Long-term phytoplankton-nutrient interactions in a shallow coastal sea: Algal community structure, nutrient budgets, and denitrification potential

In the eutrophic Marsdiep, the westernmost tidal inlet of the Wadden Sea, p hytoplankton 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 spec ies revealed that the phytoplankton community changed drastically both betw een 1976 and 1978 and again between 1987 and 1988, and that it was relative ly stable in-between (1974-1976, 1978-1987) and thereafter (1988-1994). The se major changes in phytoplankton biomass and species composition coincided with changes in absolute and relative (TN:TP) nutrient concentrations. Dur ing the summer of 1977, the Marsdiep shifted from a rich, but phosphorus-co ntrolled system to an even more eutrophic but nitrogen-controlled environme nt. The system reshifted towards P-control between 1987 and 1988. The coinc idence of the shifts in relative nutrient concentrations and phytoplankton species composition implies a strong causal relationship between TN:TP rati os and phytoplankton community structure. Among diatoms, the observed incre ase in phytoplankton biomass under eutrophic N-controlled conditions was pa rticularly 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 st ructure, suggesting enhanced loss of nitrogen to the sediment through incre ased deposition of larger algal cells.