AN EXPERIMENTAL-STUDY ON THE ROLE OF CRUSTACEAN AND MICROPROTOZOAN GRAZERS IN THE PLANKTONIC FOOD-WEB

The roles of inorganic nutrients and grazing by crustaceans and microp rotozoans as factors shaping the planktonic community of the Gulf of F inland were studied in a 6 d laboratory experiment. The removal of met azoans >100 mu m greatly enhanced microprotozoan growth during the las t 3 d of the experiment. When inorganic N and P additions were combine d with the removal of metazoans, the growth of protozoans was stimulat ed even further. On the other hand, addition of metazoans >200 mu m ke pt the protozoan populations low, even in the presence of extra N and P. In connection with high protozoan numbers, the growth of phytoplank ton >10 mu m was enhanced, while the 1-10 mu m phytoplankton populatio ns decreased towards the end of the experiment. With abundant metazoo- plankton and nutrient additions (and only few protozoans), phytoplankt on in the size range of 1 to 10 mu m flourished, while >10 mu m phytop lankton biomass did not rise above the initial level. Whether the graz er community was dominated by protozoans or metazoans did not have muc h effect on picoalgal (<2 mu m) growth, but the presence of crustacean s resulted in increased bacterial production in nonnutrient-enriched c onditions. The results suggest that total phytoplankton biomass is lar gely determined by both nutrient availability and grazer activities, a nd can be quite similar in both proto- and metazoan-dominated communit ies. However, the structure of the prey communities >2 mu m depends on the composition of the grazer populations. The biomass and compositio n of microprotozoan communities also appear largely to depend on the s haping effect of metazoan grazers. Nutrient recycling by crustaceans s eems to enhance bacterial production, and possibly also picoalgal grow th.