Marie Maar, Janus Larsen and Stiig Markager, Marine Ecological Modelling Centre, Aarhus Univ
The Western Baltic Sea is affected by eutrophication and receives nutrients from local land-based sources, atmospheric deposition and by advection from the neighbouring North Sea and Baltic Sea. In the present study, we evaluated the importance of local (Danish) versus external (Baltic Sea-North Sea) nutrient loads for surface Chl a- concentrations, total areal primary production and the vertical distributions of primary production in the W. Baltic Sea using the 3D coupled HBM-ERGOM model. This was assessed by improving descriptions of nutrient loads and primary production in the model and by conducting scenarios of different nutrient loads according to i) the eutrophication level in 1990, ii) the new Danish Water Plan (DWP) and iii) DWP + the Baltic Sea Action Plan (BSAP2). The model results showed that local nutrient reductions have a profound effect on Chl a concentrations and primary production in the coastal areas of the Western Kattegat and the Belt Sea with the highest impact <10-25 km from the coast. However, on average for the study area, Danish reductions have a limited effect. The DWP complemented the BSAP2 reductions spatially, since they had the highest impact in different areas and both were important for improving the ecological status of the ecosystem. The model suggested a significant change in the vertical distribution of primary production with less deep primary production in response to increased nutrient load. We recommend using this redistribution as an ecological indicator of eutrophication in seasonally stratified seas. The new description of primary production takes into account that carbon fixation continue, even when phytoplankton is severely nutrient limited. This model improvement increases primary production by a factor of 2.6 and brings the model estimates in agreement with measurements and the physiology of phytoplankton. We recommend that other dynamic ecosystem models using Liebig's law for primary production consider a similar approach.
D5, Descriptor 5, modelling, eutrophication,atmospheric deposition, chlorophyll, primary production, nutrient loads, Baltic sea, carbon fixation
Workshop of the Network of Experts for ReDeveloping Models of the European Marine Environment
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