Snail grazing effects on the composition and metabolism of benthic diatom communities and subsequent effects on fish growth

Abstract

Eastern mud snails (Ilyanassa obsoleta) in densities of zero, six or twelve snails were placed in flow-through-laboratory microcosms (765 cm2) and incubated for five weeks. Other tanks were raked daily to a depth of 10 mm. Grazing by low densities of snails significantly increased chlorophyll standing stock, respiration and gross photosynthesis as measured by light and dark exchange of oxygen and carbon dioxide compared to untreated tanks. The standing stocks of algal pigments, respiration and photosynthesis were depressed in the microcosms which received the 12-snail or the raking treatments. Simulating snail excretion by fertilization with ammonium increased chlorophyll standing stock by a similar magnitude, but this effect could be inhibited by raking the sediments daily. At low densities Ilyanassa's acceleration of nutrient cycling stimulates algal growth, but this effect is overwhelmed at higher densities by overgrazing and stirring inhibition. The dominant benthic algal group in the containers were pennate diatoms. Grazed containers contained a larger percentage of the non-motile classes of diatoms, as compared to the motile forms which predominated in the untreated microcosms. The snails are able to selectively graze these mobile species. Their gut contents are enriched in carbon. nitrogen and algal pigment content by 20-40 times over the surface sediments. A small, non-significant, growth effect can be seen in the snails' response to density changes, but another marsh consumer, Fundulus heteroclitus, grows faster at low snail densities when snails are absent.