Nutrient turnover at the sediment/water interface in shallow eutrophic coastal waters

The nutrient situation in eutrophic shallow coastal water systems is characterized by particular features. In addition to water exchange with the open sea, interactions at the sediment/water interface play a significant role. With the help of investigation on phosphate sorption/desorption balances and on the denitrification capacity of shallow coastal waters, the following general conclusions can be made: The phosphate concentrations in the water are primarily determined by physico-chemical reactions with the sediment, and less by the rhythm of the phytoplankton primary production. Through intensive interactions between sediment and water, a phosphate "equilibrium" concentration which fluctuates very little is maintained over the whole year. High primary production rates are possible at these low equilibrium concentrations of phosphate. The phosphate sorption capacity of sediments is a characteristic parameter of shallow systems. Sediments with a high organic matter content exhibit the highest phosphate sorption capacities. With the development of nearly anearobic conditions, nitrate reduction can occur at a rate, when the appropriate amount of nitrate is made available to the reaction. Under optimal conditions for nitrate reduction, nitrate concentration is also not a parameter characterizing the nature of the water body