Relating Seasonal Hypoxia and Nitrogen Fluxes in Hood Canal, WA.

Hood Canal, located in Washington State, is a long inland extension of the Salish Sea. The Canal is narrow, with sills that limit exchange with the rest of the Salish Sea, leading to hypoxic conditions which threaten the critical local shellfish industry. Here, we use output from the LiveOcean numerical model, run by the University of Washington, nutrient sampling from July 2022, and long term nutrient data from the Washington State Department of Ecology (DOE) to better understand the physical processes that may contribute to hypoxia in Hood Canal. Hood Canal is a nitrogen-limited system, so large fluxes of dissolved inorganic nitrogen (DIN) can lead to algal blooms and eutrophication, contributing to the development of hypoxic conditions. To assess these fluxes, we construct a control volume DIN budget, taking into account the import of DIN via estuarine inflow and rivers, the export of DIN via estuarine outflow, and the loss of DIN via denitrification. This budget is developed using measured DIN concentrations, novel laboratory estimates of the denitrification rate, and LiveOcean velocity output. To calculate the estuarine circulation DIN and volume fluxes, we separate the water column into inflow and outflow layers based on along-channel velocity output from the LiveOcean model, averaged over the residence time of the Canal. To include riverine DIN input, we use DIN measurements from the DOE and river discharge data from the USGS for the Skokomish and Duckabush rivers, both averaged over the residence time of the basin. We compare the importance of these fluxes to assess the dominant sources and sinks of DIN in Hood Canal during summer 2022. These nutrient flux calculations may be able to explain the physical processes causing hypoxic conditions in the Hood Canal. Using the time series of nutrient data from the WA DOE and LiveOcean velocity output, we also assess the seasonality of the relative importance of the fluxes described above, as the seasonality of DIN flux may influence seasonal hypoxia. The results presented in this study provide important insights into the physical drivers of hypoxia in Hood Canal, which has become more common in recent years.