Are things getting better or worse?
Water quality improvement was evident in levels of ammoniacal nitrogen (NHx-N), clarity, turbidity, and faecal coliforms – consistent with national patterns. Improvements in these parameters benefit aquatic ecosystems, and value for commercial and recreational use. Organic pollutants have decreased in New Zealand since the 1980’s, reflecting better management of point source discharges. High levels of these parameters, particularly ammoniacal nitrogen and faecal coliforms, are commonly associated with dairy shed effluent and municipal wastewater.
While ammoniacal nitrogen has decreased, other forms of nitrogen – total nitrogen and nitrate - have increased, which is also consistent across New Zealand. It appears that while point source pollution has decreased, diffuse source pollution has increased. Total nitrogen levels in the lower Grey and Buller Rivers are below those typical in New Zealand Rivers but the rate of increase has been much greater than that observed nationally. There has been a recent trend for increasing intensification of agriculture across the country. Since the 1980’s, stocking rates and fertiliser application has increased, with the use of nitrogen fertilisers doubling since the 1990’s.
Phosphorus associated with increasing intensification of agriculture could be offset by gains made through better management of point source pollution. This could have been the case in the Buller River, but not the Grey River, where a meaningful increase in phosphorus was apparent.
Despite the likely effect of anthropogenic pressures on water quality trends, the role of natural events and climatic influence should be considered as a potential factor responsible for changes in some of the water quality parameters observed over time.