Evolution of Nitrogen and Phosphorus Forms In a Subtropical Urban Coastal Watershed.

Non-point source pollution is the dominant pathway of nitrogen (N) and phosphorus (P) transport in agricultural but also urbanized and rapidly urbanizing watersheds. We used monthly concentrations data (1991–2009) of inorganic and organic forms of N and P as in stream waters draining different sub-basins, ranging in size from 19 to 350 km², of the Alafia River Watershed (total drainage area: 1085 km²) that drains to Tampa Bay Estuary, to understand N and P transport. The sub-basins were classified based on the percentage of urban land use as: three developed (18–24% residential, 1–14% built up) and two undeveloped (3–11% residential, 1–3% built up). Urban land use at two mainstem stations that drained 80–99% of the watershed was 16–17% residential and 3% built up. Total N concentrations ranged from 0.8 to 2.4 mg L^-1 and were greatest in stream waters draining developed (1.7–2.4 mg L^-1) than undeveloped (0.8–1.2 mg L^-1) sub-basins. Inorganic N (primarily NO₃-N) was the dominant form in streams draining developed sub-basins while organic N was greater in streams draining undeveloped sub-basins. Total P concentrations ranged from 0.6 to 2.5 mg L^-1 and were not different among developed (0.8–2.5 mg L^-1) and undeveloped (0.6–0.8 mg L^-1) sub-basins. Of total P, 70–90% was dissolved reactive P. Increasing total N and decreasing total P concentrations trends at the mainstem station draining 89% of the watershed over the 19-year period were observed and suggest that the development of the watershed has increased N but not P concentrations in streams. We recommend that BMPs to reduce  N loss from urban land uses in three developed sub-basins (with total N of 1.7–2.4 mg L^-1) may yield greater reductions in N concentrations at watershed outlet to achieve EPA proposed numeric criteria total N concentration of 1.798 mg L^-1. On the other hand, due to P rich geology and wastewater discharge, most developed and undeveloped sub-basins  had greater total P concentrations (0.8–2.5 mg P L^-1) than EPA proposed numeric total P value of 0.739 mg L^-1 indicating that BMPs should focus on reducing P loss from phosphate rock mined sub-basins and P inputs from wastewater.