Effects of snail grazing and nutrient release on growth of the macrophytes Ceratophyllum demersum and Elodea canadensis and the filamentous green alga Cladophora sp.

The effects of snail (Lymnaea (Galba) turricula) nutrient release and grazing on young macrophytes and filamentous green algae were examined in a laboratory experiment. Snails released an average of 24.2 mug PO4-P and 48.9 mug NH4-N g(-1) snail FW d(-1). Snails consumed Cladophora sp. at the highest rate (45 mg g(-1) snail FW d(-1)), Elodea canadensis at a lower rate (7 mg g(-1) snail FW d(-1)), and Ceratophyllum demersum at the lowest rate (2 mg g(-1) snail FW d(-1)). Snail nitrogen and phosphorus release rates increased with temperature and were affected by the food that was consumed. After 10 days of exposure in a mesocosm experiment, snails reduced the biomass of Cladophora sp. and E. canadensis, but had no effect on the biomass of C. demersum, which resulted in C. demersum becoming the dominant species. Grazing and environment-conditioning by snails stimulated the growth of Cladophora sp. and had a null or negative effect on the growth of E. canadensis. Selective grazing by snails and associated nutrient regeneration can affect the taxonomic composition of macro autotrophs in lake littoral zones.