Assessing the impact of terrestrially derived phosphorus on flowing water systems

Recent efforts to quantify the terrestrial inputs and cycling of P in agric ultural systems are allowing management strategies to be developed to minim ize the loss of P. While this approach has had clear advantages for standin g waters, potential benefits for flowing freshwater systems are more diffic ult to predict. As a consequence of the unidirectional and dynamic nature o f flow in rivers, temporal and spatial separations between the source of P and the point of potential impact are introduced when considering a whole w atershed. The ability to quantify the residence time of P within any partic ular part of a river system is therefore essential so that the specific con tribution of individual P sources to biological demand ran be determined. T he smaller algal biomass productivity measured in flowing, compared with st anding, waters at a similar supply rate of P indicates the need to consider the roles of other watershed factors in modifying the relationship between growth and P supply. The site-specific nature of responses to P suggests a need to develop a hierarchical classification system where individual rive r sections can be ranked according to their likely responsiveness to P. The ecoregion concept, combined with stream order, would integrate spatial and temporal aspects of gradient, land use, flow velocity, and water quality. For this to be successful, we must demonstrate a link between perturbations to the terrestrial P cycle, the delivery of P to flowing waters, and a bio logical impact.