Involvement of the oxygen minimum in benthic zonation on a deep seamount

Abstract

LOW oxygen concentration in the seawater column reduces the abundance of midwater consumer populations1,2, which can enhance the supply of undegraded organic matter reaching the benthos. Low oxygen concentration in the water at the bottom can exclude most tolerant species from benthic habitats3–5. The interception of the seafloor with pronounced oxygen-minimum zones can produce steep gradients in benthic assemblages. We now present evidence for this interaction on Volcano 7, an oceanic seamount penetrating the oxygen-minimum zone in the eastern tropical Pacific. Submersible observations revealed only a few benthic species at the summit (730–750 m), where oxygen levels were lowest. Just tens of metres below, megafaunal and macrofaunal abundances were extremely high. Sediment organic carbon, a benthic food indicator, was unusually high. We hypothesize that a dynamic low-oxygen interface physiologically restricts benthos on the upper summit, that the enriched sediment is a result of reduced consumption and degradation of sinking material in the oxygen-minimum zone, and that this high benthic food level supports the unusually high benthic abundance. Sharp benthic zonation associated with oxygen concentrations may also be preserved in the palaeoceanographic record4,6.