Effects of pH, humic substances and animal interactions on survival and physiological status of Asellus aquaticus L. and Gammarus pulex (L.) : A field experiment.

The mortality and physiological status (body water content) of Asellus aquaticus (Isopoda) and Gammarus pulex (Amphipoda) were measured after 25 days exposure in 20 natural streams with a pH range of 4.3-7.5 and a colour range of 8-280 mg Pt L-1. In addition, the effects of keeping the animals as single species or together were studied. The response of Gammarus to low pH was an increased mortality and lower physiological status of surviving individuals in streams with a pH lower than 6.0. In Asellus the physiological status was correlated with pH, while the mortality was not pH dependent. The effects of humus on the physiological status of Asellus was significant when fitted to a second order polynomial function. The influence of humus can, however, be regarded as small relative to pH. The interactions between the species could be described as asymmetric under optimal conditions of high pH and low humus concentrations, where the presence of Gammarus decreased the survival and physiological status of Asellus. Acid stress did not seem to reverse the direction of this asymmetry, but the presence of Gammarus improved the physiological status of Asellus at pH lower than 6.0. Since the presence of Asellus did not increase the mortality or decrease the physiological status of Gammarus, this could be explained by Asellus feeding on Gammarus that died from physiological stress solely. This mechanism suggests that food quality, and thus effects of diffuse competition, can be important for the ability to withstand acid stress. The results, though, give no support for the hypothesis that competition from Asellus is important for the disappearance of Gammarus during the acidification of streams.