Grazing impact of two aquatic invertebrates on periphyton from an Andean-Patagonian stream

Abstract

The mayfly Meridialaris chiloeensis and the snail Chilina dombeiana were observed to coexist and develop abundant populations in several Andean streams. In this study we examined and compared the mouthpart morphologies and the grazing mechanisms of these two species. In addition, through field experiments we analysed the grazing effect on periphyton composition and biomass. Results showed that the herbivores contrasted in their mouthpart morphology and foraging behaviour but would play a similar ecological role, since they both can be considered as scrapers. Experimental results indicated that the individual mayfly effect on chlorophyll-a and - ash free dry mass was lower than that of the individual snail. However, considering the spring and autumn abundances of both populations in a natural environment, their grazing impact might be similar or even higher for the mayfly. In addition, M. chi- loeensis depressed the rosette forming algae and favoured the prostrate ones. As a re- sult, the mayfly grazing produced a community dominated by Nitzschia palea instead - of Achnanthes minutissima that dominated the grazer-free controls, while the snail changed the taxonomic composition very little.