Nutrient budgets of the Western Wadden

Catharina J.M. Philippart*, Gerhard C. Cadée, Wim van Raaphorst & Roel Riegman
Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg (Texel), The Netherlands
* Corresponding author (Tel. :+31-222-369563; Fax: +31-222-319674; Email: katja@nioz.nl)

Variations in nutrient availability often effect phytoplankton dynamics, such as biomass, primary production and species composition. In the eutrophic Marsdiep, the westernmost tidal inlet of the Wadden Sea, phytoplankton biomass and production almost doubled at the end of the 1970s and remained high ever since. Furthermore, the phytoplankton community changed drastically both between 1976 and 1978 and again between 1987 and 1988. It was relatively stable in-between (1974-1976, 1978-1987) and hereafter (1988-1994). Compilation of phosphorus and nitrogen budgets (1974-1994) is applied to analyse which input or output factor is dominant in determining the variation in nutrient availability. Our results indicate that the N-budget of the area is not only correlated with nitrogen loading but also with the community structure of phytoplankton. The latter result suggests enhanced loss of nitrogen to the sediment through increased deposition of larger algal cells, i.e. an additional output of the nitrogen budget is determined by the phytoplankton community itself. Although phosphorus and nitrogen loading decreased since the mid-1980s, chlorophyll-a concentrations and primary production remained high until the present day. This unexpected observation is most probably due to compensating effects on the phytoplankton of a steady increase of silicate since the beginning of the 1990s, underlining the necessity to additionally compile silicon-budgets to fully understand phytoplankton dynamics in coastal waters.