Changes in Silica (Si) delivery to coastal systems, caused by human activities (e.g. reservoir construction and agricultural development) can cause shifts in phytoplankton community structure with impacts through the whole aquatic food web. Although nutrient exports of nitrogen (N) and phosphorous (P) and the impacts of human activity on these are relatively well understood, the extent and potential impact on the combined effects on Si, N, and P ratios are rarely evaluated simultaneously in one basin. I assessed the effects of natural and human controlling factors on nutrient loads and stoichiometry for Si, N, P in the Danube River and its tributaries.

The overall findings of this thesis indicate that land use and land cover is the major controlling factor for Si export and its ratios with N and P at a basin scale, and that this factor tends to maintain overall P limitation in the basin. Moreover, analysis of data at Reni monitoring station just upstream from the Danube Delta showed that there has been a decrease in dissolved inorganic nitrogen (DIN) and dissolved Silica (DSi) loads from the entire Danube over time (since 2000), indicating a high risk of eutrophication in the Black Sea due to DSi reduction.