FISH-FARM EFFLUENTS IN RIVERS .1. EFFECTS ON BACTERIAL-POPULATIONS AND ALKALINE-PHOSPHATASE ACTIVITY

Bacteria increased in abundance (colony-forming units and acridine-orange direct counts) and heterotrophic activity (glucose-mineralization rate) in water and sediment downstream of fish farms on two rivers in N.E. England. A fish-farm on a third watercourse, which also received a substantial discharge of effluent from a sewage-works upstream of the fish farm, had little effect on bacterial populations. Alkaline phosphatase activity (APA) was also investigated in the water of one river. This too increased downstream of the fish farm. Increase in APA was accompanied by increase in total bacteria but not by increase in phytoplankton biomass. However, partition of water samples by membrane filtration indicated that most APA was not associated with the microflora but was free in the water, hence excretion by rainbow trout in the fish farm was suggested as the origin of the extra APA in the river. © 1990.