Tracking Sources of Phosphorus in East Creek, a Coastal Plain Tributary of the Chesapeake Bay.

Non-point source pollution has contributed to eutrophication in the Chesapeake Bay since the mid 1900’s. Decades of manure and fertilizer applications in excess of crop phosphorus (P) requirements have led to widespread P accumulation in soils and adjacent areas including ditches, wetlands, and stream banks such as in Maryland’s lower Eastern Shore. Accumulated (legacy) P from past applications has potential to become a prominent long-term P source and could counteract benefits from best management practices. To distinguish between potential P sources in creek water, manure, soil, particulate, and sediment samples from the East Creek, MD watershed were collected and sequentially extracted to separate P pools. Phosphate oxygen isotope ratios (δ¹⁸O_P) of each pool were used as proxies to distinguish P sources. Comparison of δ¹⁸O_P values of different extracted P pools in potential sources as well as suspended particulate matter from the headwater to the mouth of the creek revealed interplay of biotic and abiotic factors on the formation of particulate P pools. δ¹⁸O_P values suggested that NaHCO₃-P (corresponding to loosely sorbed P) is a biologically active P pool as it showed evidence of microbial cycling. In contrast, particulate NaOH-P (corresponding to Fe- and Al- oxide bound P), a dominant P pool in creek water, was found to be inert and retained its source signature. While bioavailable particulate P pools, such as NaHCO₃-P, might be of more immediate concern for water quality, inert particulate P pools, such as NaOH-P, can be useful to track particulate P to land sources. Our results provide important insights into legacy P transport in tidal tributaries and discriminate different P sources exported to the Chesapeake Bay.