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The CE‐QUAL‐ICM (Corps of Engineers Integrated Compartment Water Quality Model) eutrophication model was applied in a 21‐year simulation of Chesapeake Bay water quality, 1985‐2005. The eutrophication model is part of a larger model package and is forced, in part, by models of atmospheric deposition, watershed flows and loads, and hydrodynamics. Results from the model are compared with observations in multiple formats including time series plots, cumulative distribution plots, and statistical summaries. The model indicates only one long‐term trend in computed water quality: light attenuation deteriorates circa 1993 through the end of the simulation. The most significant result is the influence of physical processes, notably stratification and associated effects (e.g., anoxic volume), on computed water quality. Within the application period, physical effects are more important determinants of year‐to‐year variability in computed water quality than external loads.

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A primary production model is described and compared to 3 observational data bases: light-saturated carbon fixation, net phytoplankton primary production, and gross phytoplankton primary production. The model successfully reproduces the observations while maintaining realistic calculations of algal carbon, chlorophyll, limiting nutrient, and light attenuation. Computed primary production in light-limited regions is proportional to the algal growth rate. Successful computation of primary production in nutrient-depleted waters depends on the formulation and magnitude of the model predation term. Our quadratic formulation mimics a predator population that is closely coupled to algal biomass and rapidly recycles nutrients from algal biomass to available form. KEY WORDS: Chesapeake Bay · Primary production · Phytoplankton · Mathematical modelsResale or republication not permitted without written consent of the publisher