Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 11 5 2007 10.5194/hess-11-1593-2007 http://www.hydrol-earth-syst-sci.net/11/1593/2007/ http://www.hydrol-earth-syst-sci.net/11/1593/2007/hess-11-1593-2007.html http://www.hydrol-earth-syst-sci.net/11/1593/2007/hess-11-1593-2007.pdf 1593 1607 2007-09-26 Riverine transport of biogenic elements to the Baltic Sea – past and possible future perspectives C. Humborg christoph.humborg@itm.su.se C.-M. Mörth M. Sundbom F. Wulff Dept. of Applied Environmental Science, Stockholm University, 10691 Stockholm, Sweden Dept. of Geology & Geochemistry, Stockholm University, 10691 Stockholm, Sweden Dept. of Systems Ecology, Stockholm University, 10691 Stockholm, Sweden Baltic Nest Institute, Stockholm University, 10691 Stockholm, Sweden The paper reviews critical processes for the land-sea fluxes of biogenic elements (C, N, P, Si) in the Baltic Sea catchment and discusses possible future scenarios as a consequence of improved sewage treatment, agricultural practices and increased hydropower demand (for N, P and Si) and of global warming, i.e., changes in hydrological patterns (for C). These most significant drivers will not only change the total amount of nutrient inputs and fluxes of organic and inorganic forms of carbon to the Baltic Sea, their ratio (C:N:P:Si) will alter as well with consequences for phytoplankton species composition in the Baltic Sea. In summary, we propose that N fluxes may increase due to higher livestock densities in those countries recently acceded to the EU, whereas P and Si fluxes may decrease due to an improved sewage treatment in these new EU member states and with further damming and still eutrophic states of many lakes in the entire Baltic Sea catchment. This might eventually decrease cyanobacteria blooms in the Baltic but increase the potential for other nuisance blooms. Dinoflagellates could eventually substitute diatoms that even today grow below their optimal growth conditions due to low Si concentrations in some regions of the Baltic Sea. C fluxes will probably increase from the boreal part of the Baltic Sea catchment due to the expected higher temperatures and heavier rainfall. 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