Temperature effects on body size of freshwater crustacean zooplankton from Greenland to the tropics

The body size of zooplankton has many substantive effects on the function of aquatic food webs. A variety of factors may affect size, and earlier studies indicate that water temperature may be a particularly important variable. Here we tested the hypothesis that the body size of cladocerans, calanoids, and cyclopoids declines with increasing water temperature, a response documented in an earlier study that considered only cladoceran zooplankton. We tested the hypothesis by comparing body size data that were available from prior studies of lakes ranging from 6 to 74 degrees latitude and encompassing a temperature range of 2-30 degrees C. Cladoceran body size declined with temperature, in a marginally significant manner (P = 0.10). For cyclopoids, the decline was more significant (P = 0.05). In both cases, there was considerably more variation around the regression lines than previously observed; suggesting that other variables such as fish predation played a role in affecting size. Calanoid body size was unrelated to temperature. In contrast with cladocerans and cyclopoids, perhaps calanoid body size is not metabolically constrained by temperature or is differently affected by changes in fish predation occurring with increasing temperature. The unexpected result for calanoids requires further investigation.