Assessing the Success of Regional Measures for Lowering Agricultural Nutrient Pollution in Headwater Streams

Lowland waters in Northern Ireland experience elevated agricultural phosphorus (P) inputs, and in response a variety of control measures targeting farm nutrient management have been implemented. Their efficacy in lowering nitrogen (N) and P exports and improving water quality is examined in 40 headwater streams from 1990 to 2009, and to 2014 for 24 of these. Over this period manure production in the study catchments declined by 7%, but regional chemical fertilizer inputs declined by 37% for N and 79% for P, and the regional nutrient surplus was lowered by 18% for N and 49% for P. Diminished pollution by organic wastes meant that 85% of streams exhibited chemistry suitable for salmonids in 2009 compared to 40% in 1990. Flow-weighted mean concentrations (FWMCs) of nutrients declined between 1990 and 2009, and their correlations with catchment stocking rates became stronger over time. For catchments with manure inputs <16.6 kg P ha, total P and nitrate FWMCs declined from 123 ± 19 μg P L and 1.92 ± 0.5 mg N L in 1990 at rates of 2.2 ± 0.5 and 30 ± 10 μg L yr, respectively. For catchments with higher manure inputs the respective rates of decline were greater at 5.8 ± 1.0 μg P L yr and 160 ± 20 μg N L yr from 1990 concentrations of 270 ± 25 μg P L and 5.99 ± 0.4 mg N L. Although now lower, P concentrations in the more highly stocked catchments still exceed regional nutrient standards so that the identification of further factors impinging on nutrient losses is critical if such standards are to be achieved.