A BETWEEN-RIVER COMPARISON OF EXTRACELLULAR-ENZYME ACTIVITY

River-water extracellular-enzyme activity in the lowland Rivers Ouse a nd Derwent, northeast England, had much in common. In both rivers, the mean enzyme activities over 15 months differed in the following order : leucine aminopeptidase > phosphatase > beta-D-glucosidase > beta-D-g alactosidase and beta-D-xylosidase. None of the five enzymes assayed h ad significant between-river difference in activity, and there was sig nificant between-river correlation of beta-D-glucosidase, phosphatase, and leucine-aminopeptidase activity. The common enzyme regimes were p robably more due to between-river similarity of planktonic microbiota than to similar physico-chemical conditions. The potential for glucose uptake by bacterioplankton closely followed beta-D-glucosidase activi ty in magnitude and periodicity. The potential for leucine uptake, how ever, was much less than leucine-aminopeptidase activity; hence rate o f leucine release probably did not limit leucine uptake. There was an appreciable and highly variable proportion of free (<0.2 mu m) enzyme activity in river water; ranges were beta-D-glucosidase 10-30%, phosph atase 53% to apparently 104%, and leucine aminopeptidase 22-98%. These free enzymes did not necessarily originate from planktonic microbiota and may explain the fairly loose coupling between whole-water enzyme activity and microbial variables. Marked downstream increase in enzyme activity, along about 104 km of the River Derwent, was found on only one of three sampling days; hence the single site used for regular sam pling was reasonably representative of most of the river.