Throughout tropical and subtropical seas, Trichodesmium spp. contribute significantly to marine fixation of atmospheric di-nitrogen and influence the global carbon cycle. We suggest that dissolved inorganic phosphate (DIP) availability has a predominant role in controlling Trichodesmium spp. biomass. From experimental work carried out on cruises in the SW Pacific Ocean, and by re-analysing previous data, we have defined a critical level of DIP needed for single filaments of Trichodesmium spp. to grow. Thus, seasonal variations in DIP availability could control Trichodesmium spp. growth and decay. As this critical level is below the detection limit of classical DIP measurements obtained during oceanic cruises, we suggest a re-evaluation of the phosphate availability in the oligotrophic ocean in order to determine what ultimately controls di-nitrogen fixation in the sea.

Is it possible to derive accurately Total Suspended Matter concentration or its proxy, turbidity, from remote sensing data in tropical coastal lagoon waters? To investigate this question, hyperspectral remote sensing reflectance, turbidity and chlorophyll pigment concentration were measured in three coral reef lagoons. The three sites enabled us to get data over very diverse environments: oligotrophic and sediment-poor waters in the southwest lagoon of New Caledonia, eutrophic waters in the Cienfuegos Bay (Cuba), and sediment-rich waters in the Laucala Bay (Fiji). In this paper, optical algorithms for turbidity are presented per site based on 113 stations in New Caledonia, 24 stations in Cuba and 56 stations in Fiji. Empirical algorithms are tested at satellite wavebands useful to coastal applications. Global algorithms are also derived for the merged data set (193 stations). The performances of global and local regression algorithms are compared. The best one-band algorithms on all the measurements are obtained at 681 nm using either a polynomial or a power model. The best two-band algorithms are obtained with R412/R620, R443/R670 and R510/R681. Two three-band algorithms based on Rrs620.Rrs681/Rrs412 and Rrs620.Rrs681/Rrs510 also give fair regression statistics. Finally, we propose a global algorithm based on one or three bands: turbidity is first calculated from Rrs681 and then, if < 1 FTU, it is recalculated using an algorithm based on Rrs620.Rrs681/Rrs412. On our data set, this algorithm is suitable for the 0.2-25 FTU turbidity range and for the three sites sampled (mean bias: 3.6 %, rms: 35%, mean quadratic error: 1.4 FTU). This shows that defining global empirical turbidity algorithms in tropical coastal waters is at reach.

[1] As part of the French Joint Global Ocean Flux Study Etude du Broutage en Zone Equatoriale program, we investigated the distributions of microorganisms (bacteria and protists <200 mm) in the upper 120 m of the equatorial Pacific from 8°S to 8°N, along 180°. Population distributions, determined by a combination of flow cytometry, microscopy and spectrofluorometry, were closely related to physical features across the study site. Phytoplankton biomass, ranging from 1.2 to 34.2 mg C L À1 and averaging 15.5 mg C L À1 , was most enhanced in the divergence zone. Carbon to chlorophyll ratios were also enhanced in the divergence zone and showed distinct latitudinal variations. Heterotrophic biomass, excluding ciliates, was patchy across the area, ranging from 5 to 36 mg C L À1 and averaging 13 mg C L À1 . Prokaryotic species (Prochlorococcus spp., Synechococcus spp., and heterotrophic bacteria) showed similar patterns of abundance, with the main feature being their distributional asymmetry to the south of the equator. Both autotrophic and heterotrophic biomass were enriched in the convergent zone at 4°-5°N between the South Equatorial Current and the North Equatorial Counter Current. Heterotrophic biomass exceeded phytoplankton biomass in the more nutrientimpoverished waters to the north and in the branch of a tropical instability wave eddy. Microplankton represented only a small portion of the total autotrophic carbon and was comprised mostly of dinoflagellates. Large species dominated the relatively modest diatom biomass. Food web interactions and biogeochemical fluxes in the central equatorial Pacific may be significantly influenced by temporal and spatial variability of the microbial community associated with physical features of the region.INDEX TERMS: 4231 Oceanography: General: Equatorial oceanography; 4223 Oceanography: General: Descriptive and regional oceanography; 4815 Oceanography: Biological and Chemical: Ecosystems, structure and dynamics; 4840 Oceanography: Biological and Chemical: Microbiology; KEYWORDS: microbial community structure, plankton, equatorial Pacific Citation: Brown, S. L., M. R. Landry, J. Neveux, and C. Dupouy, Microbial community abundance and biomass along a 180°t ransect in the equatorial Pacific during an El Niño-Southern Oscillation cold phase,

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