A general, size-based simulation model is developed to investigate the dynamics of carbon and nitrogen flows in plankton communities. In the model, community structure and transfer processes are all size-dependent, and all model parameters are determined by body size, using empirically determined relationships calculated from published data Major flows include carbon fixation, release of photosynthetically produced dissolved organic carbon (PDOC), nitrogen uptake, respiration, excretion, predation, senescence and sinking. Because the model is based on general ecological principles and not on a specific ecosystem or data set, it can be used to simulate interactions within plankton communities of any ecosystem. The structure of the model can easily be altered to incorporate fewer or more size classes, or different size ranges of organisms. The program adjusts interactions between different components to allow for changes in the number of size classes, and new parameters are estimated, based on mean organism size A standard simulation is described, which serves as the basis for comparing output from a sensitivity analysis. The model is robust with respect to most parameters. Important factors which affect model output include estimates of various rate parameters (which may be altered by environmental effects), shapes of initial biomass distributions (seeding effects), and wet mass to carbon conversion functions. The model is a useful tool to assist in analysing and interpreting carbon and nitrogen flows in planktonic ecosystems.

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