Bacterial production and the sinking flux of particulate organic matter in the subarctic Pacific

Abstract

We measured bacterial production and estimated the carbon consumption by bacteria in the mesopelagic zone (80–600 m) in the subarctic Pacific during May and August. Bacterial production was measured by leucine and thymidine incorporation. The two methods gave similar results. Bacterial production in the euphotic zone accounted for about 13% of primary production and in the whole water column for 20% (0–600 m). To bracket bacterial carbon consumption we made a lowest and highest estimate of bacterial production. The lowest estimate assumes zero isotope dilution for converting ¹⁴C-leucine incorporation rates into bacterial production and a 50% growth efficiency. In the mesopelagic zone, this estimate implies that bacterial account for 52 and 41% of the POC sinking flux as measured by sediment traps in May and August, respectively. The highest estimate, assuming two-fold isotope dilution of ¹⁴C-leucine and a 30% growth efficiency, yields bacterial carbon consumption values of 172 and 137% of the POC downward flux in both months. This indicates that bacteria are important, if not the major consumers of organic matter in the mesopelagic zone of the subarctic Pacific.