A total of 94 vertical profiles of size-fractionated chlorophyll a concentration and primary production rate were obtained along a meridional transect from the United Kingdom to the Falkland Islands (50°N to 50°S) during 4 cruises carried out in April and October 1996 and in April and October 1997. This data set allowed us to characterize the patterns of phytoplankton size-structure and productivity in temperate, oligotrophic, upwelling and equatorial regions. On average, picophytoplankton (0.2 to 2 µm) accounted for 56 and 71% of the total integrated carbon (C) fixation and autotrophic biomass, respectively. Enhanced biomass and productivity contributions by nano-and microplankton took place in the temperate regions and in the upwelling area off Mauritania. Small (< 2 µm in diameter) phytoplankton cells should not be regarded as a background, relatively invariant component of the microbial community, given that most of the latitudinal variability in total photoautotrophic biomass and production was driven by changes in the picophytoplankton. In temperate regions and in the upwelling area off Mauritania, small (< 2 µm) and large (> 2 µm) phytoplankton accounted for a proportion of total biomass that was similar to their shares of productivity. In the oligotrophic and equatorial regions, in contrast, large phytoplankton tended to account for a fraction of the total production that was significantly higher than their share of the biomass. We found that the equatorial upwelling causes an increase in phytoplankton biomass and productivity without altering the typical size structure found in less productive regions such as the subtropical gyres. In the oligotrophic ocean, significant changes in C fixation rates take place without accompanying variations in the magnitude of the phytoplankton standing stocks or the size structure of the microbial community.KEY WORDS: Size distribution · Phytoplankton · Chlorophyll · Primary production · Plankton food webs · Atlantic Ocean Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 216: [43][44][45][46][47][48][49][50][51][52][53][54][55][56] 2001 which determine the magnitude of different C pathways (i.e. size structure of primary production and coupling between production and grazing) are, in turn, under hydrodynamical control (Legendre & Rassoulzadegan 1996).Numerous studies have been carried out in which size-fractionated chlorophyll concentration and primary production are simultaneously quantified in particular geographical areas (see reviews in Tremblay & Legendre 1994, Legendre & Rassoulzadegan 1996. The majority of these observations, however, have been made in coastal and/or temperate environments, with relatively little attention given to tropical and subtropical open-ocean environments. In the tropical and subtropical Atlantic Ocean, only a few studies have dealt with the distribution of size-fractionated phytoplankton (i.e. Platt et al. 1983, Malone et al. 1993, Jochem & Zeitzschel 1993, and these have not c...

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...

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