Benthic fluxes on the Oregon shelf

Benthic chamber incubations were performed in the mid-shelf region on the Oregon shelf in June and August 2009 to measure fluxes of oxygen, nutrients, and iron and their effect on water column chemistry. Chamber oxygen and nitrate fluxes were into the sediments while silicate, iron and ammonium fluxes were out of the sediments. Benthic fluxes were similar between the two months, except that dissolved iron fluxes were higher at some sites in August. Bottom waters were consistently hypoxic (43–64 μM O₂) and had ammonium concentrations from 0 to 2.6 μM in the mid-shelf region. Given measured ammonium fluxes (0.2–1.4 mmol m⁻² d⁻¹), we used a simple stoichiometric model for a 10 m bottom boundary layer to calculate that benthic fluxes only contributed ∼16–41% of the bottom water ammonia. Benthic oxygen fluxes (−4.3 to −12.5 mmol O₂ m⁻² d⁻¹) were responsible for ∼38–51% of oxygen drawdown in the benthic boundary layer. In both cases, the remainder may be attributed to water column respiration. Benthic iron and nitrate fluxes have opposite effects on productivity. Iron fluxes (0–71 μmol m⁻² d⁻¹, average: 5 μmol m⁻² d⁻¹) increased bottom water concentrations while nitrate was lost (−1.2 to −2.9 mmol m⁻² d⁻¹₃NO⁻${NO}_3-$) due to denitrification. By supplying iron and consuming nitrogen, benthic diagenetic processes reinforce an iron-replete, nitrate-limited coastal ecosystem.