Changes in life-history parameters of Daphnia longispina (Cladocera, Crustacea) as a function of water chemistry

Health, distribution and life-history of aquatic crustaceans strongly depend on water hardness. However hardness is commonly correlated with alkalinity, which highlights the need to assess the joint effects of both hardness and alkalinity. This study aims to test the hypothesis that water hardness and alkalinity affect the life-history parameters (growth, reproduction and population growth rate) of D. longispina. Following this, life table experiments were carried out in order to study the effects of high levels versus low levels of water hardness and alkalinity. Low levels of hardness and alkalinity caused a significant reduction in the growth of daphnids after a 7-days period, which augmented during the 21-day-test period reaching a 14.5% reduction compared to high hardness and alkalinity. Allied to the reduced growth, daphnids reared at low hardness and alkalinity showed delayed reproduction, increased body length at first reproduction, reduced fertility at first brood and, consequently, a 36.6% reduction in total fertility, compared to daphnids reared at high hardness and alkalinity. Accordingly, daphnids with the same size produced smaller broods at low hardness and alkalinity, reflecting a direct effect of water chemistry on daphnids reproduction. The impaired growth and reproduction at low hardness and alkalinity levels was likely a consequence of increased maintenance costs, and was not related to changes in the feeding activity. Population growth rate of daphnids reared at low hardness and alkalinity was 13.4% lower than that of daphnids reared at high hardness and alkalinity. Thus, despite D. longispina can survive at low hardness and alkalinity, their life-history parameters are significantly affected. This study raises concerns about the effects of decreasing hardness and alkalinity, which has been reported in Europe and North America, on populations of D. longispina and, thus, on the structure of aquatic ecosystems.