Linking phosphorus sources to impacts in different types of water body

The inherent variability in type, composition and modes of transport/delivery of the various sources of phosphorus (P) contribute to the complex temporal and spatial patterns of P concentrations and loads reaching freshwaters. One particularly useful and common distinction is made between diffuse sources that are hydrologically driven and therefore episodic and seasonal, compared with the more bioavailable, concentrated and continuous P discharges from discrete point sources. In terms of ecological impacts, well-distributed rural point sources (village wastewater, septic tank overflows and farmyards) that discharge into smaller tributaries may be further separated from the more isolated large urban wastewater and industrial effluent discharges to larger waters. Impacts of diffuse sources may be similarly broken down into near-surface and base flow (groundwater) contributions. These differences in mechanisms of delivery, when coupled with differences in chemical form (and therefore availability) of P, combine to influence in-stream processes, kinetics of downstream transport and potential impacts. A greater understanding of the specific attributes that are responsible for controlling the dynamics of a particular P source together with some idea of its environmental relevance would help prioritize and target remedial actions. In particular, the extent to which periods of P supply overlap with those of biological demand help determine the relative significance of individual P sources and could inform programmes of measures to control P loss from these sources more effectively. The lack of any provision for interpreting P supply within an ecological context might result in poor relationships between reductions in P loading and environmental benefit.