Nutrient loads, advection and turnover at the entrance to the Baltic Sea

The monthly advective supply of nutrients and nutrient pool sizes in the Kattegat and the Belt Sea are estimated for the period 1989–1996 from a combination of time-dependent hydrodynamic model results and concentration measurements of dissolved inorganic nitrogen (DIN), phosphate (DIP) and total nitrogen and phosphorous (TN and TP). The net supply of nutrients to these seas is computed as the sum of advection and nutrient loads from atmosphere and land to the Kattegat and the Belt Sea. The average net supplies are 5000 t N per month and 500 t P per month in both areas, corresponding to approximately 10% of the amounts required for the annual primary production in the area. The net supply of N displays a small seasonal variation, while the net supply of P exhibits both a spring maximum and a winter minimum. The seasonal nutrient turnover cycle is estimated from the residual of the net supply and the seasonal variation in nutrient pools. Net release of DIP from the sediment takes place during late summer and winter, while net release of DIN occurs mainly during winter. Thus, the net release from the sediment adds to the pool of nutrients available for the spring bloom in the Baltic entrance the following year. Both advection and the autumn–winter release of P exceed the land load of P. Thus, on a short time-scale, reductions in P load cannot be expected to significantly change the availability of P for biogeochemical processes. However, the monthly N load from land and atmosphere exceeds the advective supply. Reductions in N load thus have a significant influence on availability of N for biogeochemical processes.