Coastal sediments are hotspots for biogeochemical cycles of essential elements, including carbon (C), oxygen (O), nitrogen (N), and phosphorus (P). They can regulate the water column geochemistry and ecology through benthic-pelagic exchanges. In this thesis, I investigate the sediments in the Pearl River Estuary and the adjacent coastal waters, a typical estuarine coastal ocean system under diverse geochemical regimes, aiming to quantitatively answer three questions: 1) how much does sediment oxygen uptake contribute to coastal hypoxia, 2) how does sediment nitrogen recycling and removal respond to hypoxia, and 3) how efficient sediment recycles phosphorus under diverse geochemical regimes. To answer these questions, I characterize the physicochemical gradients in the water, analyzing t...[ Read more ]

Coastal sediments are hotspots for biogeochemical cycles of essential elements, including carbon (C), oxygen (O), nitrogen (N), and phosphorus (P). They can regulate the water column geochemistry and ecology through benthic-pelagic exchanges. In this thesis, I investigate the sediments in the Pearl River Estuary and the adjacent coastal waters, a typical estuarine coastal ocean system under diverse geochemical regimes, aiming to quantitatively answer three questions: 1) how much does sediment oxygen uptake contribute to coastal hypoxia, 2) how does sediment nitrogen recycling and removal respond to hypoxia, and 3) how efficient sediment recycles phosphorus under diverse geochemical regimes. To answer these questions, I characterize the physicochemical gradients in the water, analyzing the porewater and solid-phase geochemistry of the sediments, calculating fluxes between the sediment and water, and investigating the mechanistic controls of the biogeochemical fluxes and rates. Results indicate that sediment oxygen uptake significantly contributes to the formation of hypoxia, and the contribution is strongly regulated by the thickness of the bottom boundary layer during stratification. Under hypoxia, the sediment removes less nitrogen because denitrification is limited by a reduction in nitrification. Phosphorus recycling is controlled by the balance between the supply of oxygen, nitrate, and sulfate in the water column, and the organic matter sedimentation that fuels the coupled cycling of iron and sulfur. The study not only suggests large variabilities within a single system but also offers quantitative descriptions of the mechanistic controls and parameterization for cross-system comparisons and larger-scale understanding.