River water quality trends and increased dairying in Southland, New Zealand

Trends in six water quality variables measured at 29 Southland, New Zealand, freshwater sites between July 1995 and June 2001 have been assessed. These are compared to changes in livestock numbers to assess whether water quality changes could have been driven by the intensification and expansion of dairy farming and/or by other pressures. Trends were assessed for raw and for flow-adjusted data, the latter being a better indicator of effects of changing external pressures on water quality (an observed water quality trend may be merely the result of changes in river flow). Results indicate that increased dairy farming has been associated with increasing concentrations of dissolved reactive phosphorus. There has been a worsening in other water quality variables (oxidised nitrogen, dissolved oxygen) but these also occurred in non-dairying catchments. An improvement in observed water clarity disappeared after flow adjustment. Continued monthly water quality monitoring is necessary to watch for emerging trends.