The role of dissolved organic matter (DOM) in the trophic dynamics of aquatic systems is difficult to study. One approach is to focus on the activity of the extracellular enzyme systems that heterotrophic bacteria use to obtain nutrients from DOM. These data are easy to acquire, but difficult to interpret, because we lack general models that relate these activities to the two sides of the trophic reaction: the DOM pool and bacterial production. To address this gap, we collected water samples from the Maumee River on nine dates from May 1996 through November 1996. The water was filtered and placed in bottles to which a variety of substrates were added (final concentration 20 µM unless otherwise noted): ammonium, leucine, albumin (1 mg liter-1), glucose, cellobiose, starch (1 mg liter-l), vanillin, and tannin (2 mg liter-1). After 20 h at ambient river temperature, respiration, productivity, growth efficiency, and the activities of a-glucosidase, P-glucosidase, leucine aminopeptidase, phosphatase, and esterase were measured. The glucosidases were the most responsive enzymes, followed by phosphatase and peptidase. Saccharide and albumin additions produced the largest increases in production and growth efficiency. Phenolic amendments generally depressed growth efficiency. The results suggest that production was primarily carbon limited: Peptides appeared to be a major component of the diet, but high productivities were associated with the availability of saccharides. During the summer, production appeared to be fueled by lysates and exudates generated through the microbial loop. Lower ratios of productivity (e.g., enzyme activity and positive metabolic responses to phenols) suggested that allochthonous carbon may have been important during spring and autumn.