Carbon oxidation in the deep Mediterranean Sea: Evidence for dissolved organic carbon source

Abstract

The supply and utilization of organic carbon in the deep western Mediterranean Sea was investigated based on measured electron transport system (ETS) activities of the nanoplankton and microplankton. The total carbon oxidation rate between 200 and 3000 m, as calculated from ETS activity, was 15.0 g C m−2 yr−1. This represents 21% of the primary production and is similar to published estimates of the annual new production. A vertical advection ‐ diffusion ‐ reaction model based on profiles of salinity, oxygen, and the carbon oxidation rate converted to oxygen consumption yielded a deepwater residence time of about 7 years, in close agreement with a published estimate based on bomb‐produced tritium profiles. This suggested that the ETS‐based rates in the deep waters were reasonably accurate. These deep rates were much greater than ETS‐based rates from the same depths in the Atlantic and equatorial Pacific Oceans. In the western Mediterranean, ETS‐based rates also greatly exceeded the rate predicted from the primary production rate and sediment trap relationships. The rapid rates observed in the deep western Mediterranean are not consistent with the supply of organic matter via rapidly sinking particulate material. Instead, rates may be supported by dissolved organic carbon (DOC) transported to depth by wintertime deepwater convection. In order to account for the portion of the ETS‐based rate which was not explained by the sediment trap flux, DOC concentrations in the surface waters entrained during deepwater formation would need to be only 11 μmol C L−1 greater than those in the deep waters exiting the basin. ETS activities from the equatorial Pacific (Packard et al., 1988) may also implicate DOC in supporting deep‐sea metabolism. There, ETS‐based carbon oxidation rates between 200 and 5000 m greatly exceeded rates calculated from sediment trap data in the same region. The source of the organic matter being respired may ultimately be the high rates of new production in the equatorial Pacific region, but the mechanism by which this material is transported to depth cannot be determined from these data. The ETS data from both the Mediterranean and the Pacific indicate much greater rates of carbon oxidation in the deep sea than expected from existing sediment trap results. Globally, transport of DOC into the deep sea possibly could rival the sinking particulate flux in importance for deep‐sea metabolism.