THE EFFECT OF NUTRIENT ENRICHMENT ON A FRESH-WATER COMMUNITY DOMINATED BY MACROPHYTES AND MOLLUSKS AND ITS RELEVANCE TO SNAIL CONTROL

(1) Cylindrical enclosures (1.25 m high, 0.75 m diameter) were used to investigate the effects of increased concentrations of nitrate and phosphate on populations of macrophytes and gastropod snails in a drainage channel in the Lewes Brooks, Sussex, U.K. (2) An increase in phosphate caused significant increases in the biomass of the floating plant, Lemna minor and the non-rooting plant, Ceratophylum demersum, but plants with roots in the sediment were not affected. (3) An increase in nitrate resulted in phytoplankton blooms, followed by a decline in the biomass of submerged rooted macrophytes. (4) The periphyton biomass was significantly reduced with high increases in nitrate. (5) There were significant negative correlations between the phytoplankton chlorophyll-a day-1 values and the numbers of Physa fontinalis, Planorbis planorbis and Bithynia tentaculata, when all the results were taken together. There was no such correlation in the case of Lymnaea peregra. (6) The results obtained are discussed with particular reference to the causative mechanisms and snail control.