Silicon retention in river basins: Far-reaching effects on biogeochemistryand aquatic food webs in coastal marine environments

Regulation of rivers by damming as well as eutrophication in river basins h as substantially reduced dissolved silicon (DSI) loads to the Black Sea and the Baltic Sea. Whereas removal of N and P in lakes and reservoirs can be compensated for by anthropogenic inputs in the drainage basins, no such com pensation occurs for DSI. The resulting changes in the nutrient composition (DSI:N:P ratio) of river discharges seem to be responsible for dramatic sh ifts in phytoplankton species composition in the Black Sea. In the Baltic S ea, DSI concentrations and the DSI:N ratio have been decreasing since the e nd of the 1960s, and there are indications that the proportion of diatoms i n the spring bloom has decreased while flagellates have increased. The effe cts on coastal biogeochemical cycles and food web structure observed in the Black Sea and the Baltic Sea may be far reaching, because it appears that the reductions in DSi delivery by rivers are probably occurring worldwide w ith the ever increasing construction of dams for flow regulation.