Inorganic nutrients are essential for phytoplankton

Inorganic nutrients, such as nitrogen (N), phosphate (P) and silicate (Si) are essential for phytoplankton to build their cells. This movie also shows that when phytoplankton growth takes off in early spring, it is accompanied by a rapid decline of the phosphate concentration. If phosphate or one of the other nutrient concentrations (N or Si) becomes limiting, this terminates the spring bloom. During summer, phosphate is nowadays the most limiting nutrient in the Wadden Sea due to the reduction of the phosphate input into the rivers in the eighties and nineties of the previous century. In late spring and early summer, the low concentration in the Wadden Sea leads to nutrient input from the North Sea where the phosphate concentration is clearly higher (although there it is (nearly) limiting as well). In autumn, extra phosphate is provided from the sediment. This phosphate originates from particulate organic matter (detritus) which has settled during and after the spring bloom and is mineralized by bacteria in the sediment.