Water quality trends and the drivers in livestock-intensive regions: Results from 20 years of catchment monitoring in Norway

Intensive livestock production has raised enormous water quality concerns in Europe and elsewhere around the world. There is a need to examine long-term water quality trends and understand the drivers for the trends based on detailed catchment monitoring. Given orthophosphate-phosphorus (P) is highly relevant to eutrophication in freshwater lakes and rivers, we monitored its concentration and load trends in streamwater of a livestock-intensive catchment in western Norway for a 20-year period, using the approaches of continuous flow measurements and flow-proportional composite water sampling. Precipitation and catchment-level soil P balance, as well as field-level measurements of soil P status, were monitored to examine the drivers. Trend analyses showed that both annual mean P concentration (range: 0.05–0.14 mg L^-1; mean: 0.08 mg L^-1; p = 0.001) and annual P load (range: 0.35–1.46 kg ha^-1; mean: 0.65 kg ha^-1; p = 0.0003) increased significantly over the 20-year monitoring period. The mean concentrations were positively correlated with cumulative soil P surplus (R² = 0.55, p = 0.0002). Although discharge of the streamflow significantly affected annual P load, the P surplus appeared to be an even more important factors. The study highlights that long-term P surplus plays a critical role in influencing orthophosphate-P concentration and loads in livestock-intensive regions. There is a big challenge to reduce the P surplus, which however may be achieved through integrated strategies such as reducing livestock density, manure refinement and transport to crop-intensive regions, improving livestock feeding management, and increasing crop P removal.