ABSTRACT
The role of phosphorus (P) inputs in accelerating eutrophication of freshwaters is well documented.[1]
The total load of P to a river can broadly be divided into point source inputs, typically dominated by sewage treatment effluents, and diffuse sources, often dominated by agriculture.[2,3] There is a general increase in P transport in the order of rivers draining forested-native ecosystems, intensively managed agriculture, and urban settings.[4] Point sources enter the river more continually through the year than do non-point sources, which are subject to large seasonal variation, typically as a function of overland flow.[5]
Changes in the forms and amounts of P during transport in streams and rivers can greatly influence the eventual impact of P loss on the degree of eutrophic response of receivingwaters.[5,6] These changes aremediated by physical (sediment deposition and resuspension and flow regimes), abiotic (P sorption and desorption), and biotic (microbial and plant uptake) processes.[7,8]
RIVERINE PROCESSES
Physical Processes
Fluvial sediments are derived from the erosion of surface soils, gullies, ditches, and stream banks. Because surface soils generally contain the highest concentration of P in soil profiles, and erosion preferentially removes P-rich particles, eroded surface soil represents a major source of particulate P in riverine systems.[9,10]
In areas with recent gully formation or bank erosion, subsoil is the dominant source of sediments. Consequently, sediments derived from these sources have low P content and high P sorption capacities.[11,12]
As P release and sorption are largely related to particle size, with coarser-sized particles releasing P more
readily than fine particles, which tend to sorb more P,[13] hydrologic processes controlling sediment particle size distribution have important implications to P fate in river systems.
