In order to disentangle if and when resource supply and adult and young-of-the-year (0+) fish predation affect zooplankton dynamics during spring, we monitored zooplankton during three consecutive years in a lake in southern Sweden. We also experimentally assessed 0+ fish predation rates and estimated changes in predation rates of adult fish on zooplankton. Decline in abundances of large-sized zooplankters in early spring was not caused by 0+ fish predation. Instead, this decline was most likely a combined result of size-selective predation from adult fish (stationary in the lake and from those returning from surrounding streams) and competition for diminishing algal food resources. On the other hand, the decline in medium-sized zooplankton in the lake during spring was strongly affected by 0+ fish. Hence, during spring, zooplankton are facing predation both from adult fish selecting large prey and from 0+ fish, which start feeding on small-sized prey and eventually switch to larger. Neither predation by different ontogenetic stages of fish (adult and 0+) nor resource supply shape the zooplankton spring dynamics, but rather they affect the timing and strength of these events. 0+ cyprinids tend to have stronger effect on zooplankton dynamics than other taxa of 0+ fish. A combination of predation from adult and 0+ fish during spring is the main mechanism behind the crash of the zooplankton community, which in many lakes leads to the termination of the clear-water phase.The spring, with its rapidly increasing temperature, is an important period for the development of the zooplankton summer community in temperate regions, mirrored in dramatic fluctuations in population densities. These conspicuous spring fluctuations are due to changes in reproductive rates and resource supply but also to increased feeding rates from predators. The total predation rate on zooplankton may be divided into several components. First, there is predation from adult fish present in the lake year-round, and the seasonal fluctuations in their predation rate is driven by temperature (Lessmark 1983) and is generally most intense on larger zooplankton size classes (Brooks and Dodson 1965). Second, a dramatic change in the predation rate occurs with the hatching and recruitment of young-of-the-year fish (0+) in the spring. They start their life by feeding on small zooplankton but are within a few weeks able to feed on larger size classes (Mills and Forney 1983;Mehner and Thiel 1999). Thus, smaller size classes of zooplankton should suffer from 0+ predation earlier in the season than larger ones. It has previously been suggested that 0+ predation drives succession in the zooplankton community (Cryer et al. 1986;Gliwicz and Pijanowska 1989), whereas other studies have concluded that 0+ fish predation probably is too weak to explain zooplankton spring dynamics (Cushing 1983;Boersma et al. 1996) and that instead other processes, such as competition and resource supply (i.e., bottom-up processes) are more important. In a review, Mehner and Th...
It is well known that the effects of direct (lethal) predator-prey interactions propagate through food webs (the trophic cascade). However, in the present study we show for the first time that, parallel to the trophic cascade, there exists a ''behavioral cascade'' in the sense that behavioral responses, induced by the nonlethal presence of a top predator, are transmitted down the food chain over more than one trophic link. By using a new method, horizontal echo sounding, in an enclosure study in a shallow lake in southern Sweden, we recorded the swimming activity and refuge use of young-of-the-year (0ϩ) roach (Rutilus rutilus) in the presence and the absence of a caged predatory fish. By connecting the predator avoidance behavior of 0ϩ roach with that of zooplankton throughout summer, we show that species interactions are more dynamic than had been predicted earlier by food web theory. In our study, 0ϩ roach changed their behavior by becoming less active in early summer and by hiding in a refuge in late summer in the presence of a piscivore, and this caused Daphnia to spend more time feeding in the open water than when piscivores were absent. Thus, we conclude that, to explain patterns of habitat use in natural systems, it is of crucial importance that we recognize the existence of behavioral cascades.
Summary 1. We performed both a large‐ and a small‐scale echo sounding study on the spatial and temporal distribution of fish (mainly roach, Rutilus rutilus and perch, Perca fluviatlis), as well as a small‐scale study of zooplankton distribution in the small, shallow and eutrophic Lake Hanebjerg in Denmark. In the small‐scale study, sampling was conducted in open water as well as in the edge zone immediately outside two different types of vegetation. 2. Fish daytime abundances differed between the northern and the southern parts of the lake and, on a small scale, small fish aggregated in the edge zones during day, preferably outside dense emergent vegetation. Copepods avoided emergent vegetation, while cladocerans showed no habitat preference. Both small fish and cladoceran numbers were found to be higher during night than day. 3. The relative abundance (number per sample) of cladocerans in the edge zone immediately outside vegetation was negatively correlated with the relative abundance of fish in that zone. There was no correlation between cladoceran and fish abundance in open water, or between the relative abundance of copepods and fish. 4. The presence of pelagic piscivores in combination with avoidance behaviour of both fish and zooplankton is a likely explanation for the observed distribution of small fish and cladocerans in Lake Hanebjerg. Both small‐ and large‐scale distribution patterns may be dependent on the type and distribution of complex structure in the lake. Even in a small lake, large‐scale patterns may affect the interpretation of small‐scale data.
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