Nutrients in the Western Wadden Sea: Freshwater Input Versus Internal Recycling

At present, phosphorus (P) is seen as the main limiting nutrient for phytoplankton growth in the western Wadden Sea. Six cruises were performed for water sampling at selected stations covering a full tidal cycle for later determination of dissolved and particulate nutrient concentrations. The major P sources were identified on a seasonal basis, by comparing the contribution of freshwater discharge and sediment release, calculated in a previous study, to the concentrations in the water column. A close relationship was found between the pelagic concentrations of dissolved inorganic nutrients and chlorophyll a, with a concomitant decrease in nutrients and increase in chlorophyll. This was observed in early spring and was followed by a later increase in the nutrient concentrations in spring-summer. The low concentrations found for the freshwater and seawater end-members for this period ruled out their importance as nutrient sources, suggesting that this increase resulted mainly from internal recycling in the Wadden Sea. Even though P limitation was observed during most of the year, a potential seasonal change in the limiting nutrient, from P to silica, was observed. The comparison between P supply to the Wadden Sea by freshwater discharge and sediment release showed a much higher contribution of the latter, especially in April-November. To our knowledge, this is the first study clearly presenting internal recycling as the main nutrient source to the western Wadden Sea in spring-autumn, instead of freshwater discharge or the North Sea.