Bacterioplankton distribution and production in deep Pacific waters: Large–scale geographic variations and possible coupling with sinking particle fluxes

Abstract

Bacterial abundance and leucine incorporation rate were measured throughout the water column (depth, 4,000– 6,000 m) at stations occupied in the equatorial, subtropical, and subarctic Pacific as well as in the Bering Sea during three cruises conducted between 1993 and 1997. In general, depth‐dependent decreases of bacterial abundance and leucine incorporation in the bathypelagic layer (depth, >1,000 m) were well described by a power function with remarkably uniform exponents among distant locations: average exponents were –0.900 and –1.33 for abundance and leucine incorporation, respectively. Depth profiles of bacterial properties were complex at some subarctic stations, suggesting lateral transport of organic carbon by local eddies. Organic carbon fluxes from abyssal sediment to overlying water would explain increases in bacterial abundance and leucine incorporation in near‐bottom layers. Biomass was twofold to fourfold and the production was threefold to sevenfold greater in subarctic than in subtropical regions. This latitudinal pattern was consistent with the basin‐scale distribution of sinking fluxes of particulate organic carbon (POC) reported in the literature. Rates of bacterial carbon uptake accounted for 51% (range, 31–153) and 23% (14–58) of deep sinking POC fluxes in subarctic and subtropical regions, respectively. Average turnover time of deep bacterial assemblages was estimated to be 1–30 yr. These results suggest that deep bacterial biomass and production are generally coupled with sinking POC fluxes and that organic carbon is substantially transformed within bathypelagic environments via a sinking POC → dissolved organic carbon → bacteria pathway, as previously suggested in the mesopelagic zone.