Enzymatic analysis of riverine bacterioplankton production

Bacterial production is the entry point for detrital macronutrients into aquatic food webs. Many factors affect productivity, but the heterogeneity of detrital substrates and the diversity of microbial communities confound simple relationships between carbon supply and growth. We tried to link the two by analyzing extracellular enzyme activities. Water samples were collected from three rivers and assayed for bacterial productivity and the activities of eight enzymes. Production varied among systems, peaking at 644, 170, and 68 µ mol C liter ‒1 d ‒1 in the Ottawa (Ohio), Maumee (Ohio), and Hudson (New York) Rivers. V max values were generally correlated with productivity. The mean ratios of productivity per unit peptidase and esterase activity were similar among rivers, whereas carbohydrase and phosphatase ratios varied widely. The data were used to evaluate a model that relates productivity to carbon flow by using enzyme activities as indicators and assuming an optimum resource allocation relationship among C‐, N‐, and P‐acquiring enzymes. The data supported the model, but predictive power was low. Bacterial productivity generally increased with inorganic nutrient availability, but high levels of productivity at any specific eutrophic state required sources of both saccharides and amino acids.