Importance of spatial and temporal patterns for assessment of risk of diffuse nutrient emissions to surface waters

The relationships between catchment characteristics, management and ecological response of receiving waters categorise risk of impact from diffuse pollution. We investigated spatiotemporal patterns in area-weighted nutrient loads and nutrient-flow relationships at 14 locations on 11 rivers in the catchment of Lough Mask in the west of Ireland. Relationships between river flow and the concentrations of a number of fractions of phosphorus and nitrogen differed significantly among the rivers, suggesting that risk is not static, but varies over short timescales and among catchments. Further, the relative magnitude of the nutrient-flow relationship was found to vary seasonally for both phosphorus and nitrogen. Nutrient concentrations during high flows in summer and autumn were significantly higher than during winter and spring, suggestive of disproportionate risk to surface water quality during summer high flow events owing to high potential growth rates of algal communities. Significant positive associations were found between extent of high productivity grasslands and urban areas with nutrient loads, slopes of nutrient-flow relationships and the most biologically available fractions of nutrients. These findings are important for the implementation of the European Water Framework Directive, as defining risk to degradation of ecological quality of waters from catchment activities will drive programmes of measures. Insufficient or inappropriate risk assessment will likely inhibit or prevent effective implementation.