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Mechanisms influencing zooplankton community structure in a shallow marl lake

Zooplankton play a central role in the pelagic food web oflakes. Populations are characteristically regulated by a combination of predation (top-down) and resource limitation (bottom-up) (McQUEEN et al. 1986). Influence of predation or resource limitation varies both between and within lakes and is dependent on fish recruitrnent and nutrient supply. Size-selective predation by fish can determine zooplankton community structure (BROOKS & DonSON 1965). Heavy predation by visually-foraging planktivorous fish can result in reduced zooplankton biomass, reduced size of individual zooplankton species and a reduction in large taxa such as Daphnia. Zooplankton are influenced strongly by both the quantity and quality o f their phytoplankton food supply. Food availability is often an important factor influencing zooplankton population dynamics. Daphnia, for example, are capable of responding to dramatic fluctuations in their food supply with considerable accuracy (GEORGE & REYNOLDS 1997). Food quality is also important. The elemental ratios of algae can vary considerably and will be dependent on the availability of these nutrients in the environment (SOMMER 1989). Heterotrophs, in contrast, maintain a comparatively constant, species-specific, ratio of cellular C:N:P (STERNER & HESSEN 1994). This homeostatic stoichiometry could be a significant factor influencing zooplankton community dynamics (HESSEN 1992). Species with high P requirements such as Daphnia (ANDERSEN & HESSEN 1991), will experience reduced growth, fecundity and survivorship when fed P-limited food (STERNER et al. 1993). Direct nutrient limitation could, therefore, be an important, but little studied aspect of primary consumer structure and production in pelagic foodwebs. This study investigated the principal drivers of zooplankton community structure in a shallow marl lake in westem Ireland. Both top-down and bottomup controls were considered, and their relative importance on the zooplankton population dynamics ofthe lake assessed.