Modification of phosphorus export from an eastern USA catchment by fluvial sediment and phosphorus inputs

Abstract

Phosphorus (P) export from agricultural catchments can accelerate freshwater eutrophication. Landscape-based remedial measures can reduce edge-of-field P losses. However, stream channel hydraulics and fluvial sediment properties can modify the forms and amounts of P exported by the time it reaches the catchment outlet. This study examines if land use, fluvial sediment properties, and storm flow dissolved and particulate P are interdependent within a mixed land use catchment in Pennsylvania, USA, so that remedial strategies can be most effectively targeted within the catchment to mitigate P export. Samples of the top 2–3 cm of stream-bed sediments ( n=40) were collected in April 2001, above and below tributary confluences and in areas of likely deposition. Stream water samples were collected at each of 23 sub-watershed outlets during base ( n=7) and storm flow ( n=3) events between 2000 and 2002. The P content and sorptive properties of deposited fluvial sediments varied among tributaries and flow regimes. Total P of sediments in tributaries (322 mg kg −1) was greater than below confluences (239 mg kg −1), whereas dissolved P release rate was lower (92 and 166 mg P kg −1 min −1, respectively). This was attributed to physical disturbance by turbulent mixing and presence of more sand-sized particles at confluences (747 g kg −1) than tributaries (707 g kg −1). The percent cropped ( r=0.51) and forest ( r=−0.57) land was related to the Mehlich-3 extractable P concentration of outflow sediment for each sub-catchment. This in turn influenced sediment P release, which was related to base flow P when sediments establish a quasi-equilibrium with flowing water within the catchment. However, storm flow P was not related to any sediment P properties but to percent of each sub-catchment in cropland ( r=0.58), reflecting the importance of erosion in P transport. Storm flow suspended sediment was related to sub-catchment area in crop ( r=0.78). To gain a better understanding of processes controlling P transport within and from a catchment and, thus, mitigation of losses, measures such as conservation tillage, manure management, and buffer strips, fluvial sediment properties as well as landscape management must be considered.