Rates of dissolved organic carbon production and bacterial activity in the eastern North Atlantic Subtropical Gyre during summer

Rates of particulate organic carbon (POC) production, dissolved organic carbon (DOC) production and bacterial production were measured at 8 stations in the eastern North Atlantic Subtropical Gyre during August 1998. The euphotic-depth-integrated POC rate was on average 27 mg C m -2 h -1 . The corresponding averaged integrated DOC production rate was 5 mg C m -2 h -1 , i.e. about 20% of the total primary production. No statistically significant relationship was found between the rates of POC and DOC production, suggesting that other processes besides phytoplankton exudation, such as cell lysis or protist grazing, could be implied in the release of dissolved organic materials to the oceanic environment. Euphotic-depth-integrated bacterial biomass and production were on average 214 mg C m -2 and 1.4 mg C m -2 h -1 , respectively. The lack of correlation between the rates of DOC release and bacterial activity and a bacterial carbon demand (BCD, calculated using an estimated bacterial growth efficiency ranging from 11 to 18%) in excess of DOC production suggested the existence of additional organic carbon sources (both allochthonous and/or autochthonous reservoirs), apart from in situ phytoplankton-derived DOC production, for the maintenance of bacterial activity in this region during summer 1998. This uncoupling between phytoplankton and bacterioplankton could partially explain the heterotrophic metabolic balance in this oligotrophic region during summer 1998.