This report constitutes the Master of Science thesis of the same title completed by the author in May, 1973, and accepted by the Department of Systems and Industrial Engineering. It is the result of a joint research project on "Decision analysis of watershed management alternatives," supported in part by the United States Department of the Interior, Office of Water Resources Research, as authorized under the Water Resources Research Act of 1964. / Various approaches to modeling phytoplanktonzooplankton- nutrient interactions have been investigated. A stochastic birth- death model was developed to describe changes in phytoplankton and zooplankton population levels at a given point. Tuie stochastic birth -death model was combined with a deterministic mass balance of limiting nutrient concentration to form an over -all system theoretic model that enables one to use Monte Carlo simulation to study the problem of eutrophication. A comparison made between this modeling approach and the standard differential equation approach suggested that further investigation was desirable, particularly in the area of model calibration.

Coastal eutrophication, defined as an increase in algal biomass (as chlorophyll (Chl)) is of increasing international concern. Although coastal eutrophication will likely increase as coastal populations grow, few models exist to support its management. Lake eutrophication has also long been recognized as an important environmental concern. However, effective lake eutrophication management exists, supported by regression and mass-balance models. Traditionally, these "Vollenweider" models link land-use to Chl via total phosphorus (TP), the nutrient considered to be limiting Chl. However, based on a data set of 63 lakes, Chl was more accurately predicted by models based on land-use than by those based on TP. This result provided the rationale to build Chl:land-use models for estuaries where the Chl:nutrient relations are unclear. Chl:land-use models were developed for 15 estuaries in PEI, 19 estuaries in Finland and 26 US estuaries. Land-use models predicted Chl more accurately than TP in the US estuaries and in some of the Finnish estuaries. In the Finnish estuaries, Chl was best predicted by a land-use model in estuaries dominated by nonpoint source loading whereas Chl was most accurately predicted by the Vollenweider approach in estuaries dominated by point-source loading. In the PEI estuaries, the accuracy of the land-use model was comparable to the accuracy of the TP model. The PEI estuaries had much lower yields of Chl per unit nutrient than lakes suggesting differences among systems. This Chl deficit (expected-observed Chl) was accounted for by herbivory and turbidity, neither of which factors are exclusive to estuaries. The comparison of Chl response to nutrients and land-use across lakes and estuaries demonstrated no systematic differences as a function of tidal energy, openness or salinity. The regression models based on the combined data accurately predicted Chl as a function of TP and percentage of the catchment forested and mean depth. These results sug

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