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Dissolved organic carbon (DOC) production by microbial populations was measured at 19 stations in the Atlantic Ocean to quantify the fraction of photoassimilated carbon that flows through the dissolved organic pool at basin scale and to assess the relationship between the percentage of DOC production, phytoplankton size structure, and rates of net community production. Experiments were conducted during four cruises carried out between May 1998 and October 1999, covering three upwelling regions: Benguela (SW Africa), Mauritania (NW Africa) and NW Spain, and the oligotrophic North Atlantic subtropical gyre between 30ЊN and 36ЊN. Photic zone integrated particulate organic carbon (POC) production rates ranged from 10 to 1,178 mg C m Ϫ2 h Ϫ1, thus covering a wide productivity spectrum. The percentage of DOC production with respect to total integrated primary production ranged from 4 to 42%, being larger in oligotrophic, picoplankton-dominated waters, where a balanced metabolism of the microbial community was observed, than in productive, net autotrophic waters, where large-sized cells formed the bulk of the phytoplankton biomass. A highly significant relationship was calculated between DOC and POC production rates in upwelling conditions. By contrast, the relationship between these variables in oligotrophic environments was weak, which suggests that different processes could be controlling the release of dissolved organic matter in productive and unproductive waters.Dissolved organic matter (DOM) is one of the least understood pools of marine matter and represents a major reservoir of organic carbon in the ocean. A large fraction of the dissolved organic carbon (DOC) present in the ocean ultimately derives from primary producers. However, a great deal of controversy still exists on the ecological significance and the ultimate control of DOC production in the ocean.The magnitude of DOC-related fluxes remains still largely uncertain, especially in oligotrophic regions. The high rates of DOC uptake by heterotrophic bacteria in relation to primary production recently measured in unproductive waters (e.g., Hansell et al. 1995;del Giorgio et al. 1997), largely justifies the growing biogeochemical interest of measuring and modeling DOC production in planktonic ecosystems.Initially, the radioactive carbon method for primary production estimation was modified for the measurement of direct excretion of dissolved organic compounds from algal cells. More recently, it has been recognized that several processes, besides direct excretion from intact algal cells, are 1 Corresponding author (eteira@uvigo.es). AcknowledgmentsWe thank G. Tilstone, B. Mouriño, C. Cariño, and C. Robinson for their contributions to the collection of data. Thanks to P. J. le B. Williams for the generous loan of analytical equipment used on the AMT-6 cruise. We are indebted to the captain and crew of research vessels, as well as to all the colleagues on board during the four cruises. We appreciate the comments of two anonymous referees, which improv...

A knowledge of the balance between plankton gross primary production (GPP) and community respiration (CR) in the open ocean is vital to the accurate determination of the global carbon cycle, yet the paucity of open ocean measurements severely limits our understanding. This study measured GPP, net community production, dark CR, and size-fractionated primary production in the upper 200 m of a 12,100 km latitudinal (32ЊS-48ЊN) transect in the Eastern Atlantic Ocean during May and June 1998. This comprehensive data set, which spans five contrasting plankton regimes, including two open ocean oligotrophic provinces, is used to derive a GPP : CR relationship, which suggests that net heterotrophy (GPP Ͻ CR) prevails in the eastern Atlantic when primary production falls below ϳ100 mmol O 2 m Ϫ2 d Ϫ1 . The predictive capability of this relationship is compared with that of the only other published relationship based on similar methodologies and is found to give a more representative description of the autotrophic (GPP Ͼ CR) to heterotrophic seasonal cycle in the Bay of Biscay. This improved predictive power is attributed to the increased representativeness of the current data set. Specifically, the interpretation suggests that the influence of community structure on net ecosystem metabolism implies that prediction of GPP : CR balances in pelagic ecosystems can be best achieved by use of a data set that covers a wide range of community structure and not only a wide range in the magnitude of primary production.