| Abstract: |
Anthropogenic nutrient enrichment has contributed to the depletion of oxygen from bottom waters in coastal systems worldwide. A major impediment to developing successful strategy to reduce hypoxia is the lack of adequate understanding of complicating effects due to climate variability. Previous hypoxia research has centered on the effects of nutrient loading and relied on statistical approaches. In order to predict hypoxia under changing climate and nutrient-loading conditions, a mechanistic model that resolves important physical and biogeochemical processes is required. In this work we present modeling studies of Chesapeake Bay hypoxia using a 3D coupled hydrodynamic-biogeochemical model with varying degrees of sophistication in representing biogeochemical processes. In this first model, we coupled the ROMS hydrodynamic model with a simple biogeochemical model in which the sediment oxygen demand and water column respiration rate are parameterized via temperature- and depth-dependent functions derived from observational data. We find that both vertical mixing and landward advection are equally important in supplying dissolved oxygen to the bottom layer in Chesapeake Bay. The model predicts a hypoxic region located in the deeper mesohaline region of Chesapeake Bay and shows a seasonal progression of hypoxia from May to October. The model also shows significant modulation and movement of hypoxia water at synoptic weather time scales. The downestuary and upestuary winds not only cause longitudinal movement of the hypoxic water but also cause lateral seiching that exposes hypoxic water to aeration at shallow shoals. In order to understand how climate variability affects hypoxia, we conduct model runs with different combinations of river flow and wind forcing. Increasing river flow leads to stronger stratification, weaker mixing to replenish the bottom layer with high oxygenated surface water, but increases the landward oxygen import from the lower Bay. Wind mixing is found to exert a surprisingly strong effect on hypoxia in the bottom water: the hypoxic volume is highly sensitive to the strength of vertical mixing. In the second model, we couple the ROMS hydrodynamic model with a sophisticated water quality model (RCA) that includes compartments such as dissolved oxygen, multiple forms of algae, carbon, nitrogen, phosphorus and silica, and a sediment diagnesis model. This model allows us to explore biochemical controls of bottom-water hypoxia and investigate processes such as the nitrification-denitrification cycle and the loss of sediment-buffering capacity resulting from the consumption of electron acceptors. |
