Consequences of fish predation, migration, and juvenile ontogeny on zooplankton spring dynamics

Abstract

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...

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Local environment overrides regional climate influence on regime shift in a north temperate lake

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