SEASONAL-VARIATION OF EPIPHYTIC EXTRACELLULAR ENZYME-ACTIVITY ON 2 FRESH-WATER PLANTS, PHRAGMITES-AUSTRALIS AND ELODEA-CANADENSIS

Extracellular enzyme activity of epiphyton on submerged surfaces of Ph ragmites australis and Elodea canadensis, and in surrounding water, wa s measured in a gravel-pit pond in NE England. Epiphytic enzyme activi ty was greater per unit dry weight of E. canadensis, but was greater o n P. australis when expressed on a plant surface area basis. The ranki ng of the enzymes assayed was the same on both host plants and in the pond water, i.e. phosphatase > beta-D-glucosidase > beta-D-galactosida se and beta-D-xylosidase > sulphatase. Values of epiphytic enzyme acti vity relative to activity in pond water were substantial enough to sug gest that, at well vegetated sites, epiphytic enzyme activity should m ake up an appreciable proportion of whole-pond extracellular enzyme ac tivity. Epiphytic enzyme activity on P. australis varied seasonally, w ith summer maxima, whereas there was no obvious seasonal periodicity o n E. canadensis. Multiple-regression analysis suggested that enzyme ac tivity on P. australis was potentially dependent on both epiphytic bac terial variables (metabolic activity and abundance) and characteristic s of the surrounding water (pH and temperature). In contrast, enzyme a ctivity on E. canadensis was apparently more dependent on surrounding water quality.