LARS-ANDERS HANSSON 
Abstract. 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 Gonycstomum 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 resp onse 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.
Key words: algae; aquatic ecosystems; food chain; food web; Gonyostomum semen; grazing; herbivory; induced behavior; Peridinium sp.; phytoplankton; predation; Sweden; zooplankton.
In combination with abiotic features of an ecosystem, direct interactions such as predation, grazing, and competition explain a major part of the variation in abundance, biomass, and succession of organisms. Accordingly, these processes have long been the focus of ecological research, including competition, food web, and optimal foraging theories, as well as succession models such as the PEG model for aquatic ecosystems (Sommer et al. 1986). However, during recent years several studies performed in terrestrial as well as in aquatic systems have shown that many organisms gather information from their environment by being receptive to chemical signals exuded not only by conspecifics, but also by potential predators and grazers. Waterborne chemicals are known to elicit morphological adjustments in prey organisms that reduce their predation rates. Examples are higher body shape in crucian carp (Carassius carassius; Bronmark and Miner ), spine formation in rotifers (Stemberger and Gilbert 1987), helmet and neck teeth formation in many cladoceran zooplankton species (Tolirian 1995), and colony formation in the green alga Scenedesmus (van Donk and Hessen 1993, Lampert et al. 1994, Lurling and van Donk 1997). Furthermore, behavioral responses have been demonstrated in flagellated algae, which avoid entering the water column when grazing zooplankton are abundant (Hansson 1996a, b, Rengefors et al. 1998). Unfortunately, our knowledge of the "chemical network" that transfers information from predators to their prey is still negligible. However, such signals are likely to affect the outcome of predator- prey interactions, as well as successional patterns in aquatic ecosystems....