................................................................................................................5
2.4
THE GULF OF VENEZUELA ......................................................................................................................................6
2.5
THE NAVIGATION CHANNEL...................................................................................................................................6
HYDROLOGY...........................................................................................................................................................7
3.1
THE LAKE MARACAIBO CATCHMENT ....................................................................................................................7
3.2
PRECIPITATION ........................................................................................................................................................8
3.3
HYDROLOGICAL BALANCE .....................................................................................................................................8
HYDRODYNAMICS OF LAKE MARACAIBO................................................................................................10
4.1
4.1.1
Tides ............................................................................................................................................................10
4.1.2
Winds...........................................................................................................................................................11
4.1.3
Currents ......................................................................................................................................................11
4.2
5
6
MIXING PROCESSES...............................................................................................................................................12
4.2.1
Internal Dynamics ......................................................................................................................................13
4.2.2
The influence of the earth’s rotation..........................................................................................................14
PREVIOUS STUDIES ............................................................................................................................................15
5.1
THE WOODS HOLE AND MIT STUDIES ...............................................................................................................15
5.2
THE BATTELLE STUDY ..........................................................................................................................................17
5.3
THE PARRA PARDI STUDIES ..................................................................................................................................18
5.4
THE NEDECO STUDY ..........................................................................................................................................18
5.5
THE ICLAM STUDIES ...........................................................................................................................................20
5.6
THE INTEVEP STUDIES .......................................................................................................................................20
5.7
STUDIES AT THE UNIVERSITY OF ZULIA ...............................................................................................................21
5.8
OTHER STUDIES OF INTEREST ...............................................................................................................................21
5.9
THE BECHTEL INTEGRATED ENVIRONMENTAL REMEDIATION STUDY ...............................................................22
HISTORICAL SALINITY TREND......................................................................................................................24
6.1
7
HYDRODYNAMIC FORCINGS .................................................................................................................................10
BEHAVIOUR OF THE UNDERFLOW.........................................................................................................................25
PHYTOPLANKTON COMPOSITION & SUCCESSION ...............................................................................27
7.1
HISTORICAL PHYTOPLANKTON COMPOSITION & SUCCESSION ...........................................................................28
iv
Table of Contents
8
9
PREDICTIVE WATER QUALITY MODELLING...........................................................................................32
8.1
INPUT-OUTPUT MODELS .......................................................................................................................................33
8.2
PROCESS BASED MODELLING/ECOLOGICAL MODELS .........................................................................................35
MODEL SELECTION............................................................................................................................................36
9.1
DYRESM-CAEDYM...........................................................................................................................................37
9.2
CAEDYM .............................................................................................................................................................37
9.2.1
REPRESENTATION OF PHYTOPLANKTON GROWTH WITHIN CAEDYM .......................................39
Light Limitation.................................................................................................................................................................. 40
Nitrogen Limitation ............................................................................................................................................................ 41
9.3
10
DYRESM: REVIEWING THE 1-D ASSUMPTION.........................................................................................42
APPLYING DYRESM-CAEDYM TO LAKE MARACAIBO .........................................................................48
10.1
DYRESM SETUP ...........................................................................................................................................49
10.1.1
PHYSICAL AND MORPHOMETRIC DATA.............................................................................................49
10.1.2
INITIAL PROFILE .....................................................................................................................................50
10.2
INPUT DATA .................................................................................................................................................50
10.2.1
Meteorological Forcings............................................................................................................................51
Data..................................................................................................................................................................................... 51
Meteorological Stations ..................................................................................................................................................... 51
Location .............................................................................................................................................................................. 51
10.2.2
Inflows.........................................................................................................................................................53
10.2.3
Water Quality Parameters .........................................................................................................................56
10.3
ASSUMPTIONS .............................................................................................................................................58
10.3.1
Forcing Data...............................................................................................................................................58
10.3.2
Combining Surface Exchange with River Flow.........................................................................................58
10.3.3
Outflow Condition ......................................................................................................................................59
10.4
CONFIGURING DYRESM TO MODEL LAKE MARACAIBO ...............................................................................60
10.4.1
Benthic Boundary Layer (BBL)..................................................................................................................61
10.4.2
Internal Mixing ...........................................................................................................................................62
10.4.3
Developing a New Internal Mixing Algorithm ..........................................................................................64
Background......................................................................................................................................................................... 64
The Algorithm .................................................................................................................................................................... 65
11
CALIBRATION.......................................................................................................................................................69
11.1
FIELD DATA..................................................................................................................................................69
11.2
CALIBRATION METHOD............................................................................................................................70
11.2.1
Phytoplankton Composition & Succession for the Calibration period ....................................................82
v
Table of Contents
12
13
PREDICTIVE SIMULATIONS OF MANAGEMENT OPTIONS..................................................................86
12.1
ASSUMPTIONS AND DATA USED IN PREDICTIVE SIMULATIONS .......................................................................86
12.2
TOOLS USED IN ASSESSING THE MANAGEMENT OPTIONS ................................................................................87
12.3
REFERENCE CASE (CURRENT CONDITION WITH REPEATED FORCING) ............................................................88
12.4
EXISTING BATHYMETRY: REDUCED NUTRIENTS (10%).................................................................................95
12.5
PRE-DREDGING BATHYMETRY: REDUCED NUTRIENTS (10%) .......................................................................97
DISCUSSION.........................................................................................................................................................102
13.1
EXISTING CONDITIONS ...................................................................................................................................104
Vertical Transport............................................................................................................................................................. 104
Oxygen Dynamics ............................................................................................................................................................ 104
Nutrient Dynamics............................................................................................................................................................ 105
Phytoplankton Composition and Succession .................................................................................................................. 105
13.2
EFFECT OF A 10% REDUCTION IN NUTRIENT LOADING .................................................................................106
13.3
EFFECT OF CLOSING THE SHIPPING CHANNEL ................................................................................................106
Phytoplankton Composition and Abundance .................................................................................................................. 108
14
CONCLUSIONS....................................................................................................................................................112
15
RECOMMENDATIONS FOR FUTURE WORK............................................................................................113
16
REFERENCES ......................................................................................................................................................114
17
APPENDICES........................................................................................................................................................123
vi
Motivation
1 Motivation
Historically, Lake Maracaibo has been a focal point for the people of Venezuela, providing
aesthetic, economic and recreational benefits to the residents. The catchment is approximately
83,000 km2, encompassing a wide range of land uses, including both urban and agricultural
areas. Lake Maracaibo itself provides fresh water for agriculture, supports a local fishery, and is
the location of a petroleum industry, which accounts for 80% of Venezuelan oil production.
Increasing salinisation due to the deepening of the shipping channel and increasing
anthropogenic nutrient inputs, has culminated in Lake Maracaibo showing signs of accelerated
eutrophication, indicated by intense phytoplankton blooms.
The problems associated with eutrophication are not isolated to Lake Maracaibo. In the early
1960’s it became apparent that a large number of lakes and reservoirs, particularly those located
in industrialised countries, were rapidly changing in character and becoming eutrophic, due to
the addition of nutrients originating primarily from anthropogenic sources (OECD 1982). The
eutrophication of inland water bodies has become synonymous with deteriorating water quality,
and has frequently led to considerable costs being incurred by those responsible for managing the
water bodies (OECD 1982).
Accelerated anthropogenic eutrophication can be reversed by the elimination or reduction of the
nutrient supply (OECD 1982). Thus, it is important to understand the complex relationship
between nutrient supply and the ecology of the lake, the ecological response. A thorough
understanding of the complex interaction between lake physics, chemistry, and biology provides
a solid foundation on which a sound lake management strategy can be developed in order to
obtain the desired water quality outcomes.
Water quality issues in the Lake Maracaibo system are complex and include salinity changes,
eutrophication, coliform bacteria, hydrocarbons, toxic contaminants, and anoxic conditions in the
hypolimnion. Some of these issues can be addressed individually, through source control and
management strategies. For example, pollution from oil spills and discharges of toxic chemicals,
including pesticides and heavy metals, requires a strategy that focuses on the reduction of these
releases and continued monitoring, in parallel with specific management efforts. Other issues,
particularly those related to salinity, eutrophication, and oxygen levels are inherently more
1
Motivation
complex because of their interrelationships.
Salinity, along with the methods required to reduce it, has been a central issue in many past
discussions and reports on the future of Lake Maracaibo. Presently, there is discussion on the
merits of closing the shipping channel and moving the port facilities located within Lake
Maracaibo to the Gulf of Venezuela in an attempt to restore the condition of the Lake. In general,
salinity changes can influence the Lake’s ecology and the ability to use the water for other
purposes. In addition, they can also influence other water quality parameters, e.g., dissolved
oxygen in the hypolimnion and the vertical distribution of nutrients, both of which affect primary
production.
The “Integrated Study for the Environmental Remediation of Lake Maracaibo” (Bechtel 2000a)
contributed significantly to the understanding of the above interactions in the context of the
present state of the Lake Maracaibo system. The study used both a one-dimensional (DYRESMWQ), and a three-dimensional model (MIKE-3), to evaluate the overall water quality impacts of
a range of environmental remediation options primarily aimed at reducing salinity and/or nutrient
inputs.
The abundance and composition of the phytoplankton standing crop is one of the most important
indicators of water quality and eutrophication in lakes and reservoirs. Thus, a critical issue that
must be considered when assessing the merit of any proposed remediation strategy, is the impact
on primary production and algal species composition. Neither DYRESM-WQ nor MIKE-3
modelled both of the dominant algal species present in Lake Maracaibo (cyanobacteria and
chlorophytes). DYRESM-WQ modelled cyanobacteria; conversely MIKE-3 modelled
chlorophytes. Due to the differing assumptions as to the dominant phytoplankton group, the
impact of various remediation strategies on primary production and algal species composition
could not be assessed with any confidence.
The dynamics of phytoplankton succession within Lake Maracaibo is a leading focus of this
project, which seeks to provide advice on the effect of management options proposed by Bechtel
(1999b), based on a detailed scientific study of the dynamics and ecology of Lake Maracaibo.
The use of a coupled hydrodynamic-ecological model, DYnamic REservoir Simulation ModelComputational Aquatic Ecosystem Dynamics Model (DYRESM-CAEDYM), is an integral part
2
Motivation
of this process.
Numerical models of ecological processes are increasingly being used as lake management tools
(Walters, 1986; Schladow and Hamilton, 1995; Harris, 1997). In a complex ecosystem, such as
Lake Maracaibo, models allow for exploration of possible outcomes of various management
options (e.g. closing the shipping channel and/or reducing anthropogenic nutrient inputs) without
inadvertently causing an undesirable change in the ecosystem. However, Walters (1986) suggests
that models may not represent the most efficient method available to predict the complex
relationships and interactions between each ecosystem component, because standard scientific
investigations fail to resolve many of the uncertainties implicit in models. To resolve many of the
uncertainties associated with models, the best available data set was used in calibration, while
numerous past field studies and data sets were used to verify the numerical model.
The specific aims of this study are to:
•
Accurately model the complex hydrodynamics of Lake Maracaibo using the one-dimensional
model DYRESM.
•
Couple DYRESM to the ecological model CAEDYM, and for the first time, model the
observed dynamics of the two dominant algal species within Lake Maracaibo.
•
Use the fully calibrated DYRESM-CAEDYM model to determine the effect of proposed
management options on long-term changes in water quality in Lake Maracaibo.
3
Background and Literature Review
2 Components of the Lake Maracaibo System
2.1
Lake Maracaibo
Lake Maracaibo is approximately 160km long in the N-S direction, and 110km wide in the E-W
direction, with an average depth of 25.9m and a maximum depth of 30m (Battelle 1974). The
Lake is connected with the Gulf of Venezuela through the Strait of Maracaibo and Tablazo Bay,
which together have a surface area of 1,080km2; this compares with the Lake surface area of
approximately 12,000km2 (Jha et al. 1999). The lake basin is bounded on the east, south and west
by high mountain ranges with a maximum altitude of 5,000 m (Battelle 1974). As one of the
world’s largest lakes its protection and management presents a difficult and exceedingly
important problem to which limnologists at all latitudes must make a contribution (Lewis 2000).
GULF OF VENEZUELA
Malecon WL
Punta Diablo WL
Tablazo WL
TABLAZO
BAY
Malecon WL
Cap. Chico
MARACAIBO
MARACAIBO
STRAIT
Palmas del Sur
Punta Hicotea
LAKE MARACAIBO
Catatumbo
Figure 2.1.1: The Lake Maracaibo System
4
Background and Literature Review
It is believed that until relatively recent geologic times Lake Maracaibo was a body of fresh
water with its surface above the sea level (Redfield, 1956). A river flowing along the western
side of the valley, where the Strait of Maracaibo is today, drained the lake. A rise in sea level of
about 20 m at the end of the last ice age created a link between the lake and the sea, resulting in
the periodic influx of saline water into the lake.
2.2
Strait of Maracaibo
The Strait of Maracaibo is approximately 40 km long, with an average depth of 10 m. Its width
varies from 6 km at the narrowest point in the north to 17 km at the southern end, with an
average width of 7.7 km. Tidal currents in the Strait are strong. The water in the Strait is fully
mixed for the majority of the year, however, it becomes stratified during the dry season, as
discharge through the Strait declines and saline water from the Gulf forms an underflow into the
Strait.
2.3
Tablazo Bay
Tablazo Bay connects the Gulf of Venezuela in the north with the Strait of Maracaibo in the
south. Tablazo Bay is very shallow, with depths varying between less than 1 m and 5 m
(Bautista, 1997), and with an average depth of 2.5 m. Approximately 25% of Tablazo Bay is less
than 1.5 m in depth (Parra-Pardi 1980). The following openings connect the Bay with the Gulf:
-
Boca San Carlos-Zapara, which is 1.5 km wide, and through which a navigation channel is
continuously dredged to a depth of 13.7 m;
-
Boca Ca onera, which is 0.9 km wide; and
-
Boca Ca onerita, which is 0.5 km in width.
Although natural channels in Boca Ca onera and Boca Ca onerita can reach depths of up to 5-6
m, there is limited navigational access from Tablazo Bay to the Gulf of Venezuela through the
“Bocas”, as shifting sands have narrowed and in some instances blocked these relatively unstable
inlets (Bechtel 1999a).
Most of the mixing in the Lake Maracaibo system takes place in Tablazo Bay, where the waters
from the lake, Rio Limón, and the Gulf, interact (NEDECO 1993). The Rio Limón watershed in
the north drains into Tablazo Bay in the vicinity of the navigation channel inlet and directly
5
Background and Literature Review
influences the water exchange between the Gulf and the Bay. The large amount of sediment
these rivers unload into the Bay plays a major role in shaping local coastal morphology.
2.4
The Gulf of Venezuela
The Gulf of Venezuela is approximately 180 km long and 75 km wide, with an average depth of
30 m. It is delimited on its northern boundary by the 80-m isobath, which runs between the
Gaojira Peninsula and the Paraguana Peninsula. A smaller gulf, 45 km in length and 6 m deep, is
located on the eastern boundary of the Gulf of Venezuela (Bechtel 1999a).
The Bay is separated from the Gulf of Venezuela by a series of shifting sand islands and
sandbars. Outside these islands, the coast changes continuously due to littoral drift,
predominantly in a south-westerly direction. Changes along the southern coastline of the Gulf
have been reported since the end of the 18th century (Jha et al. 1999). The Gulf of Venezuela is
connected with Tablazo Bay by the three openings (Boca San Carlos-Zapara, Boca Ca onera, and
Boca Ca onerita) described above.
2.5
The Navigation Channel
Before 1938, navigation between the Gulf and the Lake relied on the natural channels,
particularly the Larrazabal channel passing between San Carlos and Zapara Islands. Construction
of a stable channel was initiated in 1939 and a 6-m deep channel was maintained until 1947. In
1952, the Instituto Nacional de Canalizaciones (INC) commenced dredging operations for the
creation of a new navigation channel. The new channel, with a depth of 12 m and a 3-km
breakwater at the western end of Zapara Island, was completed in 1956. By 1963, the depth of
the Canal was increased to 13.5 m. Through dredging this depth has been maintained to the
present day (Bechtel 1999a).
6
Background and Literature Review
3 Hydrology
3.1
The Lake Maracaibo Catchment
River lakes such as Maracaibo, receive water from large areas (river drainages) relative to their
size. As such they are especially vulnerable to changes in the hydrology or water quality of
connected rivers (Lewis 2000). Lake Maracaibo has a total watershed catchment of 71,000 km2,
excluding its own water surface area of 12,000 km2 . Figure 3.1.1, below, depicts the total
watershed and rivers draining into Lake Maracaibo. The watershed can be sub-divided into 30
catchments. The total mean annual runoff rate from these catchments is 1623 m3/s (CGR 1994).
The distribution of this total runoff is uneven, both spatially and temporally.
Figure 3.1.1: The watershed and rivers draining to Lake Maracaibo.
7
Background and Literature Review
The largest tributary to Lake Maracaibo is the Catatumbo river, situated in the south-west corner
of the lake with a total catchment area of 32,000 km2 (45% of the total watershed), contributing
60% of the total runoff (975 m3/s) to the lake. The catchment of this river extends into the
Republic of Colombia. Santa Ana (184 m3/s) and Rio Escalante (103 m3/s) are the other two
large rivers draining into the south-west corner of the lake. Rio Chama, situated at the southern
end of the lake, is the fourth largest river (50 m3 /s) in terms of runoff. The runoffs from
Catatumbo, Santa Ana, Rio Escalante and Rio Chama account for 81 % of the total runoff from
the watershed of Lake Maracaibo. The total catchment area of these four rivers is equal to 65%
(46,000 km2) of the total watershed area. The western side of Lake Maracaibo (catchment area
60%) yields 68% of total runoff, whereas the eastern side of Lake Maracaibo catchment, with
40% of catchment area, yields 32% of the total runoff. The catchment yield reduces towards the
northern end of the lake (CGR 1994).
3.2
Precipitation
Of the elements that effect the hydrological balance of Lake Maracaibo the most variable is
precipitation. The basin-wide average is 1260mm/year, with values that range from 300mm on
the west coast and 400mm in the northern portion of the basin, to 3500mm in the Colombian
portion of the Rio Catatumbo sub-basin (Bautista 1997).
The annual regime of precipitation follows a bimodal distribution, with two peaks, one between
April and May, and the other in October. In general the dry season occurs between JanuaryMarch and the wet season between May-November, with April and December in transition.
3.3
Hydrological Balance
The hydrological balance of the lake includes five elements: (1) land drainage; (2) rainfall onto
the open waters; (3) evaporation losses; (4) ground water discharge and recharge, and (5) net
flow through the Strait. The contribution of ground water to the water balance can be neglected,
as it is relatively small. Several studies on the hydrologic balance of the Lake Maracaibo system
have shown a close relationship between the annual cycle of rainfall on the basin, and the water
exchange between the lake and the sea. In his 1964 study, Luis Corona analysed hydrologic data
from 1940 to 1960. He estimated that the nett fresh water entering the lake in 1960 was 1,565
m3/s (Corona, 1964). Battelle (1974) estimated that the fresh water entering Lake Maracaibo
was 2,095 m3/s. Using precipitation, evaporation, and stream gauge data, a MARNR study
8
Background and Literature Review
concluded that between 1958 and 1977 fresh water inflow was 1484 m3/s (Parra Pardi, 1977,
1979). Further analysis using precipitation, evaporation, and current measurements taken
between 1973 and 1983 by Rincón, estimated that the Catatumbo River discharges 1,147 m3/s, or
60% of the total drainage (1909 m3/s) into the Lake. A 1994 hydrologic study conducted for
ICLAM, CGR (1994), estimated that the average annual fresh water flowing into the lake is in
the order of 1,600 m3/s. The results of the different hydrologic balance studies are summarised in
Table 3.3.1.
Table 3.3.1: Nett fresh water inflow into Lake Maracaibo
Study
Fresh water
inflow, m3/s
1565
2095
1484
1909
1600
Corona, 1964
Battelle, 1974
Parra Pardi, 1977, 1979
Rincón, 1993
CGR, 1994
9
Period
1960
1975
1958-1977
1973-1983
1973-1990
Background and Literature Review
4 Hydrodynamics of Lake Maracaibo
In this section the hydrodynamic regimes operating in Lake Maracaibo are discussed. An
understanding of lake hydrodynamics is important because the mixing and transport processes
operating in a lake determine, to a large extent, the ecological response of the lake to the
meteorological forcing, inflows and outflows (Imberger 2001)
4.1
Hydrodynamic Forcings
4.1.1 Tides
A summary of earlier studies on tidal hydrodynamics can be found in Lynch et al. (1990), where
studies by Redfield (1961), Kjerve (1981), Molines and Fornerino (1985), Werner and Lynch
(1986), Lynch and Werner (1986, 1987) and Molines et al. (1989) are reviewed. These papers
describe both data analysis and modelling efforts to explain the tidal dynamics of the Lake
Maracaibo system.
The tide in the Caribbean Sea is mixed, but predominantly diurnal (Redfield 1961), with a micro
tidal range of 10 to 20 cm. In Aruba the mean range at spring tide is 21 cm. In the Gulf of
Venezuela, the semi-diurnal components of the tides are greatly augmented by resonance. The
tidal range initially increases to 1.1 m at Malecon but subsequently decreases to 0.6 m at Punta
de Palmas, inside Tablazo Bay. At the northern end of the lake spring tides appear to be diurnal,
while semi-diurnal components are evident during neap tides. The tidal range declines from 0.2
m at the south end of the strait to a few centimeters within the lake (Redfield 1961, NEDECO
1993). There are considerable phase lags from north to south. As tides propagate from Malecon
in the north, towards the south, they experience a delay of 50 minutes at Maracaibo, over a
distance of 40 km (INTEVEP 1998b). Along the Strait of Maracaibo, high water is nearly
synchronous. Tides in the lake are nearly 180° out of phase with respect to those found in the
Gulf of Venezuela, thus when the tide at Malecon is rising, the currents flow towards the lake at
the inlet to the Bay of Tablazo, but flow seaward in the strait (Lynch et al. 1990).
10
Background and Literature Review
4.1.2 Winds
The trade winds affecting the lake system blow predominately in the northeasterly direction from
November to April, and diminish considerably for the rest of the year (Bautista 1997). The
monthly wind speed in the bay peaks at 5-10 m/s in March, dropping to 3 m/s from May to
December. This windy period coincides with the dry season. The local wind field over the Lake
develops from the differential heating of land and water masses, and exhibits a counterclockwise pattern.
The wind field over the lake differs from that found in the bay and the gulf (INTEVEP 1998b).
The wind speed is of a smaller magnitude and never persists for more than 12 hours from any
direction.
Gusts of wind might have a magnitude four or more times the mean wind speed. The maximum
wind waves range from 1.5 m in the bay to 1.2 m in the lake. The mean wave period ranges 3.5
seconds to 4.3 seconds (INTEVEP 1998b).
4.1.3 Currents
A counter-clockwise current dominates the circulation in the lake. This current is driven
primarily by wind and is supported by the density stratification and the Coriolis force. As the
current spins the epilimnion water, the saline water in the hypolimnion is forced to rise at the
centre of the lake to within 15 m of the surface, forming a cone-shaped hypolimnion (Redfield,
1956). Measurements conducted in March 1974 show the same counter-clockwise circulation
pattern in the surface and the bottom current. The velocity at the centre of the lake was low near
the surface (7 cm/s) but increased to 12 cm/s near the bottom. On the other hand, the velocity in
the shallow areas of the lake had a large magnitude (on the order of 20 cm/s), which decreased
with the depth (Battelle 1974).
Currents play an important role in mixing and the formation of a conical hypolimnion at the
centre of the lake (Redfield 1961), the top of which, has been observed within 5 to 15 m from the
water surface (Bautista 1997). Parra Pardi (1996) In the lake, wind-driven seiche can, on some
occasions, be so severe as to upset water column stability, allowing nutrient-rich, hypolimnetic
waters to mix into the epilimnion (Parra Pardi 1996).
11
Background and Literature Review
4.2
Mixing Processes
Water quality in Lake Maracaibo is determined predominantly by an influx of phosphorus and
nitrogen particulates carried by rivers, with additional anthropogenic sources on the eastern
shores of the lake, and from population centres along the strait. These nutrients spread out
horizontally over the lake surface and feed a substantial primary production. The nutrients
attached to particles, as well as the particles formed due to primary production, then settle to the
lake bottom. At the lake bottom, whenever anoxic conditions prevail (due to salt stratification),
nutrients are released in forms which are accessible to further primary production. Trapped
within the saline hypolimnion, these nutrients are recycled by continuous entrainment, driven by
the circulation in the lake and by mixing events that vent the deep nutrient pool to the surface,
where they sustain further growth.
The above-described processes, particularly nutrient loading to the surface layer, sedimentation,
and stratification that supports anoxic conditions and controls entrainment across the density
interface, are all extremely important for predicting primary production in Lake Maracaibo.
Lake Maracaibo is subject to a spatially varying wind-field, which in association with the
Coriolis force creates a basin-scale cyclonic gyre (Redfield 1960, Bautista et al. 1997). The
interplay between the strong density stratification and the cyclonic gyre induces vertical shear.
The subsequent result is a very interesting doming the dense more saline hypolimnion. Rhines
(1995) observed a similar phenomenon in a small, rotating, stratified basin, in which vertical
shear was accommodated by thermal wind-tilting of isopycnals. The doming of the hypolimnion
raises nutrient rich, highly saline water nearer to the Lake surface, where it can be entrained into
the surface mixed layer.
Tidal action induces a periodic saltwater influx which enters the lake at is northern end and flows
down along the bottom to form a saline hypolimnion. From there it is mixed upwards into the
epilimnion. The anti-clockwise cyclonic circulation of the surface waters causes an upconing of
the hypolimnion in the centre of the Lake. As a result the hypolimnion is withdrawn from the
bottom around the margins, with the epilimnion occupying the entire water column, even to a
depth of 30m (Redfield 1960). The resultant dome-shaped hypolimnion has an apex at a depth of
5-15m, and salinity ranging between 5-15 ‰ (Jha et al. 1999). The epilimnion and hypolimnion
12
Background and Literature Review
mix most intensely at the apex of this dome. Consequently this area is the principal source of the
salt in the epilimnion and the chlorides in the Lake’s surface are highest in the centre (Redfield
1960).
Tidal and wind induced mixing generate vertical and longitudinal salinity gradients, typical of a
partially mixed estuary (Jha et al. 1999). The counter-clockwise current which circulates around
Lake Maracaibo at an average speed of half a knot, mixes the surface waters thoroughly, down to
the upper limit of the pool of saline water over the bottom, and produces almost uniform
conditions across the entire lake (Redfield 1958).
4.2.1 Internal Dynamics
Acoustic Doppler Current Profiler (ADCP) field data taken from the two northernmost stations
in Lake Maracaibo, LM10 and LM20 (see Figure 4.2.1), confirms the existence of a counterclockwise circulation of 20-30 cm s-1 (Horn et al. 2001). The velocity of the current at the
southern end of the lake is slower than that in the north. However, the flow is still in a direction
consistent with a general counter-clockwise circulation (Horn et al. 2001).
Figure 4.2.1: 1998-99 (wet-dry) Campaign Field Stations
13
Background and Literature Review
The downward curvature of isopycnals in the main basin of Lake Maracaibo, and consequent
higher surface salinity near the lake centre, first documented by Redfield (1958), is evident in the
1998 salinity data (Laval et al. 2001b). Based on the stratification measured in the field, the
internal Rossby radius of deformation was approximately 22 km (Laval et al. 2001b). This
indicates that rotation is important in the internal dynamics, further supporting the hypothesis
that the doming of the hypolimnion is due to a geostrophic balance with the cyclonic circulation
(Redfield 1958; Parra-Pardi 1983; Bautista et al. 1997).
4.2.2 The influence of the earth’s rotation
In large lakes such as Maracaibo, the earth’s rotation is often important in that it may influence
the motion in the lake. When a particle moves in a straight line on a rotating coordinate system it
appears to be deflected. The object does not actually deviate from its path, but it appears to do so
because of the motion of the coordinate system. The apparent deflection of particles moving over
the Earth is explained in terms of the Coriolis force.
The non-dimensional Burger number is often used to determine the importance of the earth’s
rotation on a lakes internal dynamics. The Burger number (Equation 4.2.1) is defined as the ratio
of time of travel of an internal wave across the lake (L/ci), to the time it takes for the lake to
rotate about its axis (f –1) (Imberger 2001).
(Equation 4.2.1)
c
Si = i
Lf
where ci is the phase speed of the wave, L is the typical “radius” or width of the lake at the depth
of the thermocline, and f is the inertial frequency at the latitude of the lake. When Si < 1 the
earth’s rotation dominates the internal dynamics and the waves have the character of an internal
oscillation with most of the energy in the wave being kinetic energy. As Si increases above one
the internal oscillations progressively takes on the characteristics of simple gravitational seiches
(Imberger 2001)
In Lake Maracaibo the Burger number ranges seasonally between 0.12 and 0.39 (Roza-Butler
2001) illustrating the importance of the earth’s rotation on the internal dynamics of Lake
Maracaibo.
14
Background and Literature Review
5 Previous Studies
This Section provides a brief overview of past studies emphasising major findings and
conclusions that are still valid and relevant. The major studies fall into one nine groups:
1. The studies by Woods Hole in the 50’s and MIT in the late 50’s and 60’s;
2. The Battelle study in 1972;
3. The Studies by Parra Pardi in the late 70’s and early 80’s;
4. The NEDECO study in the early 90’s;
5. The ICLAM studies and data collection program in the 90’s;
6. The INTEVEP studies in the 90,s;
7. The Studies at the University of Zulia;
8. Other studies of interest; and
9. The Bechtel Integrated Environmental Remediation Study
5.1
The Woods Hole and MIT Studies
In a series of papers, Alfred Redfield analysed organic matter in the sediments of Lake
Maracaibo (1956), water quality processes in the Gulf of Venezuela (1960), and tidal
characteristics of the Lake system (1961). The importance of his work lies not only in the
identification of various processes which have since been confirmed by other researchers, but
also in providing a set of historical data against which the changes in water quality parameters
can be measured. Redfield was the first to describe the existence of a counter-clockwise current
in Lake Maracaibo (on the order of half a knot), attributing the cause to the prevailing winds
(1956). He observed that the salt water intrusion into the lake occurred only during the three
months of the dry season. In the late 50’s, the lake’s surface water contained about 33‰
seawater, with a total phosphorous content of 40 _g/l. The concentrations of both chloride and
nutrients were considerably higher in the hypolimnion, with chloride >2ppt and total phosphorus
>6 _g/l (1960). A combination of hydrographic conditions and biological activity resulted in
15
Background and Literature Review
depleted hypolimnetic oxygen content, with no oxygen present in the lake centre. Redfield
(1956) determined that phosphorous and presumably other nutrients were derived almost
exclusively from land drainage. He found that in its particulate forms, from river inflows and as
organic detritus from primary production, phosphorous enters the saline hypolimnion through
sedimentation, accumulating in the “deep waters”. The available oxygen was noted to be
insufficient to oxidize this accumulation, resulting in conditions unusually favourable for the
entrapment of organic matter in sediments. One third of the phosphorous was present in
inorganic form, available for plant nutrition. Consequently, growth is not limited by the
availability of phosphorous in the photic zone (Redfield 1956).
Quoting a 1933 sediment analysis by Trask, Redfield (1956) argued that the concentrations at
which organic matter is entrapped in sediments depend upon the relative rates of sedimentation,
and of the accumulation of organic matter at the lake bottom. The principal source of sediment
input into Lake Maracaibo is the Catatumbo River. The distributions of organic carbon, nitrogen,
and organic extract in bottom sediments mimic the distribution of oxygen deficiency in the water
over the bottom, with the exception of the pattern being skewed toward the eastern side of the
lake due to that side’s greater rate of sedimentation.
Redfield also analysed tidal data (1961) and suggested that the dimensions of the Gulf of
Venezuela are such that they encourage the semi-diurnal constituents to be augmented by
resonance, while the diurnal constituents remain relatively unaltered.
An MIT study (1967) by Harleman, Corona, and Partheniades utilised simultaneous salinity
measurements in the navigation channel, and established a relation between a dimensionless
intrusion length parameter and a characteristic estuary number. The parameters include the tidal
prism, the magnitude of current in the inlet to the estuary, the average water depth, fresh water
discharge, and the tidal period. This relation was then demonstrated using simple theoretical
argument, and the assumption of a partial or complete mixed water column. If the tidal prism
decreases as a result of the partial or complete closure of the inlet, the analysis predicted reduced
saltwater intrusion.
16
Background and Literature Review
5.2
The Battelle Study
Sponsored by Creole Petroleum Corporation, the Battelle study constituted the first major
investigation into the ecology of Lake Maracaibo, south of the Straits. This study produced
baseline data on water quality, primary productivity, macrofauna, sediments, and fishery
resources of the lake, as well as data on major sources of industrial and domestic wastewater
discharged into the lake (Battelle 1974). The salinity patterns obtained at one metre depth and
one meter above the lake bottom were similar to those of Gessner (1956) and Redfield (1960),
however the values were approximately twice as high. The average phosphorous concentration in
the upper 5m of the water column ranged between 40-280 _g/L while the total nitrogen
concentration ranged between 779-1580 _g/L. These concentrations were several times higher
than those measured by Redfield (1960). Algal blooms were observed in late winter and early
spring. During the wet season, it was observed that the river discharge contained a high
concentration of suspended solids.
The information gathered in the Battelle study supported the conclusion drawn by Redfield
(1960), in that salt is primarily introduced into the epilimnion from the hypolimnion, largely near
the lake centre, whereas the freshwater input is derived from the rivers on the margin of the lake.
The cone-shaped hypolimnion previously described by Redfield (1960) was most apparent in the
wet season (November 1973). There was also a positive correlation between the salinity in the
hypolimnion and the high levels of PO4-P (up to 4900 _g/L) and NH3 (up to 24,800 _g/L)
observed in this region. Battelle (1974) found that only a small portion of nutrients released by
biological or chemical actions returned to the epilimnion through diffusion and were made
available for biological utilisation. The majority of nutrients were retained in the hypolimnion
until the next mixing event. Thus, mixing events impacted greatly on the chemistry of the lake
water.
Sediments were found to be rich in organic material, with extractable organic contents as high as
3% of the total dry weight. Phytoplankton abundance was observed to follow a cyclic pattern
with the highest levels, among the highest found in the world, occurring after mixing events. The
distribution and diversity of phytoplankton in the lake was approximately the same in winter and
summer, and from station to station, with the blue-green algae being the most dominant group,
the highest population densities being recorded along the Bolivar (north-east) coast.
17
Background and Literature Review
The Battelle report compiled a summary of the domestic waste discharges (Battelle Appendix F,
1974). The study summarises non-petroleum industrial wastes such as NH3-N, BOD, suspended
solids, COD, phosphorous, and sodium pentachlorophenate. Several large rivers (including the
Catatumbo, Santa Ana, Escalante and Motatan) were also sampled during the dry and wet
seasons in order to evaluate the river water quality. The levels of dissolved NH3-N and organic
nitrogen were found to be high (~300 _g/L). The study warned of the potential adverse impacts
of non-petroleum wastes, both domestic and industrial, on water quality.
5.3
The Parra Pardi Studies
From 1974 to 1979, Parra Pardi and his associates undertook a comprehensive study aimed at
understanding the lake ecosystem, evaluating its capacity to assimilate contaminants,
establishing scientific bases for water quality standards, and making recommendations for
corrective actions. The study was commissioned by la División de Investigaciones sobre
Contaminación (DISCA). An array of methodologies was used: physical modelling of the strait
and the bay, a mathematical model of the strait and the bay, intensive field observations in the
strait, and a hydrologic analysis of the water balance.
The work of Parra Pardi and colleagues was presented in two reports (1977 and 1979). The
reports proposed a strategy for managing water quality in the system, collecting and treating the
domestic and industrial wastewater from the City of Maracaibo, the Cities of La Silva and La
Rosa, and re-utilising industrial and agricultural wastewater.
5.4
The NEDECO Study
In the November - December 1992 wet season campaign and the March 25 to April 8 1993 dry
season campaign, wind velocity, water velocity and other biochemical and ecological data were
collected at 23 locations scattered in the gulf (3), the bay (4), the strait (4), and the lake (13). For
the same period, tidal data was collected at 14 points (the gulf: 3; the bay: 4; the strait: 3; the
lake: 4), and water flowing into the Lake from 15 major rivers was sampled. The total river flow
into the lake, including flow from the Limón, was found to be 2167.3 m3/s in the wet season and
571.4 m3/s in the dry season. During the dry season campaign, the maximum flow through the
strait was in the order of 25,000 m3/s.
The purpose of this study was twofold, to quantify agricultural, domestic and industrial water
18
Background and Literature Review
demand, and to estimate water supply. Intrinsic to the NEDECO study, an engineering evaluation
of alternative methods of lake desalinisation was undertaken. The alternatives considered
included reducing the depth of the navigation canal via the natural process of sedimentation,
construction of parallel dikes, and partial or complete closure of the inlet. By reducing the Lake
salinity, it was thought that the accelerating processes of contamination and eutrophication of the
lake over the last 25 years could be stopped, or even reversed. This study investigated both the
salinity and sediment transport through the strait under annual flow conditions typical of the
period 1973-1983. The important factors contributing to eutrophication were found to include
oil spills, industrial discharge of inorganic nutrients, ammonia compounds and toxic materials
from some 104 point sources, agricultural discharges, and the sewage discharges of 1.8 million
people living in Maracaibo. Rivers were estimated to carry 1943 m3/s water and 182,900 kg/day
total nitrogen into the lake system. The domestic and industrial wastewater, on the other hand,
delivers 16.9 m3/s water and 61.3 kg/day total nitrogen. The industrial organic pollutant loading,
concentrated in the north-east region of the lake and in the strait, makes a lesser contribution in
terms of quantity.
The report analysed water demand and supply for 1992, projecting the future demand and supply
to the year 2020. In order to meet this future demand for water, additional dams would need to be
built upstream. Considering the number of dams that are already under construction, one might
anticipate that in the near future the lake system would operate under a different hydraulic
regime.
As a part of the study, the Delft Hydraulic Laboratory conducted an investigation of the lake
system by means of a 3-D numerical model. The model incorporated hydraulic processes (tides,
currents, river discharges, and the mixing of fresh and salt water), physical-chemical processes
(nutrients, contaminants, dissolved oxygen, and suspended solids), and ecological processes
(dynamic cycle characteristics, eutrophication, and algal blooms).
The numerical simulation conducted by Delft indicated that mixing in the strait was sufficiently
intense that over the course of several years, contaminants released into the strait would be
carried toward the bay and the gulf by net water discharge from the lake. Thus, leaving only a
small portion of contaminants entering the lake (NEDECO 1993).
19
Background and Literature Review
5.5
The ICLAM Studies
The Master Plan regarding the control and management of water quality in the Lake Maracaibo
Basin prepared by ICLAM (1991) presents a considerable amount of information that has been
tabulated in the context of water quality goals to be achieved by the year 2005.
A comprehensive database on salinity and other water quality parameters has become available
in the last 5-6 years, due to the data collection efforts of ICLAM. The work of analysing this data
and correlating water quality parameters with dimensionless hydrodynamic parameters such as
the estuarine number, the lake number, and the inflow Froude number has not yet begun.
However, current research undertaken at the Centre for Water Research (University of Western
Australia) may begin to elucidate the link between non-dimensional numbers and water quality
in Lake Maracaibo.
Using data collected in July 1996, Godoy (2000) estimated the balance of total nitrogen in the
lake. The inputs to the lake are 278 ton/day, including 158 ton/day from river discharges, 80
ton/day from rainfall, and 40 ton/day from wastewater. An estimated 70-140 tons of total
nitrogen is removed from the lake by outflow each day. This results in a net gain of 138 to 208
tons of total nitrogen, which remains in the lake water and sediment.
5.6
The INTEVEP Studies
INTEVEP conducted several studies of the hydrodynamics and water quality of the system, as
well as studies on bottom sediments. A good summary of these studies is provided in three
recent reports by INTEVEP (1998a, b, c.)
The sediment sampling campaigns conducted by INTEVEP in 1996 and 1997 in the watershed of
the Catatumbo River and Lake Maracaibo focused on the content of heavy metals, such as
vanadium, nickel, chrome, zinc, barium, mercury, and iron. In general, it was observed that the
distribution patterns of metals in the sediments deposited in the lake system have the lowest
values in the strait and the highest values in the lake. This work constitutes a reference baseline
for future studies on sediment quality.
The recent report titled “Evaluation of the Physiochemical Parameters of the Water Column and
the Sediment Quality of Lake Maracaibo” (1998c) presents the results of measurements of
20
Background and Literature Review
physicochemical water parameters and sediment quality in Lake Maracaibo, obtained during a
sampling campaign performed in July 1994. The results indicate that epilimnion water of the
lake has slightly higher temperatures than the hypolimnion, low salinity (4‰), elevated dissolved
oxygen content, and alkaline pH in an oxidizing environment. The hypolimnion has cooler water,
moderate salinity (>10‰), less alkaline pH and absence of dissolved oxygen. The sediment in
the western region of the lake was characterised by lower organic matter and lower metal content
than sediments from the central, eastern and southern regions.
The report titled “Water Quality of Lake Maracaibo” (1998b) presents an analysis of the water
quality of the Maracaibo System, and also discusses the salinity measurements taken between
1963 and 1971. The report suggests that the vertical stratification of chlorides is a typical
estuarine stratification, and concludes that the stability of salinity values over the last 30 years
demonstrates that a new equilibrium has been reached.
5.7
Studies at the University of Zulia
Various studies on specific topics related to the environmental quality of the lake have been
conducted over the years by faculty and students at the University of Zulia.
In the continued effort to model numerically the Lake Maracaibo system, Molines et al. (1989)
has demonstrated two important wind effects: the development of seiche and large scale eddies
within hours of the initiation of wind. These processes, though having particular significance for
mixing and transport of waters in the bay, however seem less relevant when considering mixing
processes in the Lake.
5.8
Other Studies of Interest
An issue that has received considerable attention in the literature, is the increase in salinity of the
surface water over the years and the question of whether or not this process of salinisation in
continuing. When historic annual average salinity data, taken at 1-m below the water surface is
plotted, it becomes apparent that the salinity of the lake increased in the mid-50s (when the
navigation channel was deepened.) Whether the salinity of the Lake has now reached a new
equilibrium state or is rising to higher levels remains a subject of debate. Performing a linear
regression on the data series taken between 1954 and 1996, Bautista concluded that the water in
the lake has become steadily more saline (Bautista et al. 1997). In contrast, Parra Pardi (1996)
21
Background and Literature Review
conducted a linear regression on the data taken between 1960 and 1995 and found no convincing
evidence that the lake is in a process of salinisation. Parra Pardi further suggested that the
changing hydrology of the lake watershed in recent years (as a result of deforestation,) may be
the cause of some of the variations in salinity.
5.9
The Bechtel Integrated Environmental Remediation Study
A multi-year study conducted by Bechtel International (2000a,b,c), under the sponsorship of
Petroleos de Venezuela, S.A. (PDVSA), evaluated the current environmental condition of the
Lake Maracaibo system, assessed the sources and quantity of nutrients and contaminants
entering the Lake, and then applied two computer models (DYRESM-WQ and MIKE-3) to
determine short and long-term changes in water quality due to modifications of the navigation
channel, and a reduction in nutrient loading. The study included development and preliminary
cost analysis of alternative pathways to continue the transport of oil, coal, petrochemicals and
general cargo to and from the Lake.
The evaluations and analyses of this study lead to the conclusion that the solution to the problem
of anoxic conditions in the hypolimnion requires the reduction of dead organic matter in the
Lake, which in turn requires a reduction in primary production (phytoplankton, i.e. algae). The
natural conditions in the Lake indicate that this can not be easily accomplished.
Two remediation options were analysed in depth during this study. Both options involved a 10%
reduction in nutrient loading to the Lake through point source and non-point source pollution
control, with the second option, additionally evaluating the effect of allowing the Lake system to
return to its pre-dredging bathymetry, through closure of the navigation channel. The option of
closing the navigation channel and allowing the system return to its pre-dredging bathymetry
was found to have the greatest positive impact on salinity and nutrient levels. The models
indicated that there would be a seasonal improvement in oxygen levels in the Lake, but during
several (in some years up to seven) months per year, anoxic conditions would persist in the
hypolimnion, just as they do today. Channel closure would also reduce stratification and lead to
a subsequent reduction in the concentration of nutrients in the Lake. However, the effect on total
chlorophyll-a and algal species composition was uncertain.
MIKE 3 predicted a 35% reduction in chlorophyll-a if the navigation channel was allowed to
22
Background and Literature Review
return to its pre-dredging bathymetry, while DYRESM-WQ predicted no change in chlorophylla. The difference in the predictions by the two models was due to the fact that each modelled
only one species of algae. DYRESM-WQ simulations assumed the main bloom-forming species
were cyanobacteria, while MIKE 3 simulations assumed chlorophytes were the dominant algal
species. However, evidence from past studies indicates that both species are present at high
concentrations within the Lake, and that during different periods of the year algal dominance
changes between cyanobacteria and chlorophytes. Due to the differing assumptions, neither
model could provide any meaningful insight into the likely impact of proposed management
strategies on phytoplankton species composition, and thus water quality.
23
Background and Literature Review
6 Historical Salinity Trend
The aim of this analysis of historical data is to determine whether salinity levels in the Lake have
reached a new equilibrium following construction of the navigation channel. An analysis of the
salinity data at 1m depth has been conducted to reduce any bias due to the uneven distribution of
data in time. The data sources utilised in this analysis include those used by Bautista et al. (1997)
together with additional data from the ICLAM database for the period 1996-1998 (Appendix A).
Figure 6.1 shows the linear trend line obtained from the regression analysis of all data. The
slope of this line, i.e. the rate of salinity increase, is 0.054‰ per year, giving the impression that
Lake Maracaibo has been undergoing a process of salinisation since 1954. This relatively high
average rate of salinity increase is due, in part, to the fact that this data set includes data collected
prior to the deepening of the navigation channel, a discrete event that is generally believed to
have contributed to the increase of salinity levels in the lake. To assess the current trend in
salinity levels it is more appropriate to work with data obtained post completion of the
navigation channel.
Salinity of Lake Maracaibo at 1m depth
(Data: 1954-96 from Bautista et al. (1997) and 1996-98 from ICLAM Database)
5.5
5
]
u
s
p
[
y
t
i
n
i
l
a
S
salinity
50y r trend
20y r trend
New Equilibrium Salinity
4.5
4
3.5
3
2.5
2
1.5
1
Jan54
Jan77
Date
Figure 6.1: Salinity of Lake Maracaibo at 1m depth
24
Nov 98
Background and Literature Review
To reduce the biases due to uneven distribution of data in time, it was decided to test the time
series from 1977 to 1998, a period for which a significant data series exists. Prior to 1973 there
was only one sampling event per year, and between 1973 to 1977 only two sampling events per
year. The average of all salinity values between 1977 and 1998 is 3.81‰, with a root mean
square error of 0.059‰. Thus, it appears that the lake has reached a new state of equilibrium
over the past twenty years, with salinity in the surface waters stabilising at approximately 3.8
g/L. To better understand salinity changes in the lake, it is important to examine the processes
giving rise to saline-fresh water exchange at the lake entrance.
6.1
Behaviour of the Underflow
Prior to the deepening of the channel, periods of high rainfall flushed the Lake, and salt
stratification was replaced by a weak thermal stratification (Redfield 1961). The deepening of the
channel in the mid 1950's allowed more seawater to enter the Lake, increasing both the
epilimnetic salinity, and the salt stratification in the bottom waters (Parra-Pardi 1983). Salinity
stratification is now a permanent feature of the Lake (Laval et al. 2001a).
The salt water in the Gulf of Venezuela enters the Bay of Tablazo through the mouth of the
estuary. In the Bay of Tablazo, which has an average depth of 2.5m, the water is generally
mixed over the entire depth (NEDECO 1993). A definite stratification has been documented in
the navigation channel leading to the Strait of Maracaibo (Jha et al. 1999). In the strait, fresher
water from Lake Maracaibo meets the saline water from the bay, forming a salt wedge along the
length of the strait. The exact nature of the salt wedge, the strength of the stratification, and the
positions of the isohalines are determined by both the baroclinic and barotropic flows induced by
a combination of the:
•
Barotropic tides;
•
Nett outflow due to the fresh water inflows to the lake;
•
Baroclinic pressure gradient due to the salinity difference between the bay and the lake; and
•
Vertical diffusion of salt.
When these influences combine favourably, salt water from Tablazo Bay is able to negotiate its
way through the strait and flow down the incline into the bottom of Lake Maracaibo. Two
25
Background and Literature Review
inferences follow directly from this general description. First, the underflows of salt water into
Lake Maracaibo are most likely tidally modulated, and therefore of short duration. Second, the
periods during which spring tides can combine with other influences to produce substantial salt
water inflows into Lake Maracaibo will be seasonally modulated, with major underflow events
occurring on a small number of occasions each year (Bechtel 1999a).
Freshwater
B
E
Outflow
D
A
C
Figure 6.1.1 Schematic diagram of the salt flux path in the Maracaibo System along the navigation channel. A) Transfer across
the mouth between the Gulf of Venezuela and Tablazo Bay; B) mixing and secondary transport in Tablazo Bay; C) Unsteady salt
wedge dynamics in Maracaibo Strait; D) Plunging salt water underflow into Lake Maracaibo; E) Vertical mixing in Lake
Maracaibo.
As can be observed from Figure 6.1.1 lower density water flows out of the lake on the surface,
while denser water from the Bay of Tablazo flows towards the lake. It has been suggested that
hydraulic discharges control the balance of salinity in the lake (Laval pers. com. 2001). During
the wet season, when discharge velocities are high, the mouth of the strait is flooded with water
from the lake, preventing salt water from entering. When discharges drop during the dry season,
and aided by tidal effects, the saline water enters the lake. It was observed that when fresh water
flows into the ocean, a current of deep salt water flows in the opposite direction (Bechtel 1999a).
Tides thus produce cyclical oscillations resulting in vertical and longitudinal salinity gradients.
26
Background and Literature Review
7 Phytoplankton Composition & Succession
Seasonal changes in the abundance and composition of phytoplankton may be observed to occur
on temporal scales measured in weeks or months, during which mean specific population
densities may increase or decrease through 6-9 orders of magnitude (i.e. from 10-3 to 106 cells
ml-1 ) (Reynolds 1984). The time scale is such that the amplitude range of environmental
oscillations is itself likely to alter significantly. In temperate lakes, for instance, the
environmental variables that are subject to such profound changes include day-length, total
irradiance and its attenuation within the underwater light field, temperature, thermal structure
and nutrient availability, as well as the abundance of herbivorous animals. Reynolds (1984)
stipulates that Tropical lake communities will be influenced by broadly similar constraints to
those that operate in temperate waters. The ecological responses of the phytoplankton to these
changes are generally well defined, although factor interaction often prevents a clear appraisal of
cause and effect, and may even lead to misinterpretation. Correct interpretation is essential if
year-to-year variations and long-term responses of phytoplankton composition to climate change,
hydrological change, and eutrophication effects are to be adequately understood and predicted.
There have been a large number of studies made and descriptions given of phytoplankton
periodicity, though few have seriously attempted to account for the patterns observed. Reynolds
(1984) cites several classical studies on individual lakes, or groups of temperate lakes (e.g. Birge
and Juday 1922; Ruttner 1930; Pearsall 1932; Grim 1939; Findenegg 1943) which established
the clear year-to-year similarities in changes in abundance and composition, however there is a
dearth of such literature elucidating the patterns occurring in Tropical Lakes.
In large tropical lakes chlorophytes and cyanophytes dominate, and in some instances diatoms
also (Lewis 2000). Cyanobacteria are generally absolutely and/or relatively more abundant in
tropical lakes (Reynolds 1984) than in those at higher latitudes. Water blooms of cyanobacteria
develop during periods of stratification, low turbulence, and high temperature. The absence of
real water blooms of cyanophytes and dinoflagellates in deep large lakes may be due to the lack
of one of the prerequisites for their development; minimal water turbulence. A given wind
velocity leads to a greater turbulence per unit area in large lakes than in small water bodies.
Consequently, in low-latitude lakes, chlorophytes and diatoms dominate during the mixing
27
Background and Literature Review
period (Reynolds 1984).
7.1
Historical Phytoplankton Composition & Succession
Due to the high nutrient inputs, nearly constant sunlight, and elevated temperatures, the densities
of phytoplankton in Lake Maracaibo are extremely high. According to Dr. Fritz Gessner (1956)
“Lake Maracaibo may well be the richest in plankton of all the waters of the earth.”
It is important to understand the composition of the phytoplankton community as this provides
an indication of water quality. Phytoplankton blooms within water bodies tend to be indicators of
cultural eutrophication, and can have serious toxicological and aesthetic implications for the
water, and for organisms that inhabit those ecosystems. Phytoplankton form the basis of the food
web for aquatic fauna; however, high to excessive levels of cyanobacteria (blue-green algae) are
indicative of nutrient-rich, eutrophic conditions. The secretion of toxic nuerotoxins is more often
associated with cyanobacteria than any other algal group (Margalef 1983). Thus, high
concentrations of cyanobacteria may be harmful to some vertebrates. However, at moderate
levels the phytoplankton provides a rich food base for the entire aquatic community, thus
contributing to its richness and diversity.
Nutrient rich river inputs combined with large volumes of untreated residential waters and the
occasional petroleum spillage have resulted in Lake Maracaibo showing signs of accelerated
eutrophication. Throughout the lake system, the phytoplankton distribution correlates with the
nutrient levels in the water. At the centre of the lake, the phytoplankton population in the surface
water is low in the dry season, increasing considerably during the wet season. Algal populations
are generally higher along the lakeshore, where the water depth is shallow. In the northeastern
part of the lake, especially along the shoreline, there are high concentrations of blue-green algae,
believed to be due to a combination of river discharges, lake currents, tidal mixing, and
prevailing wind direction (Battelle 1974).
By presenting the characteristics of the successional dynamics and the temporal and spatial
variations of the lacustrine phytoplankton populations, we can attempt to establish possible
relationships between the phytoplankton blooms and the process of eutrophication in Lake
Maracaibo.
Researchers at ICLAM reviewed variations in phytoplankton populations in the lake for the
28
Background and Literature Review
years 1973, 1974, 1978, 1986, 1988, and the period of 1992 through 1994 (Latchinian et al. date
unspecified). Phytoplankton data obtained during 1973, 74, 78, and 86-88 was collected it such a
fashion that it was not possible to establish consistent seasonal patterns. Figure 7.1.1 presents the
general trend of phytoplankton development for the period 1973 -1994.
Phytoplankton Evolution (1973-1994)
100
90
n
o
i
t
i
s
o
p
m
o
C
80
Cy anobacteria
Chlorophy tes
Chry sophy tes
Other
70
60
50
40
%
30
20
10
0
1973
1994
Y ear
Figure 7.1.1: Phytoplankton Evolution in Lake Maracaibo (1973-1994)
It is evident from Figure 7.1.1 that chlorophytes have become relatively more abundant over the
period 1973-1994, however, cyanobacteria remain the dominant species throughout this period.
It is interesting to note that cyanobacteria were far more dominant in the early 70’s comprising
approximately 63% of the total phytoplankton standing crop. Thus, if the navigation channel
were to be filled, it would be valid to assume that the phytoplankton composition would shift
back towards conditions that favour cyanobacteria. Battelle (1974) also documented the
dominance of cyanobacteria during the early 70’s.
On analysing data from 1973-1994 Latchinian et al. (date unspecified) identified four dominant
groups of phytoplankton: cyanobacteria, chlorophytes, chrysophytes, and others (euglenophytes,
pyrophytes, and chryptophytes). Figures 7.1.2 shows typical periodic phytoplankton population
succession between the two most dominant groups in the lake, cyanobacteria and chlorophytes.
29
Background and Literature Review
Phytoplankton Composition at 1m Depth (1992-1994)
7000
Cy anobacteria
Chlorophy tes
Chry sophy tes
Other
Total
6000
5000
l
m
/
s 4000
l
l
e 3000
c
2000
1000
0
Mar92
Jun92
Oct92 Dec92
Mar93
Jun93
Date
Sep93 Nov 93
Apr94
Jul94
Figure 7.1.2: Phytoplankton composition at 1m depth (1992-1994).
Latchinian et al. (date unspecified) found that areas in the northwest and central locations were
dominated by cyanobacteria, with Anabaena and Microcystis being the dominant representatives
of this group. Conversely, chlorophytes were dominant in the vicinity of the discharge zones of
the rivers along the southern and central shorelines of the lake and towards the Motatán. This
finding is in accordance with Konopka et al. (1978), who observed that Anabaena and
Microcystis tend to occupy the epilimnion layers in large eutrophic lakes. These microorganisms
are particularly efficient at fixing organic nitrogen and N2, which allow them to maintain their
growth even when concentrations of dissolved nitrogen diminish in the lake. According to
Garlick et al. (1977) the exceptional capacity of realising oxic, as well as anoxic photosynthesis,
allows these organisms to exist in conditions that are uninhabitable by other alga.
For the period before 1992 Anabeana dominated over Microcystis, a situation that was reversed
after 1992, probably as a result of the increase in available nutrients, principally Nitrogen, as a
result of the progressive increase in anthropogenic contributions. Observations by Parra Pardi
(1996) support this theory.
During the NEDECO field campaign of 1992 and 1993 (NEDECO 1993), the lake waters were
dominated by green algae (chlorophytes), with associated changes due to the wet and dry
seasons. The green algae measured in the lake were species characteristic of freshwater systems.
30
Background and Literature Review
The most common genus measured was Selenastrum, however during the dry season
Scenedesmus predominated in the north and northeastern part of the lake.
NEDECO (1993) attribute the changing phytoplankton composition within Lake Maracaibo to
the increase in salinity since the deepening of the navigation channel. Latchinian et al. (date
unspecified) postulated that the change in composition could be attributed to a combination of
increasing lake salinisation, and an increase in available nutrients, primarily nitrogen, arising
from increased river and anthropogenic contributions.
Recently, Gardner et al. (1998) examined nitrogen cycling rates and light effects on the algae
growth. They found that the nutrient cycles are dominated by biological control, with nitrogen
being the most limiting nutrient for phytoplankton growth. Thus, it is likely that the increase in
anthrogenic nutrient inputs has contributed to the change in phytoplankton composition, as
suggested by Latchinian et al. (date unspecified).
31
Background and Literature Review
8 Predictive Water Quality Modelling
The need for predictive water quality modelling has arisen largely as a result of increased
eutrophication of lakes throughout the world. Eutrophication describes the biological reaction of
aquatic systems to nutrient enrichment, the eventual consequence of which is the development of
primary production to nuisance proportions (Marsden 1989). Eutrophication can arise under
natural conditions, but is more commonly recognised as a consequence of anthropogenic
activities (Moss 1988), and frequently results in large high-density growths of planktonic algae
which significantly reduce the financial and aesthetic value of lakes and reservoirs (Marsden
1989). Eutrophication also results in a reduction in species diversity in waterbodies at all trophic
levels. The frequent dominance of cyanobacteria in eutrophic waters is an additional concern due
to the common ability of these organisms to produce toxins (Codd 2000). As a corollary, the
fundamental aim in restoring eutrophic waterbodies, such as Lake Maracaibo, should be the
reduction of total phytoplankton biomass and a shift in species dominance away from
cyanophytes.
Understanding mechanisms that control lake eutrophication is an issue of both theoretical and
applied concern. Initially, it was thought that a reduction in external loading of nutrients would
be sufficient to curb lake productivity. However, in many lakes, the remineralisation of deposits
accumulated during eutrophication produces significant internal loading (Ostrovsky et al. 1996).
The release of nutrients from the sediments, and their subsequent transport upward can play a
decisive role in lake trophic status (Vollenweider 1968, Nürnberg 1984). On the other hand, in
lakes characterised by stable stratification, the metalimnetic barrier restricts direct vertical
exchange of material. Therefore, in stratified eutrophic lakes, primary producers inhabiting the
epilimnion are often restricted by the lack of dissolved mineral biogenic elements (e.g.
phosphorus), while in the hypolimnion, concentrations of these elements can be much higher due
to their release from bottom sediments (especially in anaerobic conditions) and to decomposition
of settling organic matter (Wetzel 1983). Thus, any physical mechanism responsible for
significant cross-metalimnetic upward mass transport will be important in controlling lake
productivity (Ostrovsky et al. 1996)
32
Background and Literature Review
Restoration programs must be based on a theoretical framework that will allow a prediction of
change. Numerical models can provide this necessary theoretical framework. They can also
assist in understanding the origins and mechanisms of water quality problems and can be
important tools in the development of management strategies for lakes and reservoirs (OECD
1982). Multitudes of models are available for simulating water quality problems, ranging from
simple empirically derived equations, to multi-dimensional ecosystem models (Orlob 1984).
8.1
Input-Output Models
Deterioration of recreational and drinking water quality arising from cultural eutrophication
prompted the development of a number of quantitative relationships between nutrient loading to
lakes and in-lake concentrations (Vollenweider 1968, Vollenweider 1975, Dillon and Rigler
1974), and indices of plant biomass and availability of nutrients to primary producers (Sakamato
1966).
In most temperate lakes of North America and Europe phosphorus (P) has been implicated as the
limiting nutrient for algal growth (Schindler 1977, Nürnberg 1984, Marsden 1989, Hamilton &
Schladow 1997). Hence a cornerstone strategy for the restoration of eutrophic lakes has been the
reduction in the external inputs of P (Edmondson 1970) which are commonly derived from
controllable point sources (Vollenweider 1968).
The most common modelling approach is exemplified by the empirical derivation of input-output
(mass balance) models, based on correlations between P loading and indicators of trophic status
(Hamilton & Schladow 1997, Marsden 1989, Nürnberg 1984). The effect of P-loading reduction
strategies on total phosphorous can be estimated with empirical loading models (Vollenweider
1968, 1975; Dillon & Rigler 1974) from the external P-load, hydraulic residence time, average
depth, and estimates of P lost to the sediments. These models provide long-term, steady-state
predictions after internal P alterations have reached equilibrium with an assumed constant
external loading. However, they neglect the response time to attain steady-state concentrations
after an external loading reduction (Robson et. al. in progress). This response time can be highly
variable due to differences in internal P-loading, the magnitude of the former loading, and the
hydraulic retention time of the reservoir (Jeppesen et al. 1991). Additionally, though reductions
in external P-loading may reduce concentrations of the limiting nutrient, lower algal biomass
may not necessarily result because of disruptions via food web interactions (Jeppesen et al.
33
Background and Literature Review
1991).
Input-Output models make a number of simplifying assumptions, which make them
inappropriate in modelling the long-term changes in phytoplankton (chlorophyll-a) abundance in
Lake Maracaibo, resulting from the proposed reduction in river-borne nutrient loading and
closure of the navigation channel. The empirical P-loading models discussed above assume that:
•
The system is at steady state;
•
The lake is well mixed;
•
The lake is P limited;
•
The in-lake P concentrations are < 0.2 mg/L (OECD 1982)
•
The external loading rate remains constant (i.e.- no intra or inter-annual variations in
loading);
•
The outflow concentrations are equal to the average in-lake concentration;
The Lake Maracaibo system fails to meet each and every one of these assumptions. To highlight
the inadequacy of the use of input-output models in assessing the long-term changes in water
quality it is worth noting three salient points regarding Lake Maracaibo:
1. The system rarely fully mixes due to the almost permanent salt stratification.
2. Phosphorous concentrations in the surface waters are approximately 0.4 mg/L (ICLAM
database).
3. Nitrogen and light limit the system (Gardner, et al. 1998).
For these reasons it is obvious that a simple input-output model will have no utility in evaluating
and analysing the management options under consideration in this study, and that a more
complex process based model is required.
34
Background and Literature Review
8.2
Process Based Modelling/Ecological Models
The development and refinement of deterministic ecological models has furthered the
understanding of biological and chemical processes in aquatic ecosystems, and have been used as
a vehicle to synthesise and evaluate knowledge of natural waters. An important objective of
ecological models is the ability to predict the outcome of a series of possible management
alternatives for a given aquatic system. The majority of deterministic phytoplankton models are
ordered frameworks of mechanistic or semi-empirical sub-models. Each sub-model describes a
particular process, whether it be lake circulation, nutrient limited growth rate of phytoplankton,
or the respiration rate of phytoplankton. The success or indeed failure of such models is
dependent not only upon the inclusion of all important sub-models, but also the way in which
given processes are mathematically described (Scavia and Robertson 1979).
Effects of nutrient reduction strategies on the water quality of a lake or reservoir must account
for both external loading (rivers and precipitation), internal loading (release from sediments) and
fate (uptake by phytoplankton or settling with burial to sediments). Thus, it is imperative that a
water quality model capable of accurately simulating the seasonal and inter-annual
hydrodynamics, biogeochemistry and ecology of lakes and reservoirs be utilised as a tool to
understand long-term process dynamics, and to formulate and/or evaluate optimal management
strategies (Robson et. al. in progress).
35
Model Selection
9 Model Selection
The complex biogeochemical processes that control the nutrient cycles within Lake Maracaibo
present particular challenges to any attempt at modelling the ecology of the lake.
Initial investigations show a strong link between peak river inflows and high nutrient
concentrations in the hypolimnion. The short lag between the peak inflows and peak
concentrations suggests high sedimentation rates, which must be captured by the numerical
models. The sedimentation of organic matter from the epilimnion to the hypolimnion, and the
subsequent accumulation of the unoxidized fraction of this material on the floor of the lake, may
be an important sink for nutrients within the Lake Maracaibo.
In addition to external loadings described above, there are a number of important internal
loadings within the lake. These processes are controlled by vertical fluxes of salt, oxygen, and
nutrients through the water column. Due to the density stratification, these fluxes are limited by
entrainment across the halocline. It is important that any hydrodynamic model capture these
entrainment processes. As well as determining the volume of the saline hypolimnion, the rate of
entrainment across the halocline will determine the dissolved oxygen profile, and the rates at
which nutrients are recycled from the nutrient rich waters of the hypolimnion to the photic
waters of the epilimnion.
The release of nutrients from the sediments underlying the anoxic saline hypolimnion may be an
important internal loading, and any model must include such fluxes. Importantly, these fluxes are
dependent on the dissolved oxygen concentration, reiterating the interdependence of many of
these processes, and the importance of capturing vertical fluxes through the water column. The
observed persistent low concentrations of dissolved oxygen and nitrates in the saline
hypolimnion suggests that denitrification may be a sink for nitrogen in Lake Maracaibo.
The horizontal spatial variability of nutrients and chlorophyll-a over the surface of the lake is
dependent on many factors, including the location of nutrient point sources, the primary counterclockwise currents, local wind-driven currents and the secondary radial currents. It would be
unlikely to capture these factors in sufficient detail to allow for direct comparison between model
simulations and field data over such a large domain without introducing some horizontal spatial
36
Model Selection
averaging. However, from a review of the literature it appears that the nutrient cycles within
Lake Maracaibo are dominated by vertical fluxes. Thus a one-dimensional
hydrodynamic/ecological model such as DYRESM-CAEDYM provides the best possible
methodology of simulating the horizontally averaged fluxes and concentrations.
9.1
DYRESM-CAEDYM
DYRESM-CAEDYM is a one-dimensional coupled hydrodynamic and water quality model for
lakes and reservoirs. It is used to predict the variation of hydrodynamics and ecology with depth
and time. The hydrodynamic component (DYRESM) is a process-based model that simulates the
vertical distribution of temperature and salinity in lakes and reservoirs through a series of
Lagrangian layers. The success of the model applications can be attributed to the high level of
process description, which means that the model is free from calibration, and to the Lagrangian
layer structure, which optimises computation times and negates any requirement to solve vertical
flow velocities. The water quality component (CAEDYM), however, is empirically based and as
such has many parameters requiring calibration.
9.2
CAEDYM
CAEDYM (Computational Aquatic Ecosystem DYnamics Model) is a self-contained ecological
model, which has been designed to link to a suite of hydrodynamic models. The model has been
set up largely for assessments of eutrophication, being of the ‘N-P-Z’ (nutrients-phytoplanktonzooplankton) model format, but it also contains process descriptions of oxygen dynamics,
nutrient cycling, and several other state variables. CAEDYM is a substantial advance on the
traditional N-P-Z models, in that it serves not only as a general ecosystem model (e.g. resolving
various biogeochemical processes), but also as a species- or group-specific model (e.g. resolving
various phytoplankton species) (Hamilton and Herzfeld 1999).
CAEDYM has been modified specifically to allow easy linkage to the hydrodynamic models
used within the Centre for Water Research, University of Western Australia. One of the prime
objectives of the hydrodynamic links is to allow user flexibility in resolving the appropriate
temporal and spatial scales of interest. It has been coupled previously with process
representations of biogeochemical processes (DYRESM-WQ; Hamilton and Schladow, 1997;
Schladow and Hamilton, 1997). However, the CAEDYM model represents a substantial advance
in process representation and flexibility of state variable choices over the traditional DYRESM37
Model Selection
WQ model, which was used in previous modelling of Lake Maracaibo, or other N-P-Z models.
By simulating phytoplankton at the species level, CAEDYM represents the ideal model to aid in
better understanding the ecology of Lake Maracaibo. In addition, coupling CAEDYM with the
one-dimensional hydrodynamic model (DYRESM) allows for analysis and assessment of the
effect of the proposed management options on annual and long-term (>10 years) variations in
water quality.
The major biogeochemical state variables included in CAEDYM are illustrated in Figure 9.2.1,
with differentiation of ecological state variables to species level shown in Figure 9.2.2.
Figure 9.2.1: Major State Variables included in the CAEDYM model
38
Model Selection
Species
Group
Phytoplankton
Zooplankton
Fish
Macroalgae
Invertebrates
Seagrass
Jellyfish
1
Dinoflagellates
44-100 mm
Hardyheads
Gracilliara
Bivalves
Halophila ovalis
Phyllorhiza
punctata
2
Freshwater
Cyanobacteria
100-300 mm
Perth Herring
Cystosira
Polychaetes
3
Marine
Cyanobacteria
> 300 mm
(Gladioferans)
Yellow Tail
Trumpeter
Chaetomorpha
Crustacean
Grazers
4
Chlorophytes
> 300 mm
(Sulcanus)
Black Bream
Ulva
5
Cryptophytes
> 300 mm
(Acartiura)
Sea Mullet
6
Marine Diatoms
7
Freshwater
Diatoms
Figure 9.2.2: Species representation within the major biological state variables in CAEDYM
The model offers substantial flexibility to the user in the choice of state variables. The objective
in setting up the model has been to offer the user a very wide choice of options, which can then
be reduced according to the specific application. This approach is considered to provide greater
flexibility to the user; input files of calibration parameters can be adjusted rather than modifying
the source code (Hamilton and Herzfeld 1999).
In order to predict the effect on phytoplankton composition and abundance resulting from
closure of the navigation channel or a reduction in nutrient loading, and to further the
understanding of phytoplankton dynamics within Lake Maracaibo, it is necessary to discuss the
way CAEDYM represents phytoplankton growth.
9.2.1 REPRESENTATION OF PHYTOPLANKTON GROWTH WITHIN CAEDYM
There is general agreement that algal production and photosynthesis is dependent on the effects
of light, nutrients and temperature (Lehman et al. 1975). However, the interactions between these
effects vary between models. Chen and Orlob (1975) assumed that the limiting factors are
independent of each other and are of equal weight, and thus applied a multiplicative relationship
between light, temperature and nutrient limitation functions.
CAEDYM’s representation of phytoplankton growth limitation differs from Chen and Orlob, in
39
Model Selection
that it assumes that more than one limitation value (e.g. for light or nutrients) is not limiting at
the same time. In this approach, the minimum of two or more of the limitations is used (e.g.
Hamilton and Schladow 1997).
P = µ max ↔f (T ) ↔min[ f ( I ), f (nutrients )] ↔[phyt ]
(Equation 9.2.1)
The maximum growth rate (µmax) is thus modified by the temperature function f(T) and by the
minimum of the light limitation function f(I), and the nutrient limitation functions, f(nutrients),
which can include nitrogen, carbon, phosphorus and silica as is appropriate, and then multiplied
by the phytoplankton concentration, [phyt], to get the new phytoplankton biomass P. Most
recent models use a single multiplicative function or use only the minimum limiting function
(CWR book unpublished).
Gardner et al. (1998) suggested that nitrogen and light are the factors limiting phytoplankton
growth within Lake Maracaibo.
Light Limitation
A wide range of light limitation models have been used in the past (e.g. Jørgensen 1983). Most
include the reduction of light with depth and a Michaelis-Menten expression to describe the
relationship between photosynthesis and irradiance (Robson et. al. in progress). CAEDYM uses
the simple model of Scavia and Park (1976), in which the fractional limitation of the growth rate
by light, f(I), is described as:
f (I ) = 1 − exp
(Equation 9.2.2)
I
√
IK √
↵
where, I is irradiance and IK is a light saturation parameter.
40
Model Selection
Nitrogen Limitation
CAEDYM uses a simple Michaelis-Menten term to represent nitrogen limitation:
f (N ) =
NH 4 + NO3
NH 4 + NO3 + K N
(Equation 9.2.3)
where KN is the half saturation constant for nitrogen, NO3 is the nitrate concentration and NH4
is the ammonium concentration.. The value of KN can be set to be negligible or zero for the case
of nitrogen fixing cyanobacteria (Hamilton and Herzfeld 1999).
41
Model Selection
9.3
DYRESM: REVIEWING THE 1-D ASSUMPTION
The earliest limnological studies of thermal structure of lakes (Hutchinson 1957) recognized that
during the period of greatest stratification, especially with the formation of a distinct
thermocline, there was comparatively little variation in temperature over a horizontal plane
parallel to the water surface. Strong stratification inhibits vertical motions and reduces
turbulence to intermittent bursts. Thus, for strongly stratified lakes, such as Lake Maracaibo, the
transverse and longitudinal directions play a secondary role, and only variations in the vertical
enter the first order mass, momentum, and energy balances (Imberger & Patterson 1981).
DYRESM is based on the assumption of one-dimensionality, that is, variations in the lateral
directions are small when compared with variations in the vertical. The assumption is based on
observations that the density stratification usually found in lakes inhibits vertical motions, while
horizontal variations in density are quickly relaxed by horizontal advection and convection.
Horizontal exchanges generated by weak temperature gradients are communicated over several
kilometres on time scales of less than a day, suggesting that a one-dimensional model such as
DYRESM is applicable for simulations over daily time scales.
It is this assumption that gives rise to the model’s layer construction, in which the lake or
reservoir is represented as a series of horizontal layers. There is no lateral or longitudinal
variation in the layers, and vertical profiles are obtained from the property values of each layer.
The layers represented in DYRESM are of differing thickness; as inflows and outflows enter or
leave the lake, the affected layers expand or contract, and those above move up or down to
accommodate the volume change. The vertical movement of the layers is accompanied by a
corresponding change in layer thickness as the area occupied by each layer varies according to
its vertical position. Mixing is modelled by an amalgamation of adjacent layers, with individual
layer thickness dynamically set by the model to ensure an adequate resolution is obtained for
each process
A criterion for assessing the validity of the one-dimensional assumption, and thus the
applicability of using DYRESM to model a certain lake or reservoir, was first suggested by
Water Resources Engineers, Inc. (1969), who proposed the use of the densimetric Froude
number.
42
Model Selection
The use of the densimetric Froude number as a guide to the applicability of the one dimensional
approximation was superseded by Patterson et al. (1984) who developed a set of criteria based on
the Wedderburn number, the inflow and outflow Froude number and the Rossby radius. The
Wedderburn number represents the ratio of the buoyancy force acting on the surface layer to the
shear stress due to the wind, and is defined as:
(Equation 9.3.1)
g'h2
W = 2
u* L
Where L is the fetch length, h is the thickness of the surface mixed layer, u* is the surface wind
shear velocity, and g’ is the modified gravitational acceleration across the base of the surface
mixed layer defined as
g'= g
(Equation 9.3.2)
∆ρ
ρ0
where ∆ρ is the density difference between the bottom of the surface layer and the hypolimnion,
and ρ 0 is the density of the hypolimnion. When the Wedderburn number is W >> 1 the buoyancy
force is greater than applied wind stress, resulting in only a small tilting of the isopycnals and
negligible horizontal variations. For this case the processes are essentially one-dimensional and
the algorithms of DYRESM are valid. When W ~ 1, the forces due to the applied wind stress are
similar to buoyancy forces. This is the critical condition and can be described physically as the
point where the base of the surface layer tilts toward the surface at the upwind end, defined as
upwelling. W << 1 results in broad upwelling at the upwind end of the lake with vertical motions
occurring on smaller time scales than horizontal advection. For the two latter cases the
assumption of one-dimensionality is not valid.
The Wedderburn number appears to impose the restriction that DYRESM only be applicable to
“small to medium size” lakes and reservoirs. As, if L >> h then W << 1 unless the applied wind
stress is always low. When the wind stress is large, small lakes may also violate W >> 1.
However, when this occurs the resulting mixing is large and vertical density gradients are
smoothed. Although the upwelling processes are not explicitly modelled by DYRESM the mixed
layer algorithm will predict large vertical deepening for this case, resulting in DYRESM
43
Model Selection
accurately predicting the same resulting density profile. Figure 9.3.1 illustrates schematically that
the same wind stress applied to a large and small lake will result in a large amount of mixing due
to upwelling in the small lake, while only causing small mixing due to upwelling in the large
lake.
Figure 9.3.1: Upwelling occurring in a small and large lake with the same applied wind stress.
For large reservoirs the angle of deflection of the thermocline necessary to cause upwelling is
small compared with that for small reservoirs. The flow field resulting from upwelling is highly
complicated, with horizontal motions becoming highly important. DYRESM seems to be
unsuitable in this situation, as the processes necessary to describe this flow field are not
considered in the model. However, for lakes and reservoirs of “small to medium size” the end
result of upwelling is to thoroughly mix the water body, and although DYRESM does not
explicitly model the upwelling process, its mixed layer algorithm predicts a very similar mixing
efficiency. Thus, DYRESM models reservoirs and lakes of “small to medium size” accurately,
even if the Wedderburn number criteria is violated. This situation is complicated in most large
lakes and reservoirs by the fact that upwelling may not result in complete mixing.
The inflow Froude number may be written as
Finf low =
(Equation 9.3.3)
U
g' H
Where U is the inflow velocity, g’ defined similarly to (Equation 9.3.2) with ∆ρ now being the
difference between the inflow water and the reservoir water and H is the reservoir depth. The
one-dimensional assumption is valid when Finflow < 1. Physically this means that the buoyancy
44
Model Selection
force will dampen any non-vertical motions in the density structure resulting from inflow
disturbances. Similarly for the outflow Froude number defined as
Foutflow
(Equation 9.3.4)
Q
= 1/ 2 5 / 2
g' H
where Q is the outflow discharge and g’ is the reduced gravity between the surface and the
bottom water. Again, the one-dimensional assumption is valid if Foutflow < 1. The Rossby radius is
defined as
(Equation 9.3.5)
R=
g' H
f
Where g’ is the effective reduced gravity:
g'=
(Equation 9.3.6)
∆ρ
g
ρ0
H = mean Lake depth = 25.9m (Battelle 1974)
f = Coriolis parameter = 2Ωsin(latitude) ; latitude = 10°
Ω = Earth’s angular velocity = 7.292 × 10-5 rads-1
For one dimensionality R > 1
More recently, the validity of the one-dimensional assumption has been tested against the Lake
number (LN) (Imberger & Patterson 1990). The extent of hypolimnetic mixing in lakes and
reservoirs is strongly dependent upon the balance between the forces supplying energy to mix the
system and the forcing acting to inhibit mixing. Forces active in mixing primarily include wind,
cooling, inflow, outflow, and artificial destratification devices; whereas, density stratification is
the primary force acting to inhibit mixing. If the forces acting to mix the water column are
insufficient, as is often the case during stratification, bottom waters becomes isolated from
surface mixing. In many systems, this temporal isolation is associated with hypolimnetic oxygen
depletion, and deterioration in water quality (Robertson and Imberger 1994)
45
Model Selection
Several parameters have been used to try to describe changes in the strength or intensity of
stratification, i.e., the dynamic stability of the system. The dimensionless Lake Number, LN,
formalised and discussed by Imberger and Patterson (1990) is one parameter that represents the
dynamic stability and extent of deep mixing within a lake or reservoir. LN is a quantitative index
of the dynamic stability of the water column; it is defined as the ratio of the moments about the
water body’s centre of volume, of the stabilising force of gravity (resulting from the density
stratification) to the destabilising forces from wind, cooling, inflow, outflow, and artificial
destratification devices (Robertson and Imberger 1994). If wind is assumed to be the dominate
mixing force, i.e., inflow, outflow, and any artificial destratification devices have minimal
destabilising force, LN can be defined by Equation 9.3.7 (similar to Imberger and Patterson 1990;
Robertson et al. 1990).
(Equation 9.3.7)
S ( H − h2 )
LN = 2 t3 / 2
u* A ( H − hv )
where H is the maximum depth of the water body, h2 is the thermocline depth from the bottom of
the lake, hv is height to the centre of volume of the lake, As is the lake’s surface area, St the
Schmidt stability, and u* is the water friction velocity [cm s-1] due to wind stress approximated
by the bulk aerodynamic formula in Equation 9.3.8.
u* =
2
Where: U10
CD
(Equation 9.3.8)
Qa
2
↔C D ↔U 10
Qm
= Wind velocity at 10m above the water surface [cm s-1]
= Drag Coefficient = 1.3× 10-3 [dimensionless]
Qa/Qm = ratio between air and water densities = 1.2 × 10-3 [dimensionless]
For LN » 1, stratification is strong and dominates the forces introduced by surface wind energy.
Under these circumstances, the isopycnals are expected to be primarily horizontal. Little seiching
of the seasonal thermocline and little turbulent mixing in the hypolimnion are expected. Above a
value of 1.0 increases in LN represent very little difference in terms of mixing below the seasonal
thermocline. For LN « 1, stratification is weak with respect to wind stress. Under these
46
Model Selection
circumstances, the seasonal thermocline is expected to experience strong seiching and the
hypolimnion is expected to experience extensive turbulent mixing due to internal shear
(Imberger 2001).
Table 9.3.1 summarises the non-dimensional parameters discussed above in order to validate the
assumption of one-dimensionality and describe the dynamics of Lake Maracaibo.
Table 9.3.1: Non-Dimensional Numbers for Lake Maracaibo (adapted from Laval et al. 2000a)
Non-Dimensional Number
Value [O (of the order of)]
Interpretation
Wedderburn number (W)
O (10)
Upwelling of the metalimnion
induced by wind stress is
unlikely
Lake Number (N)
O (10)
Upwelling of the hypolimnion
induced by wind stress is
unlikely
Burger Number (S)
O (1)
Suggests that rotation and
stratification are of equal
importance
Based on the non-dimensional numbers presented above, it is appears valid to apply the
assumption of one-dimensionality when modelling Lake Maracaibo.
47
Applying DYRESM to Lake Maracaibo
10 APPLYING DYRESM-CAEDYM TO LAKE MARACAIBO
The one-dimensional coupled hydrodynamic-ecological model DYRESM-CAEDYM is a
powerful tool, which can be used to investigate the interactions between physics, chemistry, and
biology in aquatic ecosystems. The model uses a number of text files as input. A substantial
quantity of data must be collected and compiled in order to prepare the text files (see Figure
10.1).
For the majority of lakes, the vast amount of data required is not available in the form required.
Various assumptions are often required in order to determine the physical and morphometric
characteristics, and create continuous time-series of inflow quantity and quality, and
meteorological data.
The sources of the input data required to model Lake Maracaibo using DYRESM-CAEDYM,
and the assumptions made in manipulating such data, are presented in the following sub-sections.
Figure 10.1: Inputs Required for DYRESM-CAEDYM
48
Applying DYRESM to Lake Maracaibo
10.1 DYRESM SETUP
All data used in preparing the DYRESM-CAEDYM files was obtained from papers, records, and
other documentation relating to “The Integrated Study for the Environmental Remediation of
Lake Maracaibo” or staff from within the Centre for Water Research who were involved in the
aforementioned project. The initial set-up required by DYRESM-CAEDYM includes the
following information: (a) physical and morphometric data of the lake basin, inflows and
outflows, (c) water quality groups/processes and corresponding rate coefficients, and (c) initial
profiles of temperature, salt and water quality variables.
10.1.1 PHYSICAL AND MORPHOMETRIC DATA
The values of the physical characteristics of Lake Maracaibo used in DYRESM-CAEDYM
include lake length and width at maximum water level, hypsographic data consisting of height,
area and cumulative volume (Table 10.1.1), and geometric characteristics of major rivers. The
hypsographic data were based on DMAHTC Sea Chart No 24481, Lago De Maracaibo.
Table 10.1.1Surface Area and Volume of Lake Maracaibo as a Function of Depth
Height from the
bottom (m)
0
1
2
3
4
5
10
15
20
25
30
Area (x1000 m2)
Volume (x1000 m3) % of Total Volume
725,760
1,614,240
3,541,000
4,402,000
5,076,000
5,736,000
6,926,000
8,585,000
10,230,000
11,220,000
11,956,320
0
945,400
2,559,600
6,531,100
11,270,100
16,676,100
48,331,100
87,108,600
134,146,100
187,771,100
245,711,900
0.0%
0.4%
1.0%
2.7%
4.6%
6.8%
19.7%
35.5%
54.6%
76.4%
100.0%
Although there are thirty catchments draining into Lake Maracaibo, 81 percent of the total runoff is accounted for by just four major rivers, with the largest of these, Rio Catatumbo,
contributing approximately 60 percent of the total inflow to the Lake. Since these rivers all flow
into the south-western part of Lake Maracaibo, DYRESM-CAEDYM modelled the river inflow
as a single source. The dynamics of the river inflow, including the entrainment of Lake water by
49
Applying DYRESM to Lake Maracaibo
the inflow, is governed by the longitudinal slope of the streambed and by its cross-sectional
shape. DYRESM assumes that the river valley is approximately triangular with some average
streambed half-angle (defined as the angle of the side slope of the stream to the vertical). Slopes
and half angles for this single inflow stream were based on bathymetry data for the Rio
Catatumbo and are shown below in Table 10.1.2. Details of the inflow time series are given later
in Section 10.2.2.
Table 10.1.2: Stream inflow parameters
Inflow Parameter
Streambed Slope (degrees)
Streambed half angle(degrees)
Streambed Drag Coefficient
River Inflow
0.001
85
0.016
Saline Underflow
0.005
85
0.016
The saline underflow from Maracaibo Strait (as discussed in section 8.1) was included in
DYRESM-CAEDYM as a separate stream inflow and the streambed slope and half angle were
based on the bathymetric data for the southern end of the Strait.
The outflow through the Strait was not specified for modelling Lake Maracaibo, but rather a
maximum water elevation specified and the outflow calculated by DYRESM-CAEDYM. The
outflow calculated by DYRESM was compared with MIKE-3 outflow and demonstrated good
agreement on a seasonal time-scale. In the absence of detailed water levels over the Lake it was
assumed that variations in Lake water level could be neglected for the purposes of assessing
long-term effects on water quality.
10.1.2 INITIAL PROFILE
Initial profiles of salinity, temperature, dissolved oxygen, and nutrients were derived by linear
interpolation of field profiles collected by ICLAM at station C11 on September 22nd 1998.
10.2 INPUT DATA
Input data required by DYRESM-CAEDYM for the duration of each simulation includes:
meteorological forcing, and inflow volumes and properties (e.g. temperature, salinity and water
quality.
50