MEPS 136:59-68 (1996)  -  doi:10.3354/meps136059

The roles of inorganic nutrients and grazing by crustaceans and microprotozoans as factors shaping the planktonic community of the Gulf of Finland were studied in a 6 d laboratory experiment. The removal of metazoans >100 um greatly enhanced microprotozoan growth during the last 3 d of the experiment. When inorganic N and P additions were combined with the removal of metazoans, the growth of protozoans was stimulated even further. On the other hand, addition of metazoans >200 um kept the protozoan populations low, even in the presence of extra N and P. In connection with high protozoan numbers, the growth of phytoplankton >10 um was enhanced, while the 1-10 um phytoplankton populations decreased towards the end of the experiment. With abundant metazooplankton and nutrient additions (and only few protozoans), phytoplankton in the size range of 1 to 10 um flourished, while >10 um phytoplankton biomass did not rise above the initial level. Whether the grazer community was dominated by protozoans or metazoans did not have much effect on picoalgal (<2 um) growth, but the presence of crustaceans resulted in increased bacterial production in non-nutrient-enriched conditions. The results suggest that total phytoplankton biomass is largely determined by both nutrient availability and grazer activities, and can be quite similar in both proto- and metazoan-dominated communities. However, the structure of the prey communities >2 um depends on the composition of the grazer populations. The biomass and composition of microprotozoan communities also appear largely to depend on the shaping effect of metazoan grazers. Nutrient recycling by crustaceans seems to enhance bacterial production, and possibly also picoalgal growth.

Crustaceans . Microprotozoans . Nutrients . Grazing . Food web shaping