The optimal size of dynamic phosphorus models for Baltic coastal areas

6 models for phosphorus and chlorophyll of different complexity were applied in 11 Swedish coastal areas. The number of state variables varied between 1 in the simplest model to 7 in the most complex. The results were compared for total phosphorus and chlorophyll in surface water compartments. The results imply that for the studied water bodies, models containing separate state variables for phosphorus in surface water and deep water are superior to models treating the water column as a completely mixed entity. The results do not justify separation of phosphorus into dissolved and particulate fractions. Modeling chlorophyll as a separate state variable did not improve the results for individual values compared to a simple regression against total phosphorus in surface water. Unless detailed descriptions or predictions of chlorophyll dynamics are required, modeling eutrophication in coastal areas may thus be considered as a matter of total phosphorus in two water compartments plus sediments.