SIZE STRUCTURE AND SUCCESSION IN PHYTOPLANKTON COMMUNITIES - THE IMPACT OF INTERACTIONS BETWEEN HERBIVORY AND PREDATION

The aim of this study was to determine whether fluctuations in the abu ndance of, and interactions between, large (macrozooplankton) and smal l (rotifer) herbivores are related to the development and succession i n the phytoplankton community. Field recordings from three highly prod uctive lakes over a 3-year period revealed a negative correlation betw een the ratio of the dominant rotifer Keratella cochlearis and macrozo oplankton vs the ratio of ''small'' and ''large'' algae. This suggests that the algal community was forced towards larger forms when rotifer s were abundant. In complementary laboratory experiments, the grazing pressure by rotifers was negligible on algae >30 mu m, but considerabl y higher on algae <30 mu m, whereas macrozooplankton mainly grazed on algae larger than 30 mu m. Moreover, experiments showed that cyclopoid copepods rapidly reduced rotifer abundance, suggesting that large zoo plankters may affect the size distribution of algae not only directly by selective grazing, but also indirectly by predation on smaller herb ivores. Hence, our study suggests that interactions among different si ze classes of zooplankters affect size distribution and succession pat terns in the phytoplankton assemblage.