Nutrient cycling through phytoplankton, bacteria and protozoa, in selectively filtered Lake Vechten water

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Abstract
Models of carbon and nitrogen cycles in the ocean are a major tool in elucidating short-and long-term patterns of chemical fluxes. Variability in space and time are usually attributed to changes in ocean physics at different scales. This paper stresses the significance of the upper (predatory) closure in these simple nutrient-plant-herbivore models. The mathematical form used to close the system and the values given to the parameters have very marked effects on the overall response. In particular the major differences between North Atlantic and Pacific patterns may depend on this aspect as much as on the physical cycles. It is shown that the selection of different closure forms in five recent modelling studies corresponds to differences in the nutrient dynamics and plankton cycles. Thus, in general, the character of the results from these models will depend on both the form of the mortality closure and the parameter values used. Our ignorance in both areas is considerable.