The effect of nutrient enrichment on a freshwater community dominated by macrophytes and molluscs and its relevance to snail control.
Cylindrical enclosures (1.25 m high and 0.75 m in diam., constructed of transparent plastic) were used to investigate the effects of increased concn of nitrate and phosphate on populations of macrophytes and snails in a drainage channel in the Lewes Brooks, Sussex, UK, between Apr. and Sep. 1988. Addition of 0.02-0.2 g m-2 week-1 of phosphate caused significant increases in the biomass of Lemna minor and Ceratophyllum demersum, but plants that were rooted in the sediment (Glyceria maxima, Sparganium erectum, Potamogeton crispus, Groenlandia densa, Callitriche obtusangula, Lemna trisulca and Ranunculus circinatus) were not affected. Addition of 0.2-2 g nitrate resulted in phytoplankton blooms, followed by a decline in the biomass of submerged rooted macrophytes. Periphyton biomass was significantly reduced by the higher level of nitrate. There were significant negative correlations between the density of phytoplankton and the numbers of some snails (Physa fontinalis, Planorbis planorbis and Bithynia tentaculata) but not Lymnaea peregra. The relevance of these findings to freshwater snail control are discussed.