Dissolved Phosphorus Transport during Storm and Base Flow Conditions from an Agriculturally Intensive Southeastern Coastal Plain Watershed

The high density of animal production in southeastern Coastal Plain watersheds has caused some soils to contain excess amounts of plant–available soil phosphorus (P). Runoff, erosion, and leaching can transport P to surface water systems and out of these watersheds. High P concentrations in downstream aquatic ecosystems can increase the risk of eutrophication. Our objectives were to determine stream dissolved phosphorus (DP) mass loads transported under storm and base flow conditions and to examine relationships between precipitation, stream flow, and DP concentrations and export loads from an agriculturally intensive Coastal Plain watershed. This watershed was separated into four subwatersheds, and stream flows at their outlets were separated into base and storm flow conditions. Over the 2–year study period, stream base flow accounted for the majority of total stream flow at all outlets (58% to 73%). Average stream total DP mass loads at the watershed outlet in 1994 and 1995 were 234 and 477 mg DP ha–1 d–1, and higher DP mass loads (57% to 71% of the cumulative total) were exported during base flow conditions. In 1995, a series of intense storm events over two months caused a large DP pulse (approximately 63% of the stream’s yearly annual DP mass load) to exit the watershed. Regression analysis showed a linear relationship (P < 0.001) between log10 instantaneous stream flow and log10 DP export. Our results showed that more DP was exported during stream base flow conditions. However, intensive summer storms can greatly accelerate stream DP export from this agriculturally intensive Coastal Plain watershed.