A simple surface water biogeochemical model 1 . ' Description , sensitivity analyses , and idealized simulations

Accurate simulation of surface productivity, dissolved nutrient and oxygen concentrations, and particulate carbon flux are critical components of any viable surface water quality model. A linear, three-component model consisting of a dissolved limiting nutrient, phytoplankton, and zooplankton coupled to a one-dimensional turbulence closure fluid flow model has been constructed and applied to a generalized surface water setting. The model transfers nutrients from the euphotic zone to the subeuphotic zone using empirical biogenic flux-depth relationships. The model does not require specification of biogenic particle settling velocity which is known to vary by at least 2 orders of magnitude in natural surface waters and thus constitutes a major source of uncertainty in many biogeochemical models. Using an empirical biogenic flux-depth function to transfer nutrients through the water column introduces spatial errors which are most extreme in deep water. The model is thus most appropriate for lakes, estuaries, and shallow (<1000 m deep) marine settings. Restricting the model to three components with seven empirical constants permits a high degree of computational efficiency. Sensitivity analyses of the empirical constants show that integrated surface productivity is responsive to the three constants related to light (initial slope of the light-productivity curve, light attenuation coefficient of the water, and photosynthetically available radiation) while being relatively insensitive to constants which are strictly related to biogeochemical transformations (remineralization rate, nutrient half-saturation constant, maximum zooplankton growth rate, and Ivlev constant for zooplankton growth). Model simulations using idealized surface temperature boundary conditions produce thermal and biogeochemical characteristics of a typical temperate-latitude lake. The simulations also suggest hat vertical mixing plays an important role in producing the late seasonal, deep chlorophyll maximum commonly observed in many surface water settings.