SYNERGISTIC EFFECTS OF FOOD CHAIN DYNAMICS AND INDUCED BEHAVIORAL RESPONSES IN AQUATIC ECOSYSTEMS

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The aim of the present study was to test the hypothesis that temporal differences in food chain composition affect lower trophic levels not only directly, by predation and grazing, but also indirectly, by inducing avoidance behavior. In a field study, the recruitment rate from the sediments to water of two algal species (Gonyostomum semen and Peridinium sp.) was higher at low than at high biomass of herbivorous zooplankton. In complementary laboratory experiments, where abiotic conditions were standardized, the presence of live, as well as dead, herbivores reduced the recruitment rate of both Gonyostomum semen and Peridinium sp. These results suggest that some algal species are able to adjust their recruitment behavior in response to the likely risk of being grazed. Together with morphological adaptations (e.g., spines and large size) common among many algal species, such an induced behavioral response is an important adaptation to reduce cell mortality. As shown in this study, this behavioral response may have a profound impact on dominance and succession patterns in algal communities. The high zooplankton biomass observed during the first year of the field study was caused by failed reproduction of the dominant fish species in the lake (roach, Rutilus rutilus). Hence, food chain interactions (low predation on zooplankton, leading to high biomass of herbivorous zooplankton) may act in concert with more indirect, predator-avoidance behavior in structuring the phytoplankton community.