Preamble
Letter from the Co-Chairs
4.0 INTERNATIONAL JOINT COMMISSION CHARGE TO THE LAKE ERIE TASK FORCE
4.1 WHAT IS HAPPENING TO LAKE ERIE?
Prototype Components
Use of Existing Models in Prototype Development
Use of the Lake Erie Prototype
Step 1: Use a "Capacity Building" Approach in Developing the Model4.5 SUMMARY
Step 2: Communicate the Model Capabilities
Step 3: Start an Iterative Correction Process
Step 4: Consult with Users
Step 5: Test the Three-Basin Concept
Figure
The current version of the Lake Erie model is a prototype ,intended to be as a framework for further model development, including the incorporation of additional issues of concern to Lake Erie and Great Lakes managers and researchers.
The model is not intended to be a management tool at this stage of its development. Instead, the model needs extensive testing, scoping and calibration. After these trials, the Task Force expects the model will have considerable utility in understanding and managing Lake Erie's ecosystem, and assisting in setting priorities for research. Ultimately, the model could advance management strategies for all of the Great Lakes.
The Task Force encourages reviewers to use the model to conduct tests, add data and explore hypotheses; however, the Task Force cautions using the model to develop management strategies at this stage of development.
The accompanying report describes the work conducted by the Lake Erie Task Force over the past two years. Our charge from the International Joint Commission (IJC) was to provide advice on the impact of the various stressors on the Lake Erie ecosystem. In traditional IJC fashion, we have worked with others from government and academia, including expert modellers and managers, who shared their time with us because of mutual concern about Lake Erie and its changing state.
The Task Force undertook its mission by seeking strong technical modelling expertise, researching to reduce the potential for duplication, and ensuring frequent interaction and consultation with key stakeholders. We believe that this combination provides the setting for a sustainable ecosystemic approach to modelling.
We convened discussion groups and workshops to benefit from expert advice from a diverse and stimulating group of managers and scientists. We harvested the ideas, intuition, knowledge and concerns of some of the best scientific and policy minds in the Great Lakes basin.
The Prototype Ecological Model was built on this advice. Numerous approaches to developing the model could have been used; after much consideration, we selected one. However, the selection is just a beginning upon which we can build a sustainable modelling process and framework, which should be valuable for years to come.
The following report describes the approach taken to develop the model, outlines the model's capabilities and scope, and recommends next steps for the International Joint Commission to consider. We look forward to continuing this work in the future.
Yours truly,
Dr. Jeffrey Reutter
U.S. Co-Chair
Lake Erie Task Force
Dr. Douglas Dodge
Canadian Co-Chair
Lake Erie Task Force
With the beginning of a new biennial cycle in October 1993, the Commissioners assigned a series of priorities and their sub-elements to various International Joint Commission (IJC) Boards and Committees. These included, "Ecosystem Framework" and "Wetlands," which were assigned to the Council of Great Lakes Research Managers, and "Pesticide Usage," "Groundwater," and "Pollution from Land Use Activities (PLUARG)," which were assigned to the Water Quality Board. These sub-elements are reported on elsewhere in the referenced sections.
The "Ecological Changes" sub-element, initially assigned to the Lake Erie Steering Committee, was subsequently expanded to include preparation of a prototype ecological model of Lake Erie. The committee was upgraded to Task Force status and available resources were redirected in order to achieve this goal.
In its Seventh Biennial Report in 1994, the International Joint Commission established the Lake Erie Steering Committee, later to become te Lake Erie Task Force. This Task Force was formed as a result of a priority identified by the Council of Great Lakes Research Managers. As facilitators and catalysts for Great Lakes research, the Council had identified the need for a comprehensive understanding of the changes in the Lake Erie ecosystem. The purpose of the Task Force was to provide advice to the Commission on the impact of various stressors affecting the health of the Lake Erie basin. In particular, the Task Force was to focus its efforts on the negative effects of stressors on the Lake Erie benthic and fish communities, and report to the IJC at its eighth Biennial Meeting on Great Lakes Water Quality.
After considerable discussions and reviewing initiatives and activities surrounding the state of Lake Erie, the Task Force determined the need to develop an ecosystem model for Lake Erie. This model would integrate current knowledge, determine linkages between stressors and ecosystem balance, and be capable of assessing and predicting potential adverse effects from nonindigenous species entering Lake Erie. Expert advice and guidance from scientists in the field were considered important before concluding on either the need for, or the purpose of, such a model.
The Task Force organized a workshop as a precursor to model development to determine the need, the scope and the boundaries such a model should take. Results from this workshop, held in June 1994, encouraged the Task Force to pursue the development of an ecosystem model for Lake Erie. Workshop participants confirmed the need for a Lake Erie model, and identified key elements of an approach to model development, including: a comprehensive inventory of existing models, focusing on their scope, linkages and data gaps; and development of a stress/response model for zebra mussels to test critical questions and linkages.
Participants recognized the benefits of the IJC taking a coordination/leadership role in a Lake Erie model development initiative, and stressed the need for involvement by those who will ultimately use the model, including Lake Erie managers in federal, state and provincial resource management and environmental control agencies, the Great Lakes Fishery Commission and others.
From a water quality perspective, the current condition of Lake Erie is a mixed success. Phosphorus loading targets are now being achieved. Reductions in posphorus loadings, coupled with the filtering effects of zebra and quagga mussels, have resulted in impressive increases in water clarity.
By 1993, phosphorus and chlorophyll levels in the eastern and central basins were approaching oligotrophic conditions. Secchi disk readings were averaging more than 4 metres (13 feet). The lake's clarity is a cause of celebration to many lakeside residents. As an overall success story, hwever, this visible progress may be deceptive.
Trends which previously demonstrated a decline in contaminant body burdens in fish and wildlifenow appear to be levelling off. Increasing levels of persistent, chlorinated hydrocarbons are being observed in some age groups of fish.
Ecosystem scientists report that populations of the invertebrate, Diporeia , have declined by 40 percent since 1965. Gammarus and oligochaete populations have increased dramatically while native unionid clams have almost disappeared. The changing trophic status suggests that the lake may not be able to support traditionally high fishery yields: stocks of white perch, yellow perch and smelt are rapidly declining.
From a fish productivity perspective a dramatic recovery of walleye and other fisheries was experienced throughout the 1980s. But this recovery also appears to be in decline.
Both fishery managers and fishers are expressing concern that this decline is due to further reductions in phosphorus loadings, together with the apparent impact of zebra and quagga mussels on energy flow and nutrient cycling.
Lake Erie is again front and centre in the minds of user groups and governments. In short, the celebration of success may be short-lived. As we move through the next decade, we must become better informed of the overall ecosystem dynamics, the interrelationships between stressors, and the net, overall effect those stressors have on the lake ecosystem. This understanding is crucial for the next set of management decisions about water quality and fisheries management in Lake Erie.
Great Lakes scientists and research and program managers are challenged by these changes in Lake Erie. There are more questions than answers. Not all changes in Lake Erie can be attributed to invasions of zebra and quagga mussels, nor to the significant reductions in loadings of phosphorous. Other factors may be causing the observed changes; for example, declining forage fish stocks may reflect the large number of predators feeding in the lake.
In 1994, most agencies had identified Lake Erie as a top priority. Several meetings and workshops were held to coordinate programs, but there was a growing concern that a large number of unconnected projects could not by themselves correctly diagnose the problems and lead to effective management actions.
Important gaps in knowledge were identified in the Lake Erie priority. These included:
Other pertinent questions are related to management decisions that must be made soon, to ensure better management of fisheries and water quality of the system. These include:
Building on the results of the June 1994 workshop, the Task Force began the Lake Erie Ecological Modelling Project (LEEMP) in January 1995. The purpose of the project was to encourage the development of an ecological model that would enhance understanding of rapid changes taking place in the Lake Erie ecosystem.
The Lake Erie Modelling Project had two main initiatives:
A binational consulting team was organized by the LURA Group, a Toronto-based environmental planning firm, Dr. Joseph Koonce, a Great Lakes modelling expert, and Dr. Ana Locci from Case Western Reserve University in Cleveland, Ohio. At the outset of the project, the consulting team established a Core Advisory Group of Lake Erie fishery and water quality managers. The Core Advisory Group provided critical advice and guidance regarding implementation of Initiatives I and II of the LEEMP.
Nine ecological modelling projects were reviewed to identify attributes applicable to the design and development of the LEEMP; these projects are briefly described below. For a detailed report, see the Task Force's " Initiative I Report: Review of Existing Ecological Models, March, 1995. " The nine models varied greatly in purpose, complexity, level of development, and the nature and amount of information available about them. They included:
These nine models were developed to study specific ecosystem functions. Some are based on the same framework or contain common subsystem models. The Puget Sound Systems Model was developed in 1975 as a tool for water quality decisionmaking. Designed to expand on the application of earlier models, it has subsequently been developed into the Water Quality Analysis Simulation for Toxics (WASP4) model. The WASP4 model framework provides the basis for several current models, including the GBMBS and the Eutrophication Model for the Louisiana Inner Shelf (Gulf of Mexico), numbers 2 and 5 above, respectively.
GBMBS relates loadings of nutrients, solids and contaminants with concentrations in the water column and sediments. The WASP4 framework is a linked submodel approach which combines two parts: exposure concentration (physical-chemical) and food chain components (U.S. EPA 1989). The goal is to be able to predict concentrations in water, sediment and biota in response to differing regulatory and remedial action scenarios.
The Eutrophication Model for the Louisiana Inner Shelf (Gulf of Mexico) uses a Limno-Tech, Inc. (LTI) modified version of the U.S. Environmental Protection Agency (U.S. EPA) WASP4/EUTRO4 model. The scope of this model is limited to considering primary production. The model was used to estimate responses of primary productivity and dissolved oxygen to reductions in nutrient loadings from the Mississippi and Atchafalaya Rivers.
The Lake Michigan Mass Balance Model, in the development stages, proposes to apply the same linked submodel approach used in Green Bay. The mass balance model for toxics in Lake Michigan will retain some of the same basic models as the GBMBS. The model will be comprised of linked hydrodynamics, eutrophication/sorbent dynamics, particle transport, contaminant transport and transformation, and bioaccumulation simulations.
The Saginaw Bay Zebra Mussels Model represents the lower food web, including zebra mussels, within a mass balance modelling framework. It is based on the original multi-class phytoplankton model used to establish the target phosphorus loading for Saginaw Bay as part of the 1978 Great Lakes Water Quality Agreement. The phytoplankton-zebra mussel model is a research tool and has not, as yet, been used by any management agencies.
The LEIFS Model is part of the Great Lakes Forecasting System (GLFS), which makes six-hour forecasts for currents, temperatures, wind waves, water levels and associated physical data. The model is easily accessed by modem or internet for enhancement of commercial and recreational activities, resource management and hazard avoidance.
The Chesapeake Bay Ecosystem Model addresses the flow of energy, nitrogen and phosphorus through the ecosystem. The model illustrates seasonal dynamics of the Chesapeake Bay ecosystem by providing information on the rates of energy transfer between the trophic components.
The SIMPLE Model, as applied in Lake Michigan and Lake Ontario, is a decisionmaking tool for fisheries management, which includes economic and social inputs.
The Network Model of Lake Ontario and Erie is similar to the prototype ecological model being developed by the Lake Erie Task Force. This network model is in early stages of development; one primary function is to forecast potential fish production for fisheries managers. This network approach to modelling should create linkages between a variety of subsystem models, one of which is the SIMPLE model. The network model is proposed to be developed for Lake Ontario first, and later adapted to Lake Erie. Due to its early state of development, details of the model's functions are yet to be determined.
All the models currently in use are in the public domain, but most require a degree of training for their use. The Chesapeake Bay Ecosystem Model is fully documented, including data requirements, a bibliography, operating instructions and guides to interpret data. The LEIFS model provides for public access to updated and historical files by modem or internet on the Great Lakes Forecasting System database at Ohio State. Two-dimensional maps can be displayed on the computer. The GBMBS represents the other extreme; there has been little documentation, and operation of the model requires extensive training or modelling experience.
While work is ongoing on many of these models, two models have potential to contribute to the present focus on Lake Erie: the Network Model of Lakes Ontario and Erie, and the Lake Erie Trophic Transfer Model. The Network Model, originally only for Lake Ontario, is designed to aid fisheries managers to predict fish productivity. The contractor was unable to use any attributes from the Lake Erie Trophic Transfer model given the timeframe for this project.
From this review, the SIMPLE model provided the most comprehensive approach to developing the sustainable fishery component of the ecosystem model. In addition, this model has sufficient structural scope to accommodate issues associated with nutrients, contaminants and exotic species. This model subsequently formed the core of the LEEMP.
Prototype Components
The prototype has six main integrated and interacting components:
The prototype is a simplified representation of the food web of the Lake Erie ecosystem. The model simulates energy flow and contaminant movement by implementing a set of rules, which approximate the feeding behaviour of individual animals (Figure 1). Primary production depends on phosphorus loading in the model, and nutrient loading thus limits the overall productivity of the simulated Lake Erie ecosystem.
The key assumptions made in developing the prototype are included in Appendix A.
Use of Existing Model in Prototype Development
The Lake Erie prototype incorporates elements of the following existing models:
![[Figure 1]](../../gifs/pr9395l1.gif)
Figure 1.
Use of the Lake Erie Prototype
In developing its model, the Task Force had the benefit of previous work by Dr. Koonce and Dr. Locci at Case Western Reserve University. The prototype is not intended to be a management tool at this stage of development. Rather it is a heuristic (see footnote) demonstration model which requires extensive calibration and testing. Although advice from the Core Advisory Group advanced prototype development to the point where it is nearly a useable tool, substantial work remains to make it a realistic representation of the Lake Erie ecosystem.
However, it is possible to show how the prototype might be used in discussions about some important ecosystem interactions in Lake Erie. The prototype has been designed to allow exploration of hypotheses or management actions in comparison to historical trends. The prototype begins a simulation in 1960 and runs through 1995. Two sample applications illustrate ways the model can be used:
Through similarly derived scenarios, the user can explore the interactions of the various model components:
Exploring these interactions is a heuristic exercise. Using the model to track complex interactions allows users the opportunity to try alternative hypotheses, or to check for consistency with otherobservations or models. This gaming with simulation may be the main application of a model of the Lake Erie ecosystem, and the prototype provides an example of this utility.
The prototype was tested in April 1995 when the Task Force hosted an interactive demonstration workshop for managers, scientists and researchers. Recommendations were made for a series of next steps in model development, which are included in this report.
From this project to develop a Lake Erie ecosystem model, and the subsequent review of the prototype by Lake Erie managers, scientists and researchers, the Task Force proposes a number of key areas for consideration by the IJC. First, the model is only a prototype and needs further work by testing, trials, demonstrations and experiments. However, a solid groundwork has been established for a long-term sustainable process to model the Lake Erie ecosystem.
At this early stage in its development, the model focuses on sustaining fish productivity as an ecosystem component and, as such, is not truly representative of the Lake Erie ecosystem. Differing approaches are being considered to ensure that the model becomes as reflective of the lake ecosystem as possible. When the Lake Erie Lakewide Management Plan Ecosystem objectives have been confirmed, these can be reviewed in the context of the model.
The model is designed to provide information based on a "whole lake" approach. There is considerable interest in disaggregating the model into three basin components, including nearshore and offshore effects. The benefits of this approach need to be evaluated.
Users may benefit from the model if it concentrates on the lower trophic levels, in sub-basin units. For example, a better link is needed between phosphorous loadings and phytoplankton and zooplankton production. These subcomponents, when developed, could connect to the lakewide model. This exercise should not proceed simultaneously with the main model development. It is important that the iterative correction process occur with the main model to determine errors prior to embarking on additional major sub-basin modelling exercises.
A number of hypothetical tests need to be conducted on the model to ensure its representativeness. Reviewers and participants in the Task Force process indicated a strong desire to run trials with the model to test certain hypotheses. The Task Force strongly supports this iterative correction process.
The Lake Erie Ecosystem Modelling Project could provide valuable insight into the challenges facing managers of the Lake Erie ecosystem. For the model to be useful, users must have confidence that it is kept up to date, and corrected as new information develops. As well, the model provides the basis for sharing information, knowledge and for developing common policy. Finally, the model provides another way for the IJC to assess progress of the Parties toward meeting the commitments of the Great Lake Water Quality Agreement.
Managers and modellers indicated a strong interest to continue involvement in model development. It is important that common goals are established among users of this model. So far, advice and input of more than 50 key advisors assisted the Task Force in developing this prototype.
The Lake Erie Task Force concludes that the model has the potential to be a valuable tool for use by Lake Erie managers, scientists and researchers, and has the potential to be adapted for use on other Great Lakes.
This modelling exercise reflects the considerable leadership of the Commission in moving towards an understanding of changes to the Lake Erie ecosystem. By starting the development of an ecosystem model, changes can be better understood and even predicted, and thus, managers can move forward with a higher degree of confidence.
Through the establishment of a special Task Force on Lake Erie, the IJC has indicated that the Lake Erie priority is of significant importance. By initiating the development of this prototype, the IJC is facilitating progress. We suggest that the IJC continue in this leadership role.
The Task Force recommends that the IJC continue efforts to develop the model through the next biennial cycle.
A series of next steps has been developed by the Task Force for consideration by the IJC. These are described below, and are drawn from the results of the workshop held in April 1995. Step 1:
Use a Capacity Building Approach in Developing the Model
Numerous user groups have an identified interest in the model. Users include government agencies, academia, the fishing industry, public interest groups and members of the general public. If the model is to be a useable, sustainable tool for mangers over the long term, the IJC should consider attracting partners to assist with model development. This approach would ensure that an integrated, collaborative approach continues, while stretching available financial resources. To ensure continuance of management style, we recommend that the IJC remain the lead partner.
Step 2:
Communicate the Model Capabilities
A guidebook to using the Lake Erie Ecosystem Model would assist users and partners to understand the basis for the model, the type of data used and include a sample log book in which to record results of tests. In identifying key attributes of other models, we discovered that, while the models may be in the public domain, few are thoroughly described and documented, such that any user can test them.
Step 3:
Start an Iterative Correction Process
An iterative correction process for the model is the necessary first step to identify weaknesses in the prototype. The prototype must be adjusted and improved through a data review/trials process. The Core Advisory Group and other experts would be asked to participate in the exercise. Areas for exploration, investigation and discovery have been identified. Since the model's debut in April 1995, several modellers and managers have used the model to test assumptions. These efforts will be coordinated to ensure that results are documented and improvements are made.
Step 4:
Consult with Users
There are three potential purposes for the model: a management tool; an aid to setting research priorities; and an aid in the understanding of Lake Erie ecosystem linkages. There may be a set of phased-use objectives - e.g. short-term use, mid-term use and long-term use, which could be developed based on expectations and needs of partners.
It is important to clarify the purpose and objectives of the model with potential users. An understanding of (and consensus on) its purpose(s) are desirable among key potential user groups. As was done in the initial phase of model development, consultation with managers and technical experts should continue as this model develops (e.g. Core Advisory Group, communications, technical experts meetings, and workshops).
Step 5:
Test the Three-Basin Concept
After various trials, tests, corrections and new data have been applied to the model in accordance with Step 3 (such that deficiencies have been found and hypotheses have been tested), it is important to develop the model further to disaggregate its whole lake perspective into three basins (east, central and west), and also into offshore and nearshore effects. Fish migration patterns should be reflected between basins, and an accounting for changes in nutrients and contaminants must be made between basins. Seasonal variations within basins coupled with a facility to understand the effects of the thermocline on productivity and phosphorus management would also be important additions to the model.
The Task Force concludes that this modelling approach has great potential as a sustainable aid in assisting Lake Erie managers to arrive at appropriate policy and management decisions. In addition, the model can identify effects of stressors on Lake Erie, in particular the fishery component. Through testing and discovery, the model structure provides for an understanding of the effects and interactions between stressors, and the linkages between stressors in the lake ecosystem. The challenge is to ensure that the modelling exercise, currently in its prototype form, continues beyond this first, rough-cut iteration. To ensure that the model becomes a useable tool should be the next step.
The Task Force has described a process which, if followed, will ensure continuance. With the collaboration of advisors, the identification a few additional "leaders," we can better use limited resources. This approach is one previously identified by the Council of Great Lakes Research Managers as the challenge for the next five years. The Task Force endorses this approach. The development of the Lake Erie Ecological Modelling Project has been a positive exercise for the Task Force. We are indebted to all the advisors who participated in making this effort worthwhile.
Four detailed, background reports were prepared under contracts from the Lake Erie Task Force. These documents are available, on request, from the IJC Great Lakes Regional Office and include:
Koonce, J.F., D.B. Jester, B.A. Henderson, R.W. Hatch and M.L. Jones. 1983. Quota management of Lake Erie Fisheries. Spec. Publ. 83-1. Great Lakes Fishery Commission, Ann Arbor, Michigan. 39 pp.
Koonce, J.F. and M.L. Jones. 1994. Sustainability of the Intensively Managed Fisheries of Lake Michigan and Lake Ontario. Final Report of the SIMPLE Task Group. Board of Technical Experts. Great Lakes Fishery Commission Project Completion Report. Ann Arbor, Michigan. 62 pp.
Locci-Hernandez, A.B. 1988. Comparative study of the response of simulated temperature and tropical fish community to fish exploration and environmental variability. Ph.D. Dissertation. Case Western Reserve Univ. Cleveland, Ohio. 210 pp.
Madenjian, C.P., S.R. Carpenter, G.W. Eck and M.A. Miller. 1993. Accumulation of PCBs by lake trout (Salvelinus namaycush) : an individual-based model approach. Can. J. Fish. Aqua. Sci., 50: 97-109.
U. S. EPA. 1989. Green Bay Mass Balance Study Plan: A Strategy for Tracking Toxics in the Bay of Green Bay, Lake Michigan (working edition). U.S. Environmental Protection Agency, Great Lakes National Program Office, Chicago, Illinois.
Canada
Dr. John M. Cooley
Great Lakes Laboratory for Fisheries & Aquatic Sciences
Department of Fisheries & Oceans
Canada Centre for Inland Waters
P.O. Box 5050, 867 Lakeshore Road
Burlington, Ontario L7R 4A6
Dr. Douglas Dodge
Great Lakes Branch
Ontario Ministry of Natural Resources
P.O. Box 5000
10401 Dufferin Street
Maple, Ontario L6A 1S9
Dr. G. Douglas Haffner
Great Lakes Institute for Environmental Research
304 Sunset
Windsor, Ontario N9B 3A9
Mr. Doug McTavish, Director
Great Lakes Regional Office
International Joint Commission
100 Ouellette Ave., Eighth Floor
Windsor, Ontario N9A 6T3
Dr. Harvey Shear
Regional Science Advisor
Environment Canada
P.O. Box 5050, 867 Lakeshore Rd.
Burlington, Ontario L7R 4A6
Dr. James W.S. Young
Senes Consultants Limited
121 Granton Drive, Unit 12
Richmond Hill, Ontario L4B 3N4
United States
Dr. Jeffrey M. Reutter, Director
Ohio Sea Grant College Program
Ohio State University
Research Centre
1314 Kinnear Road, Room 1541
Columbus, Ohio 43212
Mr. James D. Rozakis
Assistant Regional Director
Meadville Regional Office
Pennsylvania Dept. of Environmental Resources
1012 Water Street
Meadville, Pennsylvania 16335
Mr. Nelson Thomas
Environmental Research Laboratory - Duluth
U.S. EPA
6201 Congdon Blvd.
Duluth, Minnesota 55804
Section Liaisons
Bruce L. Bandurski
International Joint Commission
1250 23rd Street N.W., Suite 100
Washington, D.C. 20440
Ted Bailey
International Joint Commission
100 Metcalfe Street, 18th Floor
Ottawa, Ontario K1P 5M1
Staff Support
Douglas W. Alley
International Joint Commission
100 Ouellette Ave., Eighth Floor
Windsor, Ontario N9A 6T3
John F. McDonald
International Joint Commission
100 Ouellette Ave., Eighth Floor
Windsor, Ontario N9A 6T3
Spatial and Temporal Scaling
A major simplification in the model is the choice of spatial and temporal scales for representing interactions in the Lake Erie ecosystem. The model assumes a whole-lake spatial aggregation and simulates changes in the ecosystem at a minimum of one-year intervals. This assumption means that the model is most realistic for fish populations and progressively less realistic for zebra mussels, zoobenthos and zooplankton, whose populations exhibit substantial seasonal variability. Zooplankton and zoobenthos dynamics, therefore, are simplified to steady-state approximations of mean annual abundance and productivity. Zebra and quagga mussels are treated as a single mussel type with only annual total biomass dynamics. Fish migration is assumed to average lakewide gradients in productivity, but spatial heterogeneity of fish populations is preserved through explicit consideration of habitat overlap among fish species and lower trophic level components of the ecosystem.
Primary Productivity Linked to Phosphorus Loading
Phytoplankton and other primary producers are not explicitly represented in the model. Instead, the model assumes that phosphorus loading determines lakewide primary productivity. Productivity of zooplankton and zoobenthos thus depend on phosphorus loading through primary productivity.
Contaminants Move Through the Food Chain
The prototype considers four contaminants: PCB, DDT, Mercury and Atrazine. Contaminant loadings and mass balances are not explicitly represented. The model assumes an input data set consisting of annual mean concentrations of each contaminant in lake water. Contaminant body burdens of organisms at lower trophic levels are predicted from estimated bioaccumulation factors, and contaminant body burdens of all other individuals depends on the annual balance of contaminant uptake (ingestion and absorption) and excretion.
Bioenergetics of Growth and Reproduction
Growth of individual fish depends on feeding according to annualized, theoretical expectations from bioenergetic models. Except for rainbow trout and lake trout, all fish species rely on natural reproduction for recruitment. Predicted reproduction depends on fecundity and fertility co-efficients, which vary by species and age. Habitat limitations are imposed through coefficients affecting egg mortality.
Functional Predator and Prey Interactions
Feeding by all age groups of fish depends on a common set of rules rather than a predefined set of feeding relations. Predators are assumed to search a defined habitat volume and randomly encounter prey items. Probability of capture of a prey item depends on the ratio of prey to predator and a habitat overlap coefficient.