NUTRIENT-RELATED SELECTION MECHANISMS IN MARINE-PHYTOPLANKTON COMMUNITIES AND THE IMPACT OF EUTROPHICATION ON THE PLANKTONIC FOOD-WEB

A general increase in nutrient discharges during the last few decades has caused various changes in the algal community structure along the European continental coast. Coincidentally and maybe consequently, the foodweb structure and functioning has altered in local areas causing various phenomena like oxygen depletion, mortality of groups of organi sms, foam on beaches, and an increase in the productivity of benthic c ommunities and some commercial fish species. The observed increases in algal biomass and shifts in species composition are discussed in rela tion to the involved key mechanisms: resource competition and selectiv e grazing. Along the Dutch coastal zone of the North Sea eutrophicatio n has caused a doubling of the yearly averaged algal biomass during th e past three decades. The sudden appearance of Phaeocystis summer bloo ms coincided with a shift from P-limitation towards N-limitation in th e Dutch coastal area due to a stronger increase in P-discharge relativ e to the increase in N-discharge. Competition experiments in continuou s cultures showed Phaeocystis to become dominant under N-limitation. A dditionally, the large Phaeocystis colonies, which can reach a diamete r up to one centimetre, escape from microzooplankton grazing. A comput er model is presented which demonstrates a shift from bottom-up toward s top-down control if the pelagic environment becomes eutrophicated. I mplementation of this concept in a size-differential phytoplankton con trol model generates the prediction that algal blooms are dominated by species that escape from grazing by those zooplankton species which h ave a high potential numerical response. In marine environments these are microzooplankton species. These organisms mainly feed on cyanobact eria, prochlorophytes and some nano-algal species. One of the conseque nces for foodweb structure and the carbon fluxes in marine foodwebs is that eutrophication will lead to the dominance of poorly edible algal species. Eutrophication favours the downward transport of carbon and nutrients towards the sediments not only due to higher algal biomasses but also as a consequence of a shift towards larger algal species wit h higher sedimentation characteristics. An example is given how these new insights can be used for water quality management purposes.