Abstract
Population density, biomass, growth and reproduction rate of the gastropods Lymnaea peregra (Müller) and Potamopyrgus jenkinsi (Smith) were monitored over a 21 month period in three experimental earth channels, one carrying clean river water and the others 25 and 50% treated sewage effluent mixed with river water. Each channel was divided into riffle and pool stretches; the abundance and biomass of both species were higher in the riffles. The abundance and biomass of L. peregra were significantly greater ( P < 0.01) in the riffle in the presence of effluent. Concentration of effluent did not have a significant effect on abundance or biomass. The abundance and biomass of P. jenkinsi were significantly greater ( P < 0.001) in riffle and pool in the absence of effluent. In addition, the abundance and biomass of P. jenkinsi were significantly greater ( P < 0.05) in the 25% effluent channel than in 50% effluent. No significant differences in the growth of L. peregra between channels were observed. The average number of eggs and of egg masses laid by L. peregra was significantly greater ( P < 0.01) in riffle and pool in the presence of effluent. In the riffle, significantly more masses ( P < 0.05) and more eggs per adult ( P < 0.01) were laid in the 25% effluent channel than in 50% effluent. No differences were observed in the reproduction rate per adult of P. jenkinsi between the 0 and 25% effluent channels. It is concluded that population differences between channels are probably largely the result of differing mortality rates.
Published Version
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