The Effects of Hypoxia on Threshold Food Concentrations in Different Daphnia Species

Numerous studies have revealed a negative correlation between the body size and temperature among a variety of aquatic ectotherms. Many studies at individual and population levels indicated that this mechanism may be explained by the decrease of competitive abilities of larger- over smaller-bodied individuals, as the production of larger-bodied individuals is more limited due to greater susceptibility to decreased oxygen concentrations (i.e., environmental hypoxia) at elevated temperatures. However, this hypothesis is still not tested at the community level. To test this, we performed several experiments on the food thresholds (which is a proxy for competitive ability) of 6 zooplankton (Daphnia) species varying in body size, at high or low oxygen concentrations. Contrary to the hypothesis tested, hypoxia increased threshold food concentrations to a relatively greater extent in smaller species than in larger ones. This may be attributed to the better evolutionary adaptations of larger-bodied daphnids to oxygen-poor environments manifested in higher production of haemoglobin. The results obtained in this study cannot exclude the possibility that environmental hypoxia is responsible for the temperature-size pattern in aquatic ectotherms, as our experiments did not take into account the long-term energetic costs of expedited haemoglobin synthesis, which could shift size-dependent competitive power.