Browsing by Author "Li, Qiang"
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Item Characterization of phosphorus sources and bioavailability at different hydrodynamic gradients in coastal environments(University of Delaware, 2020) Li, QiangPhosphorus (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.Item Relative Roles of Sediment Transport and Localized Erosion on Phosphorus Load in the Lower Susquehanna River and Its Mouth in the Chesapeake Bay, USA(Journal of Geophysical Research: Biogeosciences, 2022-08-05) Li, Qiang; Gray, Katelyn E.; Jaisi, Deb P.Particle size greatly influences the fate of phosphorus (P) in estuaries as P adheres more readily to the larger surface area in smaller sized particles. Here, data on two size fractions of particulate matter, permanently suspended particulate matter (PSPM, ≤40 μm) and resuspended particulate matter (RSPM, >40 μm), from field and controlled laboratory erosion experiments were analyzed to determine their relative contribution to water column P in the mouth of the Susquehanna River in the upper Chesapeake Bay. Based on the composition of sequentially extracted P pools, C and N isotopes, and elemental data, all PSPM and the majority of RSPM are most likely derived from allochthonous sources through river transport. A minor fraction of particulate matter in the water column was derived from sediment resuspension, which had a dominant role above the sediment-water interface in the river's mouth. The proportion of biologically available P pools to recalcitrant P pools in suspended particulate matter decreased with water column depth, indicating their preferential removal or biological utilization during settling. Suspended particulate matter (SPM) mobilized during sediment erosion experiments, regardless of particle size, was richer in biologically available P pools than SPM in the field, suggesting higher mobility of these pools in the field. These complementary results from field and field-simulated laboratory erosion experiments provide unique insights into the composition of particulate matter under different hydrodynamic regimes in the river estuary. Plain Language Summary: Phosphorus (P) is often the limiting nutrient for eutrophication. Of importance is the P load from the Susquehanna River into the Chesapeake Bay, as it contributes about a fourth of the total bay load. Paired field measurements and erosion experiments showed that the majority of suspended particulate matter and P load was derived from external, upstream riverine sources. Localized sediment erosion was present only in the mouth of the river in the northernmost Chesapeake Bay. Comparative analyses showed that P was more likely to be transported in fine grained suspended particulate matter and in forms that were more biologically available. Key Points: - Concentration of total particulate matter increased downstream with water column depth but P content decreased with water depth - P pools and δ13C and δ15N values of in situ and eroded particulate matter indicated riverine input as the major source - Sediment resuspension marginally contributed to P load