1. We examined the responses of two species of Daphnia to changes in food phosphorus (P) content, with animals reared at three different water temperatures. Specifically, we measured mass-specific growth rate (MSGR), body P content and respiration rate of Daphnia magna and Daphnia pulex acclimatised to 10, 17.5 and 25°C and fed food carbon : phosphorus (C : P) ratios of either 150 or 500. 2. The responses of these three physiological variables to temperature-food quality interactions were species-specific. There was a significant interactive effect of temperature and food quality on D. magna, as the greatest proportional effect of food quality on growth was observed at 10°C and reductions in body P because of low food P content were relatively greater at 25°C. These effects may reflect the temperature dependence of mechanisms that reduce elemental constraints associated with food quality in D. magna. By contrast, there were no interactive effects between food quality and temperature on MSGR, body P or mass-specific respiration of D. pulex. 3. It thus appears that temperature can alter food quality effects on Daphnia but the nature of these alterations depends upon the daphniid species and its thermal adaptability. Significant temperature-food quality interactions will complicate efforts to understand zooplankton nutrition in nature and warrant future consideration.

Abstract. We investigated the effect of land cover on the metabolic scaling of the freshwater crayfish, Orconectes rusticus, by comparing the field metabolic rate (FMR) of populations from streams flowing through different natural and agricultural land cover. When data from all streams were pooled, the metabolic mass-scaling exponent was approximately 0.71. However, both the strength and nature of FMRmass relationships varied among streams (slopes from 0.61 to 0.91). This variability in scaling exponents was significantly correlated with two types of land cover, the proportion of monoculture (row cropping) agriculture (positive slope, P , 0.02, R 2 ¼ 0.75) and the proportion of wetlands (negative slope, P ¼ 0.05, R 2 ¼ 0.57), in the riparian zone of each stream. In a complementary laboratory study, we found the metabolic response of crayfish to differ among animals consuming plant and animal based foods. Crayfish consuming animal-based foods had higher respiration rates than conspecifics consuming plant-based foods. As O. rusticus exhibits variable feeding rates and foraging behavior, differences in the availability and quality of food that accompany changes in catchment land cover provides a potential mechanism for the observed site-dependence of FMR-mass scaling. Intraspecific variability of FMR-mass scaling in stream crayfish and its relationship to catchment land use is further evidence that organismal physiological flexibility and acclimation to specific environments complicates efforts to use general mass-scaling laws to explain disparate ecological phenomena.Key words: invertebrate ecology; land use; metabolism; Ontario; Orconectes rusticus; respiration; stream ecosystems.