Changes in nutrient structure of river-dominated coastal waters: stoichiometric nutrient balance and its consequences

We present an analysis of extensive nutrient data sets from two river-dominated coastal ecosystems, the northern Adriatic Sea and the northern Gulf of Mexico, demonstrating significant changes in surface nutrient ratios over a period of 30 years. The silicon:nitrogen ratios have decreased, indicating increased potential for silicon limitation. The nitrogen:phosphorus and the silicon:phosphorus ratios have also changed substantially, and the coastal nutrient structures have become more balanced and potentially less limiting for phytoplankton growth. It is likely that net phytoplankton productivity increased under these conditions and was accompanied by increasing bottom water hypoxia and major changes in community species composition. These findings support the hypothesis that increasing coastal eutrophication to date may be associated with stoichiometric nutrient balance, due to increasing potential for silicon limitation and decreasing potential for nitrogen and phosphorus limitation. On a worldwide basis, coastal ecosystems adjacent to rivers influenced by anthropogenic nutrient loads may experience similar alterations.