Snail Grazing Effects on the Composition and Metabolism of Benthic Diatom Communities and Subsequent Effects on Fish Growth

摘要

Eastern mud snails (Ilyanassa obsoleta) in densities of zero, six or twelve snails were placed in flow-through laboratory microcosms 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.