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A conceptual model of an aquatic ecosystem has been formulated. The formulation incorporates largely empirical deterministic relationships describing biological response to abiotic and biotic environmental parameters into a stochastic representation of birth and death events. The occurrence of these events may be described as a Poisson process. Mathematical system theory provides a methodology for organizing the available information on aquatic ecosystem processes into a coherent and logical structure. This organizational capability is demonstrated. The portion of the conceptual model describing primary productivity has been calibrated and tested on an independent data set. This model works well for the Lake Mead system but needs to be tested on other aquatic systems to evaluate its managerial utility. The modeling of the complex interactions of aquatic food web processes requires further investigation to define an acceptable set of model coefficients.

A large, multi-basin, reservoir (Garza-Little Elm Reservoir) in north central Texas was studied to determine the relative effects of various parameters on primary productivity. The basins were impounded several years apart,thus allowing the influence of age on water chemistry and biota to be considered. Another principal influence on water quality was secondary sewage effluent that entered one basin from a nearby source.

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

Thesis (Ph.D.)--Murdoch University, 2004. / Thesis submitted to the Division of Science and Engineering. Bibliography: leaves167-191.

Lake Manassas is an approximate 706 acre man-made lake located in greater Prince William County near the town of Gainesville, Virginia. The lake was created in 1968 on Broad Run to serve as the primary water supply for the City of Manassas and its residents. The Lake Manassas watershed lies within the greater Occoquan River watershed which drains into the Potomac River and Chesapeake Bay. Water within Lake Manassas and its tributaries has been monitored since 1971 and this thesis presents a comprehensive limnological analysis of the Lake with an analysis of water quality impacts over time. Lake Manassas remains an enriched or eutrophic system, meaning the levels of nutrients and biomass production in lake waters is above desired standards. Nutrient loading occurs via a stream network with the largest contributor being Broad Run which is consistent with previous water quality studies. The lake serves as the sole water source for nearly 40,000 residents and businesses and ensuring clean and safe water is of significant importance. Close monitoring in conjunction with the implementation of appropriate management practices within the watershed are necessary to prevent water quality from becoming significantly degraded. / Master of Science

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