Integrating resource utilization and temperature in metabolic scaling of riverine bacterial production

The study of metabolic scaling in stream ecosystems is complicated by their openness to external resource inputs. For heterotrophic bacteria, which are a large component of stream metabolism, it may be possible to integrate the effects of resource availability and temperature on production using metabolic scaling theory and the kinetics of extracellular enzyme activity (EEA) associated with the degradation of major nutrient pools. With this goal, we analyzed previously published data on EEA and bacterial production for two rivers in northwestern Ohio, USA. The EEA data included estimates of apparent Vmax, a measure of catalytic capacity, and apparent Km, a measure of available substrate concentration, for six extracellular enzymes (alpha-glucosidase, beta-glucosidase, aminopeptidase, protease, phosphatase, and acetyl esterase). Sampling was done over an annual cycle with a temperature range of 4 31 degrees C, while EEA assays were conducted at 20 degrees C. The EEA kinetic measures were scaled to ambient stream temperature using an activation energy (Ea) of 0.5 eV (8.01 x 10(-20) J) and converted to estimates of the turnover rate (St) of their associated substrate pools. The St values associated with protein utilization, the largest substrate pool, had the strongest relationship to bacterial production (r2 = 0.49-0.52); those for carbohydrate utilization, the smallest substrate pool, had the weakest (r2= 0.09-0.15). Comparisons of apparent Ea over the annual cycle showed that the trophic basis of bacterial production switched from relatively high carbohydrate consumption in autumn and winter to relatively high protein consumption in spring and summer, corresponding to seasonal dynamics in plant litter inputs and algal production, respectively. Over the annual cycle, the summed substrate generation rate of the six enzymes was similar in magnitude and strongly correlated with bacterial production (r2 = 0.56). This approach combines effects of substrate pool size, catalytic capacity, and temperature on bacterial production and could be used to compare ecosystems along latitudinal gradients where resource, rather than temperature, effects on metabolic scaling are of greater magnitude.