Increasing nitrogen limitation during summer in the List Tidal Basin (Northern Wadden Sea) |
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Authors: | Martina Loebl Franciscus Colijn Justus E E van Beusekom |
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Institution: | (1) Institute for Coastal Research, GKSS, Max Planck Strasse 1, 21502 Geesthacht, Germany;(2) Alfred-Wegener-Institute for Polar and Marine Research, Wadden Sea Station Sylt, Hafenstrasse 43, 25992 List/Sylt, Germany |
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Abstract: | The European Wadden Sea is characterized by high nutrient loads and turbid waters. Riverine nutrient input showed a gradual
decrease since the mid 1980s. In the List Tidal Basin (Northern Wadden Sea) the frequency of low NO3 values in summer has increased and decreasing mean annual suspended matter concentrations indicate an increasing underwater
irradiance. We used an approach developed by Cloern (Aquat Ecol 33:3–16, 1999) to analyze resource limitation of nitrogen (DIN) and underwater irradiance for phytoplankton growth in the List Tidal Basin
between 1985 and 2005. Comparing our results to other studies suggests that the List Tidal Basin is one of the most nitrogen
sensitive areas of the European Wadden Sea. In 2005, phytoplankton growth was light limited from January to May and in November
and December, co-limited by both resources in June, July and October, and nitrogen limited in August and September. Comparing
phytoplankton growth limitation in the periods 1985–1991 and 1999–2005, the duration of nitrogen limitation during summer
is significantly longer in the second period (2.1 ± 0.9 months) than in the first (0.7 ± 0.5). Moreover, light limitation
in September and October has decreased in the second period. A decreasing phytoplankton growth during summer is in line with
former studies from the List Tidal Basin, which showed that summer chlorophyll concentrations have decreased since the mid
1980s. We suggest that nowadays less food is available for higher pelagic and benthic trophic levels due an increased nitrogen
limitation during summer. |
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