Inferred effects of lake acidification on Daphnia galeata mendotae

Extensive surveys of lakes in 4 areas of Ontario representing a wide range of sulphur deposition regimes, demonstrated that the ubiquitous planktonic crustacean Daphnia galeata mendotae Birge occurred in 44-92 per cent of lakes with pH greater than 6.0, where it contributed 11-14 per cent of average herbivorous zooplankton biomass, but was absent from virtually all lakes with pH less than 5.5. In all data sets D. galeata mendotae biomass was correlated significantly with lake pH, and correlation with pH was stronger than that with other limnological parameters in low pH lakes. In bioassays conducted on laboratory cultures at pH 6.0-5.2, survival was reduced at pH less than 6.0, and absent at pH less than 5.4. The 4-day LC50 pH values ranged from 5.69-5.98, and close agreement between calculated lethal acid thresholds and the biomass distribution suggested that lake acidity due to anthropogenic activity was the major factor controlling D. galeata mendotae biomass in the study area.