Distant riverine nutrient supply and local temperature drive the long-term phytoplankton development in a temperate coastal basin

Since 1984, salinity, temperature, phytoplankton biomass and nutrients have been monitored in the List Tidal Basin (northern Wadden Sea, Europe). Nutrient dynamics follow a clear seasonal pattern with high winter concentrations and low summer concentrations. Interannual phytoplankton dynamics in the basin apparently responded to both decreasing riverine nutrient loads and the prevailing temperature regime. The mean spring diatom biomass (as Chlorophyll a) was correlated with temperature and the highest biomass was reached after cold winters. Summer phytoplankton (May–September) gradually decreased and correlated with Total Nitrogen loads from the rivers Weser and Elbe debouching about 150 km south of the List Tidal Basin into the North Sea. During summer, the frequency of observations with low nitrate concentrations (< 0.5 µM) significantly increased. We conclude that the long-term development of the phytoplankton is driven by a combination of riverine nitrogen loads and temperature. We expect that global warming and decreasing nutrient loads will reduce the magnitude of phytoplankton blooms in the List Tidal Basin.