Li, Qiang2021-02-102021-02-102020https://udspace.udel.edu/handle/19716/28650Phosphorus (P) over-loading is a leading cause of surface water eutrophication and bottom water hypoxia. Response to P loading on water quality varies with watershed size and other physico-chemical and biological parameters. As the watershed size increases, nutrient loading augments and often results in complex and non-linear response to loading and corresponding biogeochemical interactions. The limited understanding between different P pools and their bioavailability from sources to sinks has limited the development of appropriate nutrient management strategies aimed to improve water quality. To address this knowledge gap, this dissertation research investigated P sources and transformation pathways at small (~62 km2) Love Creek watershed in Rehoboth Bay and part of larger Susquehanna River estuary (~777 km2) in Chesapeake Bay. In both watersheds, multiple stable isotopes and geochemical proxies were applied to analyze colloidal, dissolved, and soil/sediment P pools and associated elements along the continuum from the sources and sinks. ☐ In the Love Creek watershed, NaOH-Pi was found to be the most dominant P pool among colloids and soil fractions. Based on the trends of carbon (C) and nitrogen (N) isotopes, concentration, and relative sizes of P pools, major colloidal source in the non-tidal section was most likely to be non-agricultural sources, such as plant debris and forest soils. Along the salinity gradient, the contribution of terrestrial sources gradually decreased from the non-tidal section to the tidal section of the creek. In the Susquehanna River estuary, NaOH-Pi was still the most dominant P pool. Deer Creek, a small tributary, contributed a disproportionately high amounts of colloids to the River. Oxygen isotope values of phosphate (δ18OP) in the NaOH-Pi and HNO3-Pi pools of different colloidal size fractions were much heavier than equilibrium values in ambient water, suggested that these two pools were resistant to biological uptake and aided to identify the potential sources of colloids. Combined C, N, and Pi isotopic compositions of colloids showed that the contribution of terrestrial sources of colloids decreased downstream. ☐ Spatiotemporal analyses of colloidal and dissolved P in the Susquehanna River estuary identified the hotspots areas that appear to be controlled from seasonal loading and recycling within the water column. To further investigate loading vs internal cycling, permanently suspended particulate matter (PSPM; ˂ 40 μm) and resuspended particulate matter (RSPM; ˃ 40 μm) were studied in controlled erosion experiment in the laboratory and paired with the water column in the field. Comparison of C and N isotopes and P pools in colloids in eroded and in-situ sediments and water column confirmed that the sediment resuspension was a minor source for particulate and colloidal P in the water column in the estuary. Overall, these findings provided an improved understanding of P sources, sinks, and internal cycling in the water column in both watersheds. These results are expected to aid in further investigations of nutrient cycling in comparable watersheds and estuaries.Thesis12371248332020-10-13en