Bacterial Respiration as a Major Term in the Mesopelagic Carbon Budget

The biological carbon pump (BCP) -­ the surface production and sinking of organic matter to the deep ocean - exerts a significant control (ca 200 ppm CO₂) over global climate. The majority of BCP processes occur in the mesopelagic, making accurate quantification of carbon fluxes in this part of the water column essential. Previous attempts to budget carbon in the mesopelagic suggest respiration terms largely exceed the supply of organic matter, however this may arise due to overestimates of bacterial respiration. To derive improved bacterial respiration estimates in mesopelagic water we applied modified radiotracer bioassays employing in situ, as opposed to saturating, concentrations of leucine. Via these methods, direct measurements of bacterial production and respiration were made in the Benguela Upwelling region and near South Georgia during an austral spring bloom as part of the COMICS project. Rates of respiration and, in particular, production declined with increasing depth yielding relatively lower rates of bacterial growth efficiency at depth relative to surface. Construction of a mesopelagic carbon budget revealed significantly different carbon cycling regimes at the two ocean regions examined. The relative magnitude of organic matter supply via particulate organic carbon and dissolved organic carbon varies significantly between the two study areas with South Georgia more dominated by particulate organic carbon flux compared to the Benguela Upwelling. Bacterial respiration however, is consistently a major term for carbon consumption and the use of modified bioassays increases our chances of accounting for all organic matter supply whilst reducing uncertainty when constructing a mesopelagic carbon budget.