Resource limitation by freshwater snail (Stagnicola vulnerata) grazing pressure: an experimental study

The freshwater snail Stagnicola vulnerata was introduced into experimental channels at five different densities to compare the effects of increasing g razing pressure on the algal community. Net community metabolism, carbon in corporation and chlorophyll-a concentration decreased (by more than 50 %) w ith increasing snail density over 14 days. The relationship between periphy ton parameters and snail biomass predicted that the periphyton density avai lable for snails would decline according to a negative exponential function as snail densities increase. Cocconeis placentula, Achnanthes spp. (lanceolata, minutissima) and Mougeot ia sp. filament densities decreased with grazing pressure. However, basal c ells of Stigeoclonium tenue were favoured at the highest snail density. The biovolume of the algal assemblages also decreased in channels with more gr azers. The dominant physiognomic classes in the pre-grazing conditions were crustose and prostrate. Filamentous forms increased in ungrazed channels a nd in those with the lowest and the highest. densities. Snail grazing simpl ified both algal taxonomic and physiognomic structure but the pattern depen ded on grazer density. When density increases, snail interference competition may be partially res ponsible for changes in functional and structural responses of periphyton a ssemblages.