22. Eutrophication

Coastal zones have experienced an increased nutrient load during the past decades. In most cases, strongest increases took place since the 1950s. First signs of consequences of the increased nutrient loads were increased phytoplankton blooms, an increase in Harmful Algae Blooms, a decrease in seagrass and an increase in green macroalgae blooms. As a consequence of the increased production and accumulation of organic matter hypoxic conditions may develop with detrimental consequences for the benthic and pelagic ecosystems. The global extent of hypoxic areas has doubled since the 1960s. Relatively few time series exist, that document the early stages of eutrophication. With new data becoming available, it is now clear that the effects of eutrophication are very complex and in many cases site specific. Moreover, other aspects of human induced global change like temperature increase, or the introduction of non-indigenous species interact with phytoplankton dynamics, posing a challenge to future coastal research. A case study for the Wadden Sea, a coastal sea that is under severe pressure by continental Westeuropean rivers, is presented that shows the eutrophication history, and recent improvements after management decisions lead to decreasing nutrient loads.