Silicon in the marine environment: Dissolved silica trends in the Baltic Sea

Dissolved silica (DSi) is an important nutrient for primary production. During the last decades, negative trends in DSi concentrations have been observed in coastal areas globally, with implications for the marine ecosystem. This tendency is mainly attributed to two processes: decrease in river loadings (as a result of water regulation and building of dams) and eutrophication in the coastal zone, with subsequent silica depletion. One such area affected by both an intensive water regulation and eutrophication is the Baltic Sea. The present study focused on the pelagic DSi trends in different parts of the Baltic Sea for the last three decades. It was found that there are decreasing trends for most of areas and water depths ranging between 0.05 and 1.2 mmol Si l 1 yr 1. The trends are levelling out during the last decade. Since the riverine load of dissolved silica has not changed at a corresponding rate during the time period, the decrement is assumed to be mainly a result of the ongoing eutrophication within the marine water body. This assumption was strengthened by the increasing trends of inorganic nitrogen and phosphorus.