Agriculture and Water Quality: A Regional Study

ABSTRACT Water quality, measured on a fisheries ecosystem scale of 1 (good/salmonid) to 6 (bad/fish absent), of forty‐two lowland streams in two Northern Ireland river catchments was inversely correlated with the stocking rate of grazing animals. A decrease in water quality of one class was associated with an increase in the combined grazing/stocking rate of cattle and sheep of 0.6 dairy cow equivalents/ha. This dairy cow equivalent stocking rate was significantly correlated with maximum BOD and total amm.N concentrations and minimum dissolved‐oxygen levels. The worst pollution events, with BOD concentrations in excess of 100 mg/1, occurred at the end of May and were caused by discharges of silage effluent. Smaller BOD peaks, which occurred in late winter and early spring, were related to the land spreading of animal slurries. It was concluded that poultry and pig farms were not having a major impact on water quality.