Delivery and cycling of phosphorus in rivers: a review.

Phosphorus (P) supply (concentration and flux) is an important driver for biological activity in flowing waters and needs to be managed to avoid eutrophication impacts associated with urbanisation and agricultural intensification. This paper examines the role of in-stream retention and cycling in regulating river P concentrations in order to better understand the links between P sources and their ecological impacts. In terms of their composition (solubility and concentration), patterns of delivery (mode and timing) and therefore ecological relevance, P sources entering rivers are best grouped into wastewater discharges > runoff from impervious surfaces (roads, farmyards) > runoff from pervious surfaces (forestry, cultivated land and pasture). The localized impacts of soluble P discharges during ecologically sensitive periods can be distinguished from the downstream impacts associated with particulate P discharges under high flows due to the different processes by which these sources are retained, transformed and assimilated within the river channel. The range of physico-chemical processes involved in P cycling and the variable importance of these processes in different river environments according to stream size, stream geomorphology and anthropogenic pressures are summarised. It is concluded that the capacity to retain (process) P within the river channel, and hence regulate the downstream delivery of P without stressing the aquatic communities present, is considerable, especially in headwaters. To help achieve good water quality, there is scope to better manage this ecosystem service through regulation of P supply whilst optimising in-stream P retention according to subsidy-stress theory. Further research is needed to develop in-stream management options for maximising P subsidies and to demonstrate that regulation of downstream P delivery will reduce the incidence of eutrophication in connected waterbodies.