WATER RESOURCES BULLETIN VOL. 9, NO. 6 AMERICAN WATER RESOURCES ASSOCIATION DECEMBER 1973

THE EFFECTS OF WATER RESOURCES DEVELOPMENT ON ESTUARINE ENVIRONMENTS'

Jack C Nelson, Donald G. Rauschuber, and Lial F. Tischler2

ABSTRACT. The Texas Water Development Board, the principal water resource planning agency of the State, has been conducting extensive estuarine data collection activities and associated research to determine the required quantity and quality of fresh water inflows necessary to maintain various environmental conditions in Texas estuaries to preserve the estuarine ecosystems. These activities are a consequence of a statutory directive to the Board to make provisions in its State Water Plan for the effects of upstream water resource development on the associated estuaries.

This paper reports on the results of the first phase of an extensive estuarine research project. The objectives of the research project are to (1) define the interrelationships between estuarine ecosystems and fresh water and nutrient inflows, and (2) develop and test quantitative simulation techniques which describe these relationships. In order to accomplish the first objective, physical and chemical water quality data and biological data on the estuarine ecosystems are being collected, compiled and analyzed. The second objective is being satisfied by the development of hydrodynamic and ecologic simulation models of the estuarine environment. (KEY TERMS: limnology; oceanography; estuarine systems; planning; water resources development; water quality; data collection; hydrology; simulation; optimization; management)

The physical characteristics of Texas' surface water resource systems present a tremendous challenge to water resource planning because of the extremely complex interrelationship between rivers and their associated estuaries. Texas has approximately 400 miles of coastline (373 miles of open-ocean or Gulf shoreline and 1,419 miles of bay shoreline, for a total of slightly over 1,900 miles of coastal shoreline).

The Texas coastal zone contains seven major estuarine systems and several smaller estuaries. Eleven major river basins, ten with headwaters originating within the boundaries of the State, have estuaries of major or secondary importance. These estuarine systems, with a total surface area of more than 1.3 million acres, include many large shallow bays behind barrier islands. Additional thousands of acres of adjacent marsh and bayous provide habitat for juvenile forms of important marine species and also produce nutrients for the estuaries and bay areas.

'Paper No, 73105 of the Water Resources Bulletin. Discussions are open until July 1, 1974. 2Respectively, Chief and Hydrologist, both of the Coastal Zone Studies Branch, Operations

Division; and Director, Systems Engineering Division, Texas Water Development Board, Austin, Texas 78711.

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The ecosystems which have developed within these estuarine systems are in large part dependent upon the amount and seasonal and spatial distribution of inflows of fresh water and associated nutrients from the rivers, although coastal tributary streams and marsh areas within the adjacent coastal basins also play a key role in the ecological structure of the estuaries.

The productivity of Texas estuaries is illustrated in the 1969 commercial catch data. In 1969, fishing fleets of the states bordering the Gulf of Mexico contributed 38 percent of the total United States fisheries catch by volume, representing 29 percent of the total value of domestic fisheries production. Texas fisheries contributed 10 percent by volume and 31 percent by value of the Gulf fisheries production (King, 1971).

According to King, a significant portion of this production is based on estuarine -dependent species - principally the brown shrimp (Penaeus aztecus), spotted seatrout (Cjwoscion nebulosus), blue crab (Callinectes sapidus), red drum (Scianop ocellata), black drum (Pogonias cromis), southern flounder (Paralichthys lethostigma), and sheepshead (Archosargus probatocephalus). Texas’ average annual production of these species for the period 1962-1969 was 81,131,000 pounds valued at $35,362,000,

A Texas estuary may be defined as the region from the tidally effected reaches of terrestial inflow sources to the Gulf of Mexico. Shallow bays, tidal marshes and bodies of water behind barrier beaches are included under this definition. These estuarine systems are made up of subsystems, lesser but recognizable units with characteristic chemical, physical, and biological regimes. Estuaries are composed of component parts, primary, secondary and tertiary bays, which require separate treatment for understanding and management.

In terms of energy input to the estuarine systems the most productive and most dynamic of the estuarine habitats are teritary bays. The tertiary bays are generally shallow areas where sunlight can effectively penetrate the water and support submerged vegetation. Substantial chemical energy is produced in these areas due to photosynthetic processes. These biostimulants are distributed through the estuarine system by tide and wave action.

A characteristic of Texas estuarine systems is that many of the organisms are not permanent residents, but spend only part of their life cycle in the estuary. Migrations form an integral part of the life history of many marine organisms. These migrations occur in annual or seasonal cycles, many being for the purpose of reproduction. Larval and post larval organisms immigrate into the estuary, due to physiological requirements or the need for protection from competing species, predators, and parasites which may eliminate them in the open Gulf waters. The juveniles use the estuarine nursery areas during early development, and then emigrate back to the Gulf as adults.

Estuaries, due to their dynamic nature, are in a continual state of environmental stress. This dynamic nature, however, is what makes Texas estuaries highly productive ecosystems. Severe droughts, floods, and hurricanes are the main limiting parameters that control and influence estuarine ecosystems. The number of species remain low, while numbers of organisms within a species fluctuates with the seasonal regime, and with drought and wet annual cycles. This type of regime provides for a continuing shift in dominant organisms, therefore, preventing a specific species from maintaining dominance, as compared to a lake, where through the process of eutrophication its biological population become stagnant and dominated by undesirable populations.

These ecosystems are biologically productive due to the seasonal and long term fluctuations in fresh water inflows. The inundation of the tidal lakes and their associated

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marsh areas replenish these habitats periodically with nutrients and organic materials. These biostiniulants along with the freshening by the rivers are the major controlling factors in these estuarine systems. Defining the optimal freshwater inflows and corresponding exchanges with the Gulf of Mexico is a complex hydrological problem.

In carrying out its statutory directives, and in recognition of the fact that continued development of the water resources of the State is both necessary and inevitable to meet increasing demands for fresh water by an expanding population and to support’state and National economic growth, the Board completed the Texas Water Plan in November 1968. The Plan, as formulated, is the official guide for orderly development and management of Texas’ water resources to the year 2020. It is the first large-scale comprehensive water plan to recognize the need for allocation of a firm supply of fresh water for a vast estuarine system, consonant with progressive development and allocation of inland water supplies for other necessary and beneficial uses. Although the Plan tentatively provides a specific amount of water for supplemental estuarine inflow on an annual basis, it was, and is, clearly recognized that the amount so specified is no more than a preliminary estimate. Furthermore, the optimum seasonal and spatial distribution of these supplementary inflows could not be determined in the planning studies because of insufficient knowledge of the estuarine ecosystems. The acute need for a reliable set of criteria and techniques for defining the response of estuarine ecosystems to varying amounts and regimes of fresh water and nutrient inflows are clearly obvious.

Studies by the Board of past and present fresh water inflows to Texas’ estuaries have used all available sources of information on the physical, chemical, and biological characteristics of these estuarine systems in an effort to define the relationship between fresh water and nutrient inflows and estuarine environment. The Board realized during its planning activities that, with the exception of data of the Texas Water Quality Board’s Galveston Bay Study, very little reliable data were available on the estuaries of Texas. Several limited programs were underway; however, these were largely independent of one another. The data collected under any one program were not comprehensive, and since sampling and measurement of physical parameters under different programs were not accomplished simultaneously, the resulting data could not be reliably corrected. In some estuaries, virtually no data had been collected.

A program was, therefore, initiated by the Board to collect the data considered essential for analyses of the physical and water quality characteristics and ecosystems of Texas’ bays and estuaries. To begin this program, the Board consulted with the U. S. Geological Survey and initiated a reconnaissance-level investigation program in September 1967. Specifically, the initial objectives of the program were to define: ( 1 ) the occurrence, source and distribution of nutrients; (2) current patterns, directions, and rates of water movement; ( 3 ) physical, organic, and inorganic water quality character- istics; and, (4) the occurrence, quantity, and dispersion patterns of all water (fresh and Gulf) entering the estuarine system. To avoid duplication of work and to promote coordination, discussions were held with all other State, Federal, and local agencies having interests in Texas estuarines and their management.

The initial program approach to establishing a data network to maintain a continuing appraisal of the estuarine systems consisted of three phases: ( 1 ) reconnaissance surveys to locate an optimum data-collection network; (2) repetitive surveys throughout this network to determine the general physical and water quality characteristics of the estuarine systems; and (3) data collection at a reduced number of representative sites to maintain definition of these characteristics. At each data-collection site, field measure-

1252 Nelson, Rauschuber and Tischler

ments are made at various depths. The number of measurements ultimately made and subsequently reported depends upon the depth of water, degree of stratification if present (thermal, chemical, dissolved oxygen, etc.), or other physical or geomorphological characteristics.

Field data consist of defining such parameters as specific conductance, temperature, dissolved oxygen, and pH. On the basis of these field data, a number of representative samples are routinely collected for laboratory analysis. Laboratory analyses include calcium, magnesium, sodium, bicarbonate, sulfate, chloride, silica, biochemical oxygen demand (BOD), chemical oxygen demand (COD), total organic carbon (TOC), nitrate, nitrite, ammonia nitrogen, and phosphate (ortho and total). Other properties or constituents periodically determined on selected samples include suspended sediment, turbidity, and trace constituents such as aluminum, lithium, strontium, manganese, iron, zinc, copper, lead, bromide, iodide, arsenic, selenium, mercury and cadmium. The program was expanded in scope in 1970 to include periodic determination of pesticides in bottom sediments, color, turbidity, organic nitrogen, and phenolic compounds at selected sites among those already established.

Texas Parks and Wildlife Department studies in the coastal zone monitor selected sport and commercial species of fish and shellfish, along with associated hydrographic and chemical parameters. Information was lacking regarding the responses of primary and secondary producers in the marine food chain to fresh water inflow. These data are necessary to elucidate the mechanisms responsible for fluctuations in the abundance of the predator species most important to the commercial and sport fisheries of the area under study. In order to properly define the complex ecological interactions so that meaningful correlations could be made between inflows and the productivity of the ecosystems, the scope of the biological sampling program was expanded. This was accomplished by the Board in 197 1 through interagency contracts for investigations to determine standing crops of phytoplankton, zooplankton, and benthic organisms as such relate to variations in fresh water inflow and physical and water quality conditions of the estuarine systems.

The large amounts of data collected for the bay and estuary program are being analyzed to elucidate relationships between the diverse biological, chemical and physical properties of each estuary. These relationships must be known if a simulation approach is to be used to estimate the fresh water and nutrient requirements of estuarine ecosystems. Several types of techniques are being used in this data analysis effort. These include:

Species diversity index Primary productivity analyses Regression and correlation techniques including principal component and cluster

Sensitivity analysis analysis

Concurrent with the cooperative U. S. Geological Survey and Texas Parks and Wildlife Department data collection programs, the Board initiated in June of 1970 development and verification of mathematical models of the bay and estuarine systems. These models, which are being developed with contractual assistance, also aid in guiding the data collection programs previously described. Two types of models are developed for each of the bay systems. One is a tidal hydrodynamic model and its complement is a mass transport model of the corresponding estuarine-bay system. Both types of models are

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designed to be used on a high-speed digital computer. The mathematical hydrodynamic model is designed to determine the effects of

alternative estuarial development and management plans on the circulation and tidal patterns of shallow, irregular, nonstratified estuaries such as are found along the Texas coast. This model computes the temporal and spatial distribution of the velocity and water surface elevation, at each nodal point of an imaginary computational grid that is superimposed on an estuary. The estuarial properties along any given vertical line are assumed to be homogeneous. The model takes into account bottom friction, submerged reefs, ovefflow over low-lying barrier islands, fresh water inflow, waste inflows, ocean tides, wind, rainfall, and evaporation. The model can be used to study intra-tidal behavior, changes in scour and sedimentation patterns produced by construction, filling or dredging, and to evaluate the dispersion characteristics of waste outfalls. The output from the hydrodynamic model is a time history of water surface elevation and velocity patterns throughout the estuary-bay system. These data may be saved on magnetic tape for later use in describing the advection terms of the estuary mass transport model.

The estuary mass transport model can be used to analyze the distribution of salinity, or any other conservative material, in shallow, nonstratified, irregular estuaries under various tidal and fresh water inflow conditions. The model is dynamic and takes into account location, magnitude and quality of fresh water inflows, location and magnitude of withdrawals, evaporation and rainfall, and advective transport dispersion within the estuary. Primary outputs from this model are the tidal-averaged concentrations of the constituent of interest at each nodal point of the same computational grid used by the hydrodynamic model. In addition to analyzing salinity changes in the estuaries due to increased withdrawals and/or decreased fresh water inflows, the model can be used to evaluate the effects on the salinity distributions of intra-estuarial projects, such as dredging and filling operations, that alter the circulation pattern within the estuary. Although the model as presently structured considers only conservative constitutents such as salinity, the capability to handle nonconservative constituents can easily be obtained by attaching the appropriate nonconservative mechanisms to the appropriate mass transport equations. The hydrodynamic and transport models of the San Antonio and Matagorda Bay Systems are now completed and calibrated.

To predict and evaluate the effects of water resources development and management on the aquatic ecosystems of an estuary, it is necessary to consider all of the significant interactions between the dominant organisms in each of the trophic levels and simultaneously, their interactions with the aquatic environment. Figure 1 indicates the significant relationships in a typical aquatic ecosystem, all of which must be simulated if prediction of the effects of various stresses on the environment is desired. The environmental variables which control the activities of these aquatic organisms include meteorological conditions and water quality. The water quality variables include the nutrients, nitrogen, and phosphorus; the basic cellular building block, carbon; any toxic compounds which might be present; trace elements necessary for biological growth; and dissolved oxygen for respiratory organisms. The hydrodynamic and transport models of streams, reservoirs, and estuaries can provide some of this information. However, a Considerably more elaborate modeling techndique must be used to evaluate the behavior of living organisms and their effect upon each other and the environment.

The concept of modeling aquatic ecological phenomena has been pursued by various investigators. Until recently, these studies have generally been concerned with a single, or at most two, trophic levels. Recently, Water Resources Engineers, Inc., with funding from

1254 Nelson, Rauschuber and Tischler

M A N - I N D U C E D H Y D R O - N A T U R A L W A S T E L O A D S D Y N A M I C S I N P U T S

B A C T E R I A P HY T O P L A N K T O N

W A T E R n Q U A L I T Y 0 0 U

D E T R I T U S Z O O P L A N K T O N

Figure 1 . Definition of an Ecosystem (After Water Resources Engineers, Inc.)

the Office of Water Resources Research, developed an ecological model which described all significant trophic levels in a typical aquatic ecosystem (Chen, et al. 1972). The current TWDB research project, discussed below, will refine this basic model to describe the organisms present in an example set of ecosystems, adapt it to existing hydrodynamic and transport models and use the entire modeling technique in a water resource and environmental management planning strategy.

To provide the necessary information on the tributary inflows to the estuaries and quantity and quality of these inflows will be simulated using river basin and reservoir water quality and quantity simulation models. These are similar in concept to the estuarine models and have been reported on elsewhere (TWDB, 1971 ; Masch, 1971; Chen et a1 1972).

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R I V E R / C A N A L WATER Q U A L I T Y

S I M U L A T I O N MODEL

The Board has recently received funding from the Office of Water Resources Research, U. S. Department of the Interior, in order to develop, with the data available, an aquatic ecological model of an entire river basin and estuary system which can be used to analyze the effect of water resources development on estuarine and river basin ecology. The type of model proposed is what is termed a dominant species or “key” species type model and would represent the effects that changes in environmental conditions would have on certain important organisms in the river and estuarine ecosystems. All of these modeling activities are being carefully coordinated wih the Coastal Management Program being conducted in the Office of the Governor and the coastal programs of other Texas state agencies.

The basic approach is to apply an integrated set of digital simulation models in an interactive manner as depicted in figure 2. Alternative water resources management policies can be analyzed on an individual basis under varying hydrologic conditions using the approach described below.

R E S E R V O I R H A T E R Q U A L I T Y

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T I M E S;REAM OF R l V E R / t l N A L F L O Y S AN0 R E S E R V O I R O P E R A T I O N S

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AND Q U A L l r V O F I N F L O W TO THE E f l U A R Y .

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Figure 2. Proposed Interactive Technique for Evaluating the Ecological Effects of Water Resources Development

One of the keys to satisfying the objectives of this research will be the development of a technique for integrating the stream, reservoir, and estuary (quantity/quality/ecological) models into an efficient, interactive modeling system. The computational requirements of the models which will be used make it prohibitive to attempt to devise a single “model” to analyze the system, or even for that matter, a part of the total system such as an entire river basin. Rather, a modular system, composed of multiple computer programs operated

1256 Nelson, Rauschuber and Tischler

sequentially, will be used to analyze the ecological responses to alternative water resources management and development policies.

The sequence of simulation model usage is depicted graphically in figure 2. The interaction between the river and reservoir models in the simulation of the river basin is of particular importance. The prototype river basin consists of a network of alternating stretches of river reaches and reservoirs with potential interconnection between the elements by means of canals or pipelines. The simulation/optimization (quantity) models will operate on the entire network to reflect the flows in all rivers, canals, and pipelines and will provide the history of reservoir operations in the river basin system. Two possible modeling techniques may be used in the quantity/ecological analysis of the river basin in order to estimate the quality of the estuarine inflows. These are:

simulate the entire river basin system in sequence (e.g., stream-reservoir-stream, etc.) down to the estuary; or

aggregate the system into a composite reservoir, or reservoirs, and a composite river stretch, or stretches, representative of the actual system, to give the necessary estuarine inflow water quality data.

The latter technique is attractive from the standpoint of computational efficiency and ease of use. However, the feasibility of aggregating the entire system for modeling purposes is probably dependent upon the homogeneity of the environment in the river basin. The former technique, which would provide the most detailed analysis of the basin, would be time-consuming and complicated on a large basin with a large number of water resources projects. Both techniques will be developed and tested for their validity. The interactive model usage techniques will permit the use of either of the two models. It is quite probable that both techniques would be used for analyzing a river basin. The composited element technique could be used to analyze a large number of hydrologic and project conditions with a minimum amount of time while the more detailed method would be used to analyze specific critical conditions.

After the appropriate hydrologic and meteorologic conditions are selected, the interactive modeling techniques can be applied to the water resource system. The proposed development and operating policies will be analyzed using these selected conditions and a history of the estuarine ecology’s response to the system operation will be computed. This response can then be evaluated to determine whether or not the proposed water resources plan will have a deleterious or beneficial effect on the estuary.

This evaluation phase can be divided into three basic activities:

analyzing the effects of a proposed water resources system on ecosystems in terms of changes in the productivity and/or life cycles of “key” species;

evaluating the reliability of the simulation results;

changing the development and/or management policies and reanalyzing the system response.

The output from the ecological model will consist of information on the growth and death rates and relative abundance of selected key species of the estuarine biota. The

EFFECTS OF WATER RESOURCES DEVELOPMENT 1257

interpretation of these data will include comparison of simulated productivities of various species with their current known productivities using the above mentioned statistical techniques, as necessary. The model results should also aid in identifying potential imbalances in the ecosystem such as nutrient accumulation in the sediment, etc. A methodology will be devised to perform this post-modeling evaluation so that each analysis will be done in a consistent manner.

An important part of this evaluation procedure is evaluation of the reliability of the simulation results. The techniques of sensitivity analysis, as previously described, will be used in conjunction with statistical, correlation, and spectral analysis methods to estimate the reliability of the results of the interactive modeling technique. This reliability is a function of the characteristics of the input data as well as the veracity of the relationships used in the models. Inadequate or inaccurate hydrologic data could cause a considerably more significant error than an incorrect growth rate coefficient for a particular organism.

The complete evaluation of the significance of a large-scale water resources project must involve an estimation of the social and economic costs and benefits due to ecological changes. However, this type of analysis is outside the scope of this research project. It is the next logical step after developing the capability of aquatic ecological simulation of an entire river basin system.

The next step in the planning procedure, after the evaluation of the ecological effects of a particular water resources system has been made, is to modify the proposed water resource development and management policies to increase the benefits and reduce the detrimental effects. This type of modification might involve changing the seasonal operating rules for certain reservoirs, modifications to the physical structure of the estuary, importation of fresh water from another source, or many other alternatives. The modified plan would then be reanalyzed using the interactive modeling techniques to determine the changes it would have on the estuarine ecology. By comparing the simulation results of alternative plans, the “best” plan could be chosen. This best plan would combine meeting demands for water with minimum shortages at the minimum cost while maintaining an ecosystem at a level of productivity consistent with State and National goals.

ACKNOWLEDGMENTS

The authors wish to thank the many individuals at the Texas Water Development Board who assisted in the work described herein and the review of this paper. Mr. Seth Bumitt, Director, Operations Division, Mr. Allen White, Engineer, Systems Engineering Division, and Mr. Lewis Seward, Principal Engineer - Project Development are due special credit for their efforts in and direction of this research project. Some of the work on which this paper is based is supported in part by funds provided by the Office of Water Resources Research, U.S. Department of the Interior.

LITERATURE CITED

Chen, C. W. and Orlob, G. T., 1972. Ecological simulation for aquatic environments - final report,

King, B. D., 111, 1971. Study of migratory patterns of fish and shellfish through a natural pass, Texas - Masch, F. D., 1971. Simulation of water quality in streams and canals, theory and description of the

Texas Water Development Board (TWDB), 1968. The Texas water plan. Texas Water Development Board (TWDB), 1971. Stochastic optimization and simulation techniques

Water Resources Engineers, Inc., Walnut Creek, California.

Parks and Wildlife Department, Tech. Ser. 9.

QUAL-1 mathematical modeling systems, Texas Water Development Report 128.

for management of regional water resource systems, a completion report, Report 131.