Size structure and succession in phytoplankton communities: the impact of interactions between herbivory and predation

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The aim of this study was to determine whether fluctuations in the abundance of, and interactions between, large (macrozooplankton) and small (rotifer) herbivores are related to the development and succession in the phytoplankton community. Field recordings from three highly productive lakes over a 3-year period revealed a negative correlation between the ratio of the dominant rotifer Keratella cochlearis and macrozooplankton vs the ratio of "small" and "large" algae. This suggests that the algal community was forced towards larger forms when rotifers were abundant. In complementary laboratory experiments, the grazing pressure by rotifers was negligible on algae >30 µm, but considerably higher on algae <30µm, whereas macrozooplankton mainly grazed on algae larger than 30 µm. Moreover, experiments showed that cyclopoid copepods rapidly reduced rotifer abundance, suggesting that large zooplankters may affect the size distribution of algae not only directly by selective grazing, but also indirectly by predation on smaller herbivores. Hence, our study suggests that interactions among different size classes of zooplankters affect size distribution and succession patterns in the phytoplankton assemblage.

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