A dynamic box model was attempted in the study of phosphorus cycling in Xiamen Western Sea.

State variables involved in the model were dissolved inorganic phosphorus (DIP), dissolved organic phosphorus (DOP), particulate phosphorus (PP) and biomass of phytoplankton. The evolution of the state variables were considered to be controlled by physical transport, external inputs, and biogeochemical processes. Physical transport was caused by residual current and eddy diffusion. External inputs included benthic flux, atmospheric deposition, loading from Jiulong River and sewage discharge. The central point of biogeochemical processes was plankton activities, including phytoplankton photosynthesis, respiration and secretion, and zooplankton grazing and excretion. Plankton mortality, organic matter mineralization and sinking of phytoplankton and detritus were accounted for as well.

The above processes were parameterized and synthesized into a cycling model of phosphorus. The model was run for a period of two months from March to May, 1994. When compared to the field data, the computed dynamics of DIP, DOP, PP and phytoplankton biomass appeared generally realistic. However, some of the distribution features could not be reproduced, partly due to the simplified description of the behaviour of zooplankton and organic matter, and the unknown influence of aquaculture.

Successful simulation meant that the model contained the most important processes and the parameterization of these processes were reasonable. Therefore, the significance of such a kind of modeling study was first shown in methodology. Furthermore, flowing rates of phosphorus between the functional compartments were obtained, giving quantitative views of phosphorus cycling in the study area. Benthic flux was shown to be an important source of phosphorus in this area. It was also indicated that heavier sewage discharge or weakened tidal dispersion might be dangerous most possibly in the north part of Xiamen Western Sea. Future models might be improved once rich data and better knowledge about biogeochemical processes were obtained.