NUTRIENT CYCLING AND FOODWEBS IN DUTCH ESTUARIES

In this review several aspects of the functioning of the Dutch estuaries (Ems-Dollard, Wadden Sea, Oosterschelde, Westerschelde, Grevelingen and Veerse Meer) have been compared. A number of large European rivers (especially Rhine) have a prevailing influence on the nutrient cycling of most Dutch estuaries. Owing to the increased loading of the estuaries with nitrogen and phosphorus compounds, effects of eutrophication on the biological communities could be demonstrated, mainly in the western Wadden Sea. The causality, however, of the relation between increased nutrient loading and increased biomass and production of primary producers in the turbid tidal Dutch ecosystems is questioned. The most obvious biological effects of eutrophication have been observed in a non-tidal brackish lagoon, Veerse Meer. The estuarine food web received major attention. Budget studies of the main primary producers revealed a dominance of phytoplankton in all Dutch estuaries, followed by microphytobenthos in the tidal systems and macrophytes in the lagoons. The quantitative distribution of primary producers and primary and secondary consumers shows remarkable similarities along the physical and chemical estuarine gradients, notwithstanding the large variability in space and the considerable inconstancy over time. Among the secondary consumers (waterfowl, marine fish, larger invertebrates) the levels of organic carbon consumption - expressed in g C m-2 y-1 - are almost the same, when tidal estuaries are compared with non-tidal lagoons, notwithstanding the fact that the consumer populations show large qualitative differences. The transfer from primary consumers to secondary consumers reveals a bottle neck: especially during late winter, when macrozoobenthos reaches its lowest biomass, food may be a serious limiting resource for large numbers of migratory waders foraging on the intertidal flats. The consequences of the Deltaplan , the closure of several estuaries in the southwest of the Netherlands and their subsequent transfer into non-tidal lagoons, offer complicated case studies of ecosystem changes. Several examples of long-term trends in ecosystem development in Grevelingen lagoon have been discussed.