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

Nutrient limitation and grazing control of the planktonic community were studied in the northern Baltic Sea off the SW coast of Finland during 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 nitrogen (NH4+) additions and the removal of metazooplankton by 100-mum prefiltration. In each experiment, the responses of phytoplankton, bacterioplankton, heterotrophic nanoflagellates, and protozooplankton were followed for 2 d. Orthogonal multiple regression analysis was used to reveal which manipulations had statistically significant effects. Nitrogen was found to be the basic limiting nutrient for phytoplankton throughout the productive season. During early summer, only the combined addition of P and N evoked a clear increase in the growth of phytoplankton. In general, bacterial productivity was not highly affected by the manipulations. In summer the removal of metazooplankton caused a rapid increase in the amount of protozooplankton in the units with 100-mum prefiltration or prefiltration combined with N addition. In the absence of metazooplankton, the nutrient-induced increase in primary productivity was channeled to protozooplankton, whose growth in the units where metazooplankton was present was severely limited by food competition or by direct metazooplankton grazing.