Introduction to the special section bio-optical and biogeochemical conditions in the South East Pacific in late 2004: the BIOSOPE program

International audienceThe objectives of the BIOSOPE (BIogeochemistry and Optics SOuth Pacific Experiment) project was to study, during the austral summer, the biological, biogeochemical and bio-optical properties of different trophic regimes in the South East Pacific: the eutrophic zone associated with the upwelling regime off the Chilean coast, the mesotrophic area associated with the plume of the Marquises Islands in the HNLC (High Nutrient Low Chlorophyll) waters of this subequatorial area, and the extremely oligotrophic area associated with the central part of the South Pacific Gyre (SPG). At the end of 2004, a 55-day international cruise with 32 scientists on board took place between Tahiti and Chile, crossing the SPG along a North-West South-East transect. This paper describes in detail the objectives of the BIOSOPE project, the implementation plan of the cruise, the main hydrological entities encountered along the ~8000 km South East Pacific transect, and ends with a general overview of the 32 other papers published in this special issue

Two High-Nutrient Low-Chlorophyll phytoplankton assemblages: the tropical central Pacific and the offshore Perú-Chile Current

International audienceThe phytoplankton (>15 µm) composition and abundance was investigated along a ~8000 km transect between the Marquesas Islands Archipelago and the Chilean coasts off Concepción. In the southern limit of the central Equatorial Pacific (at 8° S, 141° W), in High-Nutrient Low-Chlorophyll (HNLC) warm waters, the microphytoplankton assemblage was dominated by the lightly silicified diatoms Pseudo-nitzschia delicatissima and Rhizosolenia bergonii. The morphology of these species, a small pennate diatom that exhibited a tendency to form "ball of needles" clusters and large centric diatom (>500 µm long), are interpreted as two anti-grazing strategies in an environment dominated by small micrograzers. Surprisingly, this a priori typical HNLC phytoplankton assemblage was also found in the temperate offshore waters of the Perú-Chile Current between 2000 and 600 km off Chile. This observation suggests that a common set of environmental factors (obviously other than temperature and salinity) are responsible for the establishment and maintaining of this distinctive phytoplankton in these geographically and hydrologically distant regions. Both regions are characterized by a surface nitrate-silicic acid ratio ranging from 1–3. Occasionally Rhizosolenia bergonii showed frustules anomalously fragmented, likely the result of extreme weakly silicified phytoplankton. We suggest that silicon deficiency may be responsible of the occurrence of HNLC phytoplankton assemblage in the tropical central Pacific as well as the offshore Perú-Chile Current during the austral summer

Phytoplankton biomass cycles in the North Atlantic subpolar gyre: A similar mechanism for two different blooms in the Labrador Sea

An analysis of seasonal variations in climatological surface chlorophyll points to distinct biogeographical zones in the North Atlantic subpolar gyre. In particular, the Labrador Sea appears well delineated into two regions on either side of the 60°N parallel, with very different climatological phytoplankton biomass cycles. Indeed, north of 60°N, an early and short spring bloom occurs in late April, while south of 60°N, the bloom gradually develops 1 month later and significant biomass persists all summer long. Nevertheless, at climatological scale, the first-order mechanism that controls the bloom is identical for both bioregions. The light-mixing regime can explain the bloom onset in both bioregions. In the Labrador Sea, the blooms seem to rely on a mean community compensation irradiance threshold value of 2.5 mol photon m−2 d−1 over the mixed layer

Calcite production by Coccolithophores in the South East Pacific Ocean: from desert to jungle

International audienceBIOSOPE cruise achieved an oceanographic transect from the Marquise Islands to the Peru-Chili upwelling (PCU) via the centre of the South Pacific Gyre (SPG). Water samples from 6 depths in the euphotic zone were collected at 20 stations. The concentrations of suspended calcite particles, coccolithophores cells and detached coccoliths were estimated together with size and weight using an automatic polarizing microscope, a digital camera, and a collection of softwares performing morphometry and pattern recognition. Some of these softwares are new and described here for the first time. The coccolithophores standing stocks are usually low and reach maxima west of the PCU. The coccoliths of Emiliania huxleyi, Gephyrocapsa spp. and Crenalithus spp. (Order Isochrysidales) represent 50% of all the suspended calcite particles detected in the size range 0.1–46 µm (21% of PIC in term of the calcite weight). The latter species are found to grow preferentially in the Chlorophyll maximum zone. In the SPG their maximum concentrations was found to occur between 150 and 200 m, which is very deep for these taxa. The weight and size of coccoliths and coccospheres are correlated. Large and heavy coccoliths and coccospheres are found in the regions with relative higher fertility in the Marquises Island and in the PCU. Small and light coccoliths and coccospheres are found west of the PCU. This distribution may correspond to that of the concentration of calcium and carbonate ions

Contrasting distribution of aggregates >100 µm in the upper kilometre of the South-Eastern Pacific

International audienceLarge sinking particles transport organic and inorganic matter into the deeper layers of the oceans. From 70 to 90% of the superficial particulate material is disaggregated within the upper 1000 m. This decrease with depth indicates that remineralization processes are intense during sedimentation. Generally, the estimates of vertical flux rely on the sediment trap data but difficulties inherent in their design, limit the reliability of this information. During the BIOSOPE study in the southeastern Pacific, 76 vertical casts using the Underwater Video Profiler (UVP) and deployments of a limited number of drifting sediment traps provided an opportunity to fit the UVP data to sediment trap flux measurements. We applied than the calculated UVP flux in the upper 1000 m to the whole 8000 km BIOSOPE transect. Comparison between the large particulate material (LPM) abundance and the estimated fluxes from both UVP and sediment traps showed different patterns in different regions. On the western end of the BIOSOPE section the standing stock of particles in the superficial layer was high but the export between 150 and 250 m was low. Below this layer the flux values increased. High values of about 30% of the calculated UVP maximum superficial flux were observed below 900 m at the HNLC station. The South Pacific Gyre exported about 2 mg m-2 d-1. While off Chilean coast 95% of the superficial matter was remineralized or advected in the upper kilometer, 20% of the superficial flux was observed below 900 m near the Chilean coast. These results suggest that the export to deep waters is spatially heterogeneous and related to the different biotic and abiotic factors

Prochlorococcus and Synechococcus: A comparative study of their optical properties in relation to their size and pigmentation

Three unialgal strains of Prochlorococcus and four of Synechococcus were grown in batch culture at low irradiances. The spectral values of light absorption, scattering and backscattering of intact cells in suspension were determined, together with cell counts, size distribution and pigment composition (via HPLC). The spectral efficiency factors Qa, Qb, Qbb for light absorption, scattering and backscattering respectively, were derived, as well as the corresponding chlorophyll-specific coefficients a*, b* and bb*. The pigment used when normalizing is “true” chlorophyll a for Synechococcus, and divinyl-chlorophyll a for Prochlorococcus. In correspondence with small sizes (0.6 μm, on average) Prochlorococcus exhibits Qb values below those of Synechococcus (size about 0.9 μm, on average). In contrast, Qa is higher for Prochlorococcus than for Synechococcus, in response to high internal divinyl-chlorophyll content. In the blue part of the spectrum the probability for photons of being absorbed by a Prochlorococcus cell exceeds that of being scattered. Such a combination has never been found before for other algal cells, consistently more efficient as scatterers than as absorbers. The magnitude of the three efficiency Q-factors, as well as their spectral variations, can be understood and reconstructed in the frame of the Mie theory. The impact of these small organisms, dominant in oligotrophic environment, upon the optical properties of such waters are discussed on the basis of their chlorophyll-specific optical coefficients. Their absorption capabilities (per unit of chlorophyll) are not far from being maximum, to the extent that the package effect is rather reduced. With respect to scattering, Prochlorococcus cells have a minute signature compared to that of Synechococcus. This point is illustrated using vertical profiles of fluorescence, attenuation coefficient, cell number, Chl a and divinyl-Chl a concentrations, as observed in an oligotrophic tropical situation. Even if the backscattering-to-scattering ratio is, as theoretically expected, higher for Prochlorococcus than for all other algae (including Synechococcus), their light backscattering capacity definitely remains negligible

Contribution of picoplankton to the total particulate organic carbon (POC) concentration in the eastern South Pacific

International audienceProchlorococcus, Synechococcus, picophytoeukaryotes and bacterioplankton abundances and contributions to the total particulate organic carbon concentration (POC), derived from the total particle beam attenuation coefficient (cp), were determined across the eastern South Pacific between the Marquesas Islands and the coast of Chile. All flow cytometrically derived abundances decreased towards the hyper-oligotrophic centre of the gyre and were highest at the coast, except for Prochlorococcus, which is not detected under eutrophic conditions. Temperature and nutrient availability appeared important in modulating picophytoplankton abundance, according to the prevailing trophic conditions. Although the non-vegetal particles tended to dominate the cp signal everywhere along the transect (50 to 83%), this dominance seemed to weaken from oligo- to eutrophic conditions, the contributions by vegetal and non-vegetal particles being about equal under mature upwelling conditions. Spatial variability in the vegetal compartment was more important than the non-vegetal one in shaping the water column particulate attenuation coefficient. Spatial variability in picophytoplankton biomass could be traced by changes in both Tchla and cp. Finally, picophytoeukaryotes contributed with ~38% on average to the total integrated phytoplankton carbon biomass or vegetal attenuation signal along the transect, as determined by direct size measurements on cells sorted by flow cytometry and optical theory. The role of picophytoeukaryotes in carbon and energy flow would therefore be very important, even under hyper-oligotrophic conditions

Contribution of picoplankton to the total particulate organic carbon concentration in the eastern South Pacific

International audienceProchlorococcus, Synechococcus, picophytoeukaryotes and bacterioplankton abundances and contributions to the total particulate organic carbon concentration, derived from the total particle beam attenuation coefficient (cp), were determined across the eastern South Pacific between the Marquesas Islands and the coast of Chile. All flow cytometrically derived abundances decreased towards the hyper-oligotrophic centre of the gyre and were highest at the coast, except for Prochlorococcus, which was not detected under eutrophic conditions. Temperature and nutrient availability appeared important in modulating picophytoplankton abundance, according to the prevailing trophic conditions. Although the non-vegetal particles tended to dominate the cp signal everywhere along the transect (50 to 83%), this dominance seemed to weaken from oligo- to eutrophic conditions, the contributions by vegetal and non-vegetal particles being about equal under mature upwelling conditions. Spatial variability in the vegetal compartment was more important than the non-vegetal one in shaping the water column particle beam attenuation coefficient. Spatial variability in picophytoplankton biomass could be traced by changes in both total chlorophyll a (i.e. mono + divinyl chlorophyll a) concentration and cp. Finally, picophytoeukaryotes contributed ~38% on average to the total integrated phytoplankton carbon biomass or vegetal attenuation signal along the transect, as determined by size measurements (i.e. equivalent spherical diameter) on cells sorted by flow cytometry and optical theory. Although there are some uncertainties associated with these estimates, the new approach used in this work further supports the idea that picophytoeukaryotes play a dominant role in carbon cycling in the upper open ocean, even under hyper-oligotrophic conditions

Optical backscattering properties of the "clearest" natural waters

International audienceDuring the BIOSOPE field campaign October–December 2004, measurements of inherent optical properties from the surface to 500 m depth were made with a ship profiler at stations covering over ~8000 km through the Southeast Pacific Ocean. Data from a ~3000 km section containing the very clearest waters in the central gyre are reported here. The total volume scattering function at 117°, ßt(117°), was measured with a WET Labs ECO-BB3 sensor at 462, 532, and 650 nm with estimated uncertainties of 2×10-5, 5×10-6, and 2×10-6 m-1 sr-1, respectively. These values were approximately 6%, 3%, and 3% of the scattering by pure seawater at their respective wavelengths. From a methodological perspective, there were several results: – bbp distributions were resolvable even though some of the values from the central gyre were an order of magnitude lower than the lowest previous measurements in the literature; – Direct in-situ measurements of instrument dark offsets were necessary to accurately resolve backscattering at these low levels; – accurate pure seawater backscattering values are critical in determining particulate backscattering coefficients in the open ocean (not only in these very clear waters); the pure water scattering values determined by Buiteveld et al. (1994) with a [1 + 0.3S/37] adjustment for salinity based on Morel (1974) appear to be the most accurate estimates, with aggregate accuracies as low as a few percent; and – closure was demonstrated with subsurface reflectance measurements reported by Morel et al. (2007) within instrument precisions, a useful factor in validating the backscattering measurements. This methodology enabled several observations with respect to the hydrography and the use of backscattering as a biogeochemical proxy: – The clearest waters sampled were found at depths between 300 and 350 m, from 23.5° S, 118° W to 26° S, 114° W, where total backscattering at 650 nm was not distinguishable from pure seawater; – Distributions of particulate backscattering bbp across the central gyre exhibited a broad particle peak centered ~100 m; – The particulate backscattering ratio typically ranged between 0.4% and 0.6% through the majority of the central gyre from the surface to ~210 m, indicative of "soft" water-filled particles with low bulk refractive index; and – bbp at 532 and 650 nm showed a distinct secondary deeper layer centered ~230 m that was absent in particulate attenuation cp data. The particulate backscattering ratio was significantly higher in this layer than in the rest of the water column, reaching 1.2% in some locations. This high relative backscattering, along with the pigment composition and ecological niche of this layer, appear to be consistent with the coccolithophorid F. profunda. Moreover, results were consistent with several expectations extrapolated from theory and previous work in oceanic and coastal regions, supporting the conclusion that particulate and total backscattering could be resolved in these extremely clear natural waters

Ecology and biogeochemistry of contrasting trophic environments in the South East Pacific by carbon isotope ratios on lipid biomarkers

International audienceThe distribution of lipid biomarkers and their carbon isotope composition was investigated on suspended particles from different contrasting trophic environments at six sites in the South East Pacific. High algal biomass with diatom-related lipids was characteristic in the upwelling zone, whereas haptophyte lipids were proportionally most abundant in the nutrient-poor settings of the centre of the South Pacific Gyre and on its easter edge. Dinoflagellate–sterols were minor contributors in all of the studied area and cyanobacteria-hydrocarbons were at maximum in the high nutrient low chlorophyll regime of the subequatorial waters at near the Marquesas archipelago. The taxonomic and spatial variability of the relationships between carbon photosynthetic fractionation and environmental conditions for four specific algal taxa (diatoms, haptophytes, dinoflagellates and cyanobacteria) was also investigated. The carbon isotope fractionation factor (ep) of the diatom marker varied over a range of 16‰ along the different trophic systems. In contrast, ep of dinoflagellate, cyanobacteria and alkenone markers varied only by 7–10‰. The low fractionation factors and small variations between the different phytoplankton markers measured in the upwelling area likely reveals uniformly high specific growth rates within the four phytoplankton taxa, and/or that transport of inorganic carbon into phytoplankton cells may not only occur by diffusion but by other carbon concentrating mechanisms (CCM). In contrast, in the oligotrophic zone, i.e. gyre and eastgyre, relatively high ep values, especially for the diatom marker, indicate diffusive CO2 uptake by the eukaryotic phytoplankton. At these nutrient-poor sites, the lowest ep values for haptophytes, dinoflagellates and cyanobacteria infer higher growth rates compared to diatoms