Temporal and spatial patterns of extreme low flows and effects on stream ecosystems in Otago, New Zealand

The temporal and spatial patterns of summer extreme low flows and effects on stream ecosystems were evaluated throughout the Otago Region of the South Island of New Zealand during a severe drought in 1998–1999. Flows, water quality, and aquatic biology were monitored bimonthly at 12 locations as part of a long-term regional monitoring programme and results were evaluated and compared among summer 1998–1999 and all previous summers, as well as among three major subregions. Flows during the drought were extremely low for prolonged periods in many locations, particularly in North Otago. At most sites temperatures were slightly higher for a longer period than during other summers. In predominantly agricultural/pastoral catchments, widespread bacterial contamination of streams occurred due to increased livestock use of watercourses and decreased dilution during low flows. Concentrations of other contaminants derived from non-point sources, including nitrogen, phosphorus, and sediment, decreased in many locations due to the lack of rainfall and runoff events. Electrical conductivity generally increased as a result of the lack of dilution and increased evaporation and groundwater inputs. Overall water quality was worst in agricultural catchments in South Otago, and returned to conditions prior to the low flows by late autumn in most areas.