Seasonal Variation and Potential Sources of Particulate Phosphorus in the Susquehanna River in the Chesapeake Bay Watershed.

Phosphorus (P) has been a major cause of surface water eutrophication and bottom water hypoxia. The incomplete understanding of different P pools and their corresponding bioavailability in the continuum from sources and sinks has limited the development of appropriate nutrient management strategies. Here we apply phosphate oxygen isotope (?18OP) method to track P sources and identify specific P pools in the suspended particulate matter that are bioavailable in the Susquehanna River sub-watershed within the Chesapeake Bay watershed. Water samples (56-80 L in each location) were collected along the physicochemical and hydrodynamic gradients in the agriculture-source runoff dominated Deer Creek, lower Susquehanna River, and the river mouth in the Chesapeake Bay in summer and winter seasons. Three different size fractions of particulate matter (1 µm – 0.45 µm; 0.45 µm – 0.1 µm; 0.1 µm – 0.05 µm) were separated by centrifugation and different P pools in each fraction were extracted using a modified Hedley extraction method. Concentrations of inorganic P (Pi) pools in all sites varied in the same order: NaOH-Pi > NaHCO3-Pi > HNO3-Pi > H2O-Pi, suggesting that NaOH-Pi pool which corresponds to majorly of Fe/Al oxide-bound P is the most dominant P pool. The d18OP values of NaOH-Pi and HNO3-Pi pools were out of the ranges of equilibrium isotope composition, which suggests the recalcitrancy of these two pools against biological cycling in the water bodies. On the other hand, NaHCO3-Pi pool was found to be rapidly cycled and reached near-equilibrium isotope composition. These results indicate NaOH-Pi and HNO3-Pi pools may preserve source signatures and thus allow identification of sources of suspended particulate matter