[1] The study presents results of decadal simulations of the dynamics of the ecosystem of the Baltic Sea with a three-dimensional coupled physical chemical biological model for the 1980s. The model consists of a circulation model, which is an implementation of the Modular Ocean Model (MOM 2) for the Baltic Sea and an embedded ecosystem model based on a nine-component model (ERGOM [Neumann, 2000]). The model was driven with realistic atmospheric forcing, river loads, and atmospheric deposition of nutrients. A simulation with the realistic forcing was carried out as a control run in order to study in a further experimental simulation the response of the model ecosystem to a 50% reduction of riverine nutrient loads. It was found that the model food web reacts on the load reduction in a complex manner. The changes in dissolved nutrients and in phytoplankton display a spatial variability and different species-specific reactions. While the total biomass and nutrient inventories are reduced, there are significant regional differences. In particular, the role of cyanobacteria, which can fix atmospheric nitrogen, is important, and the biomass of cyanobacteria increases in response to the reduced loads. Although the model system needs further improvements, the present study is a step toward an understanding of the quantitative effects of a reduction of nutrient river loads to the Baltic Sea ecosystem based on a full three-dimensional coupled model.

[1] A better understanding of the fate of nutrients entering the Baltic Sea ecosystem is an important issue with implications for environmental management. There are two sources of nitrogen and phosphorus: riverine input and atmospheric deposition. In the case of nitrogen, the fixation of dinitrogen by diazotrophic bacteria represents a third source. From an analysis of stable nitrogen isotope ratios it was suggested that most of the riverine nitrogen is sequestered in the coastal rim, specifically along the southern Baltic Sea coast with its coarse sediments, whereas nitrogen from fixation dominates the central basins. However, pathways of nutrients and timescales between the input of the nutrients and their arrival in different basins are difficult to obtain from direct measurements. To elucidate this problem, we use a source attribution technique in a three-dimensional ecosystem model, ERGOM, to track nutrients originating from various rivers. An "age" variable is attributed to the marked elements to indicate their propagation speeds and residence times. In this paper, we specifically investigate the spreading of nitrogen and phosphorus from the riverine discharges of the Oder, Vistula, Neman and Daugava. We demonstrate which regions they are transported to and for how long they remain in the ecosystem. The model results show good agreement with source estimations from observed d15 N values in sediments. The model results suggest that 95% of nitrogen is lost by denitrification in sediments, after an average time of 1.4 years for riverine nitrogen. The residence time of riverine phosphorus is much longer and exceeds our simulated period of 35 years.

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