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

Between 2004 and 2013 phosphorus concentrations decreased at 65% of the 304 sites monitored. Our research found the most probable causes of improvement were that strategies were mitigating P loss from land, guidelines were directing where to best use strategies, and policy instruments were including P management. We found little evidence that this improvement was caused by a decrease in fertilizer use or change in fertilizer form, or that greater nitrate loads were assimilating P from groundwater or sediments.