Modelling the nitrogen cycle in a eutrophic coastal marine environment influenced by sewage effluents  <B>Modelling the nitrogen cycle in a eutrophic coastal marine environment influenced by sewage effluents</B><P>G.E. Tsirtsis</B><I><P> Aquatic Ecosystems Laboratory, Department of Environmental Studies, University of Aegean, Lesvos, Greece</I><P> Abstract<P> A simulation model was developed for the quantitative description of the dynamics in a coastal marine environment influenced by eutrophication, caused by sewage effluents. The emphasis was placed on the nitrogen flow through the microbial foodchain. The calibration and evaluation of the model have been carried out using data selected from the coastal area of Mytilini, island of Lesvos, Greece. The phytoplanktonic growth was nutrientlimited in summer, due to shortage of ammonium, while in wintertime the system seemed to be lightlimited, perhaps because of the selfshading effect. The regenerated production was over 70% of the total during the whole year and almost 100% of the total during the winter. Ammonium also seemed to be the limiting nutrient for bacterial growth. Bacterial biomass showed seasonality with high numbers in summer. The mineralization of nitrogenous organic compounds seemed to be a remarkable source of ammonium, especially after the phytoplanktonic blooms. The ammonium concentration resulting from the above process was 50 to 60% of the total during periods of high productivity. The goodnessoffit of the data to the model supports the view that this kind of models can be powerful tools for the quantitative assessment of eutrophication in coastal areas.<P>