Abstract. Marine N2 fixing microorganisms, termed diazotrophs, are a key functional group in marine pelagic ecosystems. The biological fixation of dinitrogen (N2) to bioavailable nitrogen provides an important new source of nitrogen for pelagic marine ecosystems and influences primary productivity and organic matter export to the deep ocean. As one of a series of efforts to collect biomass and rates specific to different phytoplankton functional groups, we have constructed a database on diazotrophic organisms in the global pelagic upper ocean by compiling about 12 000 direct field measurements of cyanobacterial diazotroph abundances (based on microscopic cell counts or qPCR assays targeting the nifH genes) and N2 fixation rates. Biomass conversion factors are estimated based on cell sizes to convert abundance data to diazotrophic biomass. The database is limited spatially, lacking large regions of the ocean especially in the Indian Ocean. The data are approximately log-normal distributed, and large variances exist in most sub-databases with non-zero values differing 5 to 8 orders of magnitude. Reporting the geometric mean and the range of one geometric standard error below and above the geometric mean, the pelagic N2 fixation rate in the global ocean is estimated to be 62 (52–73) Tg N yr−1 and the pelagic diazotrophic biomass in the global ocean is estimated to be 2.1 (1.4–3.1) Tg C from cell counts and to 89 (43–150) Tg C from nifH-based abundances. Reporting the arithmetic mean and one standard error instead, these three global estimates are 140 ± 9.2 Tg N yr−1, 18 ± 1.8 Tg C and 590 ± 70 Tg C, respectively. Uncertainties related to biomass conversion factors can change the estimate of geometric mean pelagic diazotrophic biomass in the global ocean by about ±70%. It was recently established that the most commonly applied method used to measure N2 fixation has underestimated the true rates. As a result, one can expect that future rate measurements will shift the mean N2 fixation rate upward and may result in significantly higher estimates for the global N2 fixation. The evolving database can nevertheless be used to study spatial and temporal distributions and variations of marine N2 fixation, to validate geochemical estimates and to parameterize and validate biogeochemical models, keeping in mind that future rate measurements may rise in the future. The database is stored in PANGAEA (doi:10.1594/PANGAEA.774851).

The latitudinal distributions of phytoplankton biomass, composition and production in the Atlantic Ocean were determined along a 10,000-km transect from 503N to 503S in October 1995, May 1996 and October 1996. Highest levels of euphotic layer-integrated chlorophyll a (Chl a) concentration (75}125 mg Chl m\ ) were found in North Atlantic temperate waters and in the upwelling region o! NW Africa, whereas typical Chl a concentrations in oligotrophic waters ranged from 20 to 40 mg Chl m\ . The estimated concentration of surface phytoplankton carbon (C) biomass was 5}15 mg C m\ in the oligotrophic regions and increased over 40 mg C m\ in richer areas. The deep chlorophyll maximum did not seem to constitute a biomass or productivity maximum, but resulted mainly from an increase in the Chl a to C ratio and represented a relatively small contribution to total integrated productivity. Primary production rates varied from 50 mg C m\ d\ at the central gyres to 500}1000 mg C m\ d\ in upwelling and higher latitude regions, where faster growth rates ( ) of phytoplankton ( '0.5 d\ ) were also measured. In oligotrophic waters, microalgal growth was consistently slow [surface averaged 0.21$0.02 d\ (mean$SE)], representing (20% of maximum expected growth. These results argue against the view that the subtropical gyres are characterized by high phytoplankton turnover rates. The latitudinal variations in were inversely correlated to the changes in the depth of the nitracline and positively correlated to those of the integrated nitrate concentration, supporting the case for the role of nutrients in controlling the large-scale distribution of phytoplankton growth rates. We observed a large degree of temporal 0967-0637/00/$ -see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 -0 6 3 7 ( 9 9 ) 0 0 0 8 7 -4 variability in the phytoplankton dynamics in the oligotrophic regions: productivity and growth rates varied in excess of 8-fold, whereas microalgal biomass remained relatively constant. The observed spatial and temporal variability in the biomass speci"c rate of photosynthesis is at least three times larger than currently assumed in most satellite-based models of global productivity.

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...

Abstract. Phytoplankton identification and abundance data are now commonly feeding plankton distribution databases worldwide. This study is a first attempt to compile the largest possible body of data available from different databases as well as from individual published or unpublished datasets regarding diatom distribution in the world ocean. The data obtained originate from time series studies as well as spatial studies. This effort is supported by the Marine EcosystemPublished by Copernicus Publications. K. Leblanc et al.: A global diatom databaseModel Inter-Comparison Project (MAREMIP), which aims at building consistent datasets for the main plankton functional types (PFTs) in order to help validate biogeochemical ocean models by using carbon (C) biomass derived from abundance data. In this study we collected over 293 000 individual geo-referenced data points with diatom abundances from bottle and net sampling. Sampling site distribution was not homogeneous, with 58 % of data in the Atlantic, 20 % in the Arctic, 12 % in the Pacific, 8 % in the Indian and 1 % in the Southern Ocean. A total of 136 different genera and 607 different species were identified after spell checking and name correction. Only a small fraction of these data were also documented for biovolumes and an even smaller fraction was converted to C biomass. As it is virtually impossible to reconstruct everyone's method for biovolume calculation, which is usually not indicated in the datasets, we decided to undertake the effort to document, for every distinct species, the minimum and maximum cell dimensions, and to convert all the available abundance data into biovolumes and C biomass using a single standardized method. Statistical correction of the database was also adopted to exclude potential outliers and suspicious data points. The final database contains 90 648 data points with converted C biomass. Diatom C biomass calculated from cell sizes spans over eight orders of magnitude. The mean diatom biomass for individual locations, dates and depths is 141.19 µg C l −1 , while the median value is 11.16 µg C l −1 . Regarding biomass distribution, 19 % of data are in the range 0-1 µg C l −1 , 29 % in the range 1-10 µg C l −1 , 31 % in the range 10-100 µg C l −1 , 18 % in the range 100-1000 µg C l −1 , and only 3 % > 1000 µg C l −1 . Interestingly, less than 50 species contributed to >90% of global biomass, among which centric species were dominant. Thus, placing significant efforts on cell size measurements, process studies and C quota calculations of these species should considerably improve biomass estimates in the upcoming years. A first-order estimate of the diatom biomass for the global ocean ranges from 444 to 582 Tg C, which converts to 3 to 4 Tmol Si and to an average Si biomass turnover rate of 0.15 to 0.19 d −1 . Link to the dataset:

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We present ocean, basin-scale simultaneous measurements of N 2 -fixation, nitrate diffusion, and primary production along a south-north transect in the Atlantic Ocean crossing three biogeographic provinces: the south subtropical Atlantic (SSA; , 31uS-12uS), the equatorial Atlantic (EA; , 12uS-16uN), and the north subtropical Atlantic (NSA, , 16uN-9uN) in April-May 2008. N 2 -fixation and primary production were measured as 15 N 2 and 14 C uptake, respectively. Dissipation rates of turbulent kinetic energy (e) were measured with a microstructure profiler. The vertical input of nitrate through eddy diffusion was calculated from the product of diffusivity, derived from e, and the gradient of nanomolar nitrate concentration across the base of the euphotic zone. The mean N 2 -fixation rate in EA was 56 6 49 mmol N m 22 d 21 , whereas SSA and NSA had much lower values (, 10 mmol N m 22 d 21 ). Because of the large spatial variability in nitrate diffusion (34 6 50, 405 6 888, and 844 6 1258 mmol N m 22 d 21 in SSA, EA, and NSA, respectively), the contribution of N 2 -fixation to new production in the SSA, EA, and NSA was 44% 6 30%, 22% 6 19%, and 2% 6 2%, respectively. The differences between SSA and NSA in the contribution of N 2 fixation were partly due to the contrasting seasonal forcing in each hemisphere, which likely affected both N 2 fixation rates and vertical nitrate diffusion. The variability in the nitrogen budget of the Atlantic subtropical gyres was unexpectedly high and largely uncoupled from relatively constant phytoplankton standing stocks and primary production rates.

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