Sedimentary Biogeochemistry of the Salish Sea: Springtime Fluxes of Dissolved Oxygen, Nutrients, Inorganic Carbon, and Alkalinity

Abstract

To better understand the significance of sediments in driving bottom water hypoxia and eutrophication, benthic fluxes of dissolved oxygen (DO), dissolved inorganic carbon (DIC), pH, total alkalinity, and nutrients between the sediment and the overlying water were measured at 42 sites in the Salish Sea, in April and May 2018. Fluxes of DO were generally larger than DIC fluxes, likely due to the oxidation of reduced compounds stored in sediments. The average phosphorus flux was close to zero, indicating phosphorus storage in sediments. At all but two sites, denitrification was present, with a median rate of 1 mmol m−2 d−1. The spatial variation observed in benthic fluxes and denitrification rates was primarily driven by water column depth and bottom water DO. Denitrification rates were estimated to remove 11% of the dissolved inorganic nitrogen supplied to the bottom water in Hood Canal, whereas in the Main Basin only 1% was removed. This difference was due to the long bottom water residence time in Hood Canal. Essentially all particulate phosphorus that reached the seafloor during early spring was stored in the sediment. Sediment oxygen uptake accounted for approximately 19% of bottom water DO removal in Hood Canal and 18% in the South Sound, compared to estimates of 4.1 to 10% in the other two basins. These results illuminate the varying contributions that sediments make to bottom water chemistry and the environmental factors that drive this variation.