NUTRIENT LIMITATION AND GRAZING CONTROL OF THE BALTIC PLANKTON COMMUNITY DURING ANNUAL SUCCESSION

Nutrient limitation and grazing control of the planktonic community we re studied in the northern Baltic Sea off the SW coast of Finland duri ng the phytoplankton growth season of 1985. In situ experiments based on a 2(3) factorial design were performed in mesocosm enclosures on 1 0 occasions. The manipulations used included phosphorus (PO43-) and ni trogen (NH4+) additions and the removal of metazooplankton by 100-mum prefiltration. In each experiment, the responses of phytoplankton, bac terioplankton, heterotrophic nanoflagellates, and protozooplankton wer e followed for 2 d. Orthogonal multiple regression analysis was used t o reveal which manipulations had statistically significant effects. Ni trogen was found to be the basic limiting nutrient for phytoplankton t hroughout the productive season. During early summer, only the combine d addition of P and N evoked a clear increase in the growth of phytopl ankton. In general, bacterial productivity was not highly affected by the manipulations. In summer the removal of metazooplankton caused a r apid increase in the amount of protozooplankton in the units with 100- mum prefiltration or prefiltration combined with N addition. In the ab sence of metazooplankton, the nutrient-induced increase in primary pro ductivity was channeled to protozooplankton, whose growth in the units where metazooplankton was present was severely limited by food compet ition or by direct metazooplankton grazing.