Long‐term phytoplankton‐nutrient interactions in a shallow coastal sea: Algalcommunity structure, nutrient budgets, and denitrification potential

Abstract

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. Principal component analysis of 21-yr (1974-1994) high-resolution time series of the 32 most numerous marine algal species revealed that the phytoplankton community changed drastically both between 1976 and 1978 and again between 1987 and 1988, and that it was relatively stable in-between (1974-1976, 1978-1987) and thereafter (1988-1994). These major changes in phytoplankton biomass and species composition coincided with changes in absolute and relative (TN : TP) nutrient concentrations. During the summer of 1977, the Marsdiep shifted from a rich, but phosphorus-controlled system to an even more eutrophic but nitrogen-controlled environment. The system reshifted towards P-control between 1987 and 1988. The coincidence of the shifts in relative nutrient concentrations and phytoplankton species composition implies a strong causal relationship between TN : TP ratios and phytoplankton community structure. Among diatoms, the observed increase in phytoplankton biomass under eutrophic N-controlled conditions was particularly due to an increase of the abundance of larger algae. Our results indicate that the N budget of the area is correlated with the community structure, suggesting enhanced loss of nitrogen to the sediment through increased deposition of larger algal cells.In shallow coastal marine waters, the total primary production and biomass of phytoplankton is generally assumed to be hyperbolically related to nutrient loadings from land and subsequent availability of these nutrients in the water column (e.g., Borum and Sand-Jensen 1996). However, understanding the effect of nutrient enrichment on living resources requires detailed knowledge of how nutrients enter and leave these waters. Dissolved and particulate materials as well as living organisms are exchanged between the coastal waters and the open sea, and net material fluxes appear to depend on physical and biological responses within these systems to changes in nutrient loadings from land (e.g., Dame and Allen 1996). Phytoplankton species obviously respond in different ways to nutrient enrichment, most probably depending on their specific life-history characteristics such as growth curves and storage capacities (Grover 1997;Roelke et al. 1997). Current theories on plankton ecology predict that an eutrophic and nitrogen-controlled environment gives rise to AcknowledgmentsWe thank J. Hegeman for his help in collecting the phytoplankton samples for more than 20 years, I. Loos for digitizing the phytoplankton data, M. Rademaker for her advice in applying different methods for estimating carbon contents of phytoplankton cells, U. Tillmann for his kindness to provide his unpublished formulae on phytoplankton cell volumes and carbon contents, and P. V. M. Bot and J. Doekes of the Dutch Ministry of Transport and Public Works for kindly supplying the nutrient data. Furthermore, ...

Climate‐related changes in recruitment of the bivalve Macoma balthica

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