An Application of Phosphate Oxygen Isotope Ratios to Understand Sources and Cycling of Phosphorus in East Creek, a Chesapeake Bay Watershed.

Chesapeake Bay is one of the largest and biologically productive estuaries in North America. Non-point nutrient sources primarily from agricultural fields are considered as major input sources that cause environmental problems such as eutrophication in the bay. However, variable P sources and biogeochemical processes involved during the transport of P from source to sink have not been fully characterized. To understand the biogeochemical processes and source-sink relationship during the export of P from an agricultural field to the bay, we analyzed temporal variation in concentrations and phosphate oxygen isotope ratios (δ¹⁸O_p) of different P pools in soils, sediments, and surface waters along a major stretch of the East Creek, MD. High concentration and distinct δ¹⁸O_p values of porewater P near agricultural fields suggest that the soils near the fields act as P sink with a significant downward movement of P compared to the wetland near the bay where P is most likely released from sediments. While P is exported from agricultural fields to the bay via creek water and yet we are unable to perform any quantitative assessment of vertical and horizontal P fluxes, our results identify previously unknown P flux pathways. Further research on temporal variation of P concentration and isotopic composition in the surface water along with corresponding changes in soil/sediment P will provide valuable information on mixing of P sources, biological cycling, and source-sink relationships.