Different Pathways of Phosphorus Cycling in the Upstream and Downstream of a Creek in an Agricultural-Runoff Dominated Watershed.

Phosphorus (P) loss from agricultural fields is a major cause of environmental problem such as eutrophication. However, the impact of active P loading from agricultural fields on the sediment P pools and pathway of P cycling is poorly understood largely due to methodological limitations. This, in fact, has been a major obstacle preventing the accurate assessment of nutrient loads released to open waters. In this study, we analyzed concentrations and phosphate oxygen isotope ratios (d¹⁸O_P) in dissolved and suspended particulate matter P in the surface water, porewater, and sediment P pools in river bed along two district zones (agricultural runoff dominated headwater and wetland dominated mouth) of East Creek, a tidal tributary of the Chesapeake Bay. Our results show high concentrations of dissolved inorganic (P_i) and organic P (P_o) in the water column and porewater as well as high particulate and sediment P_i pools in the headwater region. Concentrations of all P pools in the wetland zone were low and found to be impacted by advancing tides both in surface and subsurface waters. Isotopic compositions of porewater and sediment P pools in headwater region suggested dominance of the remineralization pathway of P cycling and the overall complete biological cycling in the wetland region. New insights gained from these findings on the pathway and extent of biological cycling are expected to be useful to address water quality issues in the Chesapeake Bay and its watershed.