Why are median phosphorus concentrations improving in New Zealand streams and rivers?

The enrichment of phosphorus (P) in streams and rivers can impair water quality. Between 1994-2013, 41% of sites showed signs of decreasing total P concentrations, while 65% were decreasing between 2004-2013; similar improvements were seen for filterable reactive P (FRP). We used a range of datasets and sources to provide a semi-quantitative analysis of the cause of this improvement. We found little evidence that the improvement was caused by a decrease in soil Olsen P concentrations or imported P as fertilisers and feed, that farmers were changing to low water soluble P-fertilisers, or that a greater nitrate loading was assimilating FRP in groundwater and stream and river sediments. Possible causes of a decrease in P concentrations were attributed to land use change resulting in lower erosion, the greater assimilation of P in New Zealand soils – associated with more use of nitrogen-fertilisers, and the better awareness of P as an environmental issue. However, the most probable causes of the improvement were: the greater use of strategies to mitigate P loss from land and water; the use of guidelines to direct the use of strategies; and policy instruments that require better water quality outcomes. These findings support an emphasis on the development and implementation of strategies to mitigate P losses, supported by a mix of voluntary guidelines and regulation to ensure water quality outcomes result. However, the strength of these findings can be improved if data is able to be referenced to the equivalent, and more finely resolved, spatial and temporal scales.