Eutrophication, phase shift, the delay and the potential return in the Greifswalder Bodden, Baltic Sea

Between the 1950s and 1980s concentrations of inorganic nutrients increased in the coastal areas of the Baltic Sea, including the Greifswalder Bodden. As a consequence, phytoplankton concentrations and suspended solids increased strongly and the light penetration decreased substantially. An increased turbidity led to a phase shift from a macrophyte-dominated to a phytoplanktondominated ecosystem. Within 30 years, macrophyte cover declined from 90% to 15%. Macrophyte depth limit decreased from 14 m to 6 m in the Bodden. After 1985, nutrient loading declined strongly. Although nutrient loads in the Greifswalder Bodden were reduced (a 50% decrease in phosphate concentration and a 40% decrease in nitrogen concentration) in the last 15 years, an expected improvement in water quality did not coincide. Light conditions improved slightly and macrophytes showed no recovery. Possible reasons for the slow improvement are internal loading and subsequent release of iron-bound phosphorus from sediments. This process sustains continuous high nutrient concentrations in the water column and facilitates the high growth and dominance of phytoplankton. During summer, an abundant Cyanobacteria community fuels phytoplankton dominance. In addition, heavy losses in macrophyte cover result in an increase in sediment resuspensation, which leads to a reduced light penetration. There is no sign of any change in current conditions.