Grazing Effects on Periphyton byTheodoxus fluviatilis(Gastropoda) in a Lowland Stream

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Abstract
The development of attached algal communities in a lowland stream in central Sweden was investigated with reference to differing levels of grazing pressure. An enclosure-exclosure method was used to measure periphytic biomass accural and community structure in the presence of low, natural, and high densities of the gastropod Theodoxus fluviatilis. A highly significant difference (p < 0.01) in chlorophyll a, carbon, and nitrogen was observed between chambers with low grazer density and those with natural and high grazer densities. Chambers with natural and high densities of T. fluviatilis had lower percent composition of filamentous green algae (mainly Cladophora) and lower C/N ratios in the periphyton.