DECREASE IN DAPHNIA EGG VIABILITY AT ELEVATED PH

The effect of high pH on the reproduction of two Daphnia galeata clone s was experimentally investigated in the laboratory. We observed that the mortality of juveniles and adults did not increase with increasing pH in the range pH 9.0-10.5, which agrees with what is generally repo rted in the literature for cladoceran zooplankton and suggests that th e threshold value for mortality is between pH 10.5 and 11.5. However, both egg mortality and the proportion of stillborn neonates increased at pH 10.0 and above, and the two clones differed in their sensitiviti es to pH. Consequently, pH already affects population growth rate mark edly from pH 10.0 onward. Because pH values greater than or equal to 1 0.0 are common during spring and summer in many eutrophic and hypertro phic lakes due to intense photosynthetic activity, we expect that high pH has a much larger effect on the population structure of Daphnia an d the community composition of microcrustacean zooplankton in such wat er bodies than was assumed previously.