A TIME-DEPENDENT BUDGET MODEL FOR NUTRIENTS IN THE BALTIC SEA

Overall budgets for nutrient and humus are described for the Baltic Sea as well as for the subsystems, i. e., the Baltic proper, the Bothnian Bay and the Bothnian Sea. The residence times for total phosphorus, total nitrogen, silicate and humus are 13.3, 5.5, 11.2 and 9.6 years respectively, compared to 21.8 years for a conservative substance (salt). About 90% of the nutrient losses are due to biogeochemical sinks within the Baltic Sea. Thus only about 10% is exported to external areas (the Kattegat/Belt Sea). For humus the corresponding figures are about 75 and 25%, respectively. This means that the Baltic Sea to a large extent can be regarded as a closed system and perturbations in the water exchange with the North Sea should have little effect on the nutrient budgets. The sinks are parameterized by an expression borrowed from limnology where the net nutrient loss is a function of the winter surface concentration. A budget model is run in a prognostic, hindcast mode with the assumed timedependent phosphorus and nitrogen loading of the Baltic proper. The computed development of the winter surface concentrations of total P and total N for the period 1950-1988 appears quite realistic. The possibility of having variable sinks which are functions of the surface winter concentrations of nitrogen and phosphorus is described using calculations based on data from the different Baltic subareas. Such sinks should significantly decrease the N: P ratio in the surface water when the nutrient loading increases with time. With better descriptions of in particular the pools of nutrients in the sediment, it would be possible to model future changes of nutrient concentrations in the water column in relation to loading.