Benthic flux of oxygen and nutrients across Oregon/California shelf sediments

A free vehicle benthic chamber has been used to define benthic nutrient fluxes on the Southern Oregon and Northern California shelf (90–200 m). We measured fluxes on two occasions, in the spring and fall of 2007, to assess variability in fluxes and bioirrigation rates. Chambers were also employed to assess the impact of changing oxygen content on benthic fluxes. Oxygen uptake rates are similar to within 30% between sites in Oregon, Northern California and Central California, suggesting that a large portion of the Western US continental shelf experiences the same oxygen demand. Oxygen uptake (−2 to −10 mmol m−2 d−1) at S. Oregon/N. California sites is likely responsible for <50% of the organic carbon (Corg) oxidized in these sediments as denitrification and other electron transport processes generate much of the observed TCO2 flux (4 to 31 mmol C m−2 d−1). Much like other coastal regions, these sediments are generally a net sink of water column fixed N as nitrate uptake rates (−0.6 to −2.2 mmol N m−2 d−1) typically exceed ammonium effluxes (0 to 1.9 mmol N m−2 d−1). As chamber oxygen concentrations decline during incubation, the concentration vs. time trends in phosphate, nitrate, and silicate remain linear. We interpret this to signify that there is no short-term change in the flux of these species as a response to changing oxygen concentration. However, there is a non-linear response for ammonium, with some ammonium fluxes increasing non-linearly by as much as a factor of 14. This occurs after an incubation period of 5–15 h when oxygen levels are 50%–20% of their ambient values. This change in flux may be related to rather small changes in chamber oxygen concentration, suggesting that under hypoxic conditions, there could be an enhanced efflux of ammonia to the water column. This would provide a positive feedback leading to enhanced primary productivity and increasing hypoxia.

► Benthic fluxes on Oregon/California shelf are similar to other west coast shelves.
► Bio-irrigation is very important, correlates positively with oxygen uptake flux.
► Most nutrient fluxes are constant with time as oxygen decreases by 50%–80% of ambient.
► Ammonium flux is non-linear with time, indicating a burst of NH4+ is possible.
► Burst of ammonium flux may be triggered by hypoxia, leading to positive feedback.