Functions used in biological models and their influences on simulations

The processes taking into account in biogeochemical modelling are getting m ore complex, but the photosynthesis, respiration, grazing of zooplankton on phytoplankton, natural mortality are the most important. There are just a few kinds of descriptions of respiration and natural mortality, whereas the functional forms used to describe the photosynthesis-light relationship, t he nutrient limitation and zooplankton grazing are quite different in diffe rent biogeochemical models. Several types of functions showing light and nu trient limitation on photosynthesis and zooplankton grazing generally used in biogeochemical models are analysed. Based on a simple NPZD [nutrient (N) , phytoplankton (P), zooplankton (Z), and detritus (D)] biological model, t he influences of different functional types on model results are shown. It is indicated that functional forms may exert a great influence on model res ults. Simulations with different expressions of photosynthesis showed that the highest values are more than 5 to 10 times larger than the lowest ones for nutrient, phytoplankton, zooplankton and detritus. Different zooplankto n grazing functions gave values of several times difference for nitrogen, p hytoplankton and zooplankton. Therefore, it is necessary and very important to choose optimal functions to fit a specific study of marine ecosystem.