Limnol. Oceanogr., 44(8), 1999, 1882–1893

To test the hypothesized inhibition of high-latitude marine bacteria living at lower temperatures when organic matter is scarce, the effects of temperature and organic substrate concentration on pelagic microbial heterotrophy were investigated in the perennially cold surface waters of a summertime arctic polynya. Utilization (incorporation plus respiration) of radiolabeled amino acids was measured as a function of increasing added substrate concentration at in situ (subzero) temperature and under short-term warming. Results analyzed using analysis of variance (ANOVA) showed that increased substrate concentration had a significant effect on utilization rates at all stations, suggesting that communities were always living well below saturating levels of substrate. About half of the communities sampled revealed a significant temperature response using ANOVA; interactions between temperature and substrate concentration were detected only rarely. Kinetic parameters, used to link microbial activity and substrate utilization with temperature sensitivity, exhibited mixed responses to short-term warming. Maximum specific utilization rates showed the greater temperature sensitivity, with Q10 values ranging from 0.25 to 13. Specific affinities responded to temperature significantly at only about half of the stations in the polynya. Psychrophilic behaviors (e.g., highest specific affinities and oligotrophic capacities at lower incubation temperatures) were observed at stations most likely to be influenced by direct Arctic Ocean outflow. Complete agreement with the hypothesis of enhanced substrate requirement by bacteria living at subzero temperature was not found. An improved understanding of the diversity of cold-tolerant and cold-loving microorganisms is needed before generalizing or predicting the role of temperature in the cycling of organic matter at high latitudes. Temperature–substrate interactions may underlie seasonal cycles of marine bacterial activity at all latitudes, greatly influencing the cycling of organic matter through marine food webs (Pomeroy and Wiebe 1993). A bacterial growth limitation at low temperatures is thought to occur at the level of substrate uptake via reduction of reaction rates and membrane fluidity (see Gounot 1991 for review) and by slower diffusion rates (Jumars et al. 1993). In some perennially cold regions, a correspondence has been observed between in situ temperature and substrate concentrations needed for measurable bacterial production and substrate utilization (Pom-