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Morel A.,University Pierre and Marie Curie | Claustre H.,University Pierre and Marie Curie | Gentili B.,CNRS Oceanography Laboratory of Villefranche
Biogeosciences | Year: 2010

The cores of the subtropical anticyclonic gyres are characterized by their oligotrophic status and minimal chlorophyll concentration, compared to that of the whole ocean. These zones are unambiguously detected by space borne ocean color sensors thanks to their typical spectral reflectance, which is that of extremely clear and deep blue waters. Not only the low chlorophyll (denoted [Chl]) level, but also a reduced amount of colored dissolved organic matter (CDOM or "yellow substance") account for this clarity. The oligotrophic waters of the North and South Pacific gyres, the North and South Atlantic gyres, and the South Indian gyre have been comparatively studied with respect to both [Chl] and CDOM contents, by using 10-year data (1998-2007) of the Sea-viewing Wide field-of-view Sensor (SeaWiFS, NASA). Albeit similar these oligotrophic zones are not identical regarding their [Chl] and CDOM contents, as well as their seasonal cycles. According to the zone, the averaged [Chl] value varies from 0.026 to 0.059 mg mg 3, whereas the I (443) average (the absorption coefficient due to CDOM at 443 nm) is between 0.0033 and 0.0072 mg 1. The CDOM-to-[Chl] relative proportions also differ between the zones. The clearest waters, corresponding to the lowest [Chl] and CDOM concentrations, are found near Easter Island and near Mariana Islands in the western part of the North Pacific Ocean. In spite of its low [Chl], the Sargasso Sea presents the highest CDOM content amongst the six zones studied. Except in the North Pacific gyre (near Mariana and south of Hawaii islands), a conspicuous seasonality appears to be the rule in the other 4 gyres and affects both [Chl] and CDOM; both quantities vary in a ratio of about 2 (maximum-to-minimum). Coinciding [Chl] and CDOM peaks occur just after the local winter solstice, which is also the period of the maximal mixed layer depth in these latitudes. It is hypothesized that the vertical transport of unbleached CDOM from the subthermocline layers is the main process enhancing the CDOM concentration within the upper layer in winter. In summer, the CDOM experiences its minimum which is delayed with respect to the [Chl] minimum; apparently, the solar photo-bleaching of CDOM is a slower process than the post-bloom algal Chl decay. Where they exist, the seasonal cycles are repeated without notable change from year to year. Long term (10 y) trends have not been detected in these zones. These oligotrophic gyres can conveniently be used for in-flight calibration and comparison of ocean color sensors, provided that their marked seasonal variations are accounted for. © Author(s) 2010.

Dolan J.R.,CNRS Oceanography Laboratory of Villefranche | Stoeck T.,University of Kaiserslautern
Environmental Microbiology Reports | Year: 2011

Diversity metrics and descriptors of protistan community structure were calculated from 12 samples of 10l each collected from the Bay of Villefranche in the NW Mediterranean Sea. Variability of the sampling was on scales of minutes and meters. The individual samples were compared with each other and compared with a pooled data set from the total volume of 120l, considered as the 'true' community. We focused on a single group of planktonic protists, tintinnids, a coherent functional and phylogenetic group in which morpho-species identifications by light microscopy are unambiguous. Tintinnid abundance in the samples ranged from 217 to 321 cells of 16-21 species with the number of rare species in a sample (<1% of abundance) positively related to species richness of the sample. Rarefaction estimates of total species richness in the 12 samples ranged from 21±3.5 to 37±3.6 compared with the 34 species of the pooled data set. The measures of similarity reflected the differences between samples in both the numbers and identities of the least abundant or rare species. The species abundance distribution using pooled data was best fit by a log-series or geometric distribution; eight species accounted for about 90% of total cells and most species, the remaining 22 out of 34, were 'rare' (concentration<1% of total cells). Among the samples, 5 were best fit by a geometric model, 1 by a log-series distribution, 2 by a log-normal or log-series model, and 4 could not be clearly assigned a particular distribution. Our results suggest that single sample estimates of species richness are relatively robust compared with measures of taxonomic similarity and species abundance distribution. When measuring differences among populations sample variability should be considered. © 2011 Society for Applied Microbiology and Blackwell Publishing Ltd.

Moutin T.,Aix - Marseille University | Prieur L.,CNRS Oceanography Laboratory of Villefranche
Biogeosciences | Year: 2012

We studied a longitudinal transect in the Mediterranean Sea (MS) and along this transect, the influence of anticyclonic eddies at three long duration (LD) stations. The deep chlorophyll maximum depth, the euphotic layer depth and the top of the nitracline depth are clearly correlated outside of the eddies, and deepen from the oligotrophic western to the ultraoligotrophic eastern MS. We provide evidence that the locations of the three LD stations studied were near the axis of the eddies. Their diameters were close to 100 km and the studied areas were less than 10 km from the centre of the eddies. The positions of the LD stations are marked by an increase in the flux function and a decrease in apparent oxygen utilization (AOU) and in excess density σ), as expected for anticyclonic eddies. Integrated mean primary production measured in situ inside the three studied eddies confirms the previous conclusion that integrated primary production (IPP) about 150 mgC mg-2 dg -1 may appear as a lower limit for IPP during strong oligotrophic conditions. The mesoscale activity is strong enough to locally modify the very well-documented western-to-eastern gradient of trophic conditions in the MS. We proposed a new calculation for mixed layer depths (MLDs) enabling the determination of MLD to take into consideration processes occurring with time scales ranging from a few hours to several days, and also the winter MLD. Studying the main physical, chemical and dynamical characteristics of the three eddies enables us to consider that the vorticity barrier prevents any strong mixing and advection of outer water inside the eddy and explains why the depth range of eddies starts from the surface. As a first approximation, the anticyclonic eddies could be considered as closed systems dating back to the previous winter, making possible to draw first-order budgets. The daily new N-input in the photic zone is virtually identical to the N-export measured at 230 m by drifting traps. This means that the eddies are close to an equilibrium state where input is equal to loss. The annual N-input by winter convection, which is a fundamental criterion for new nutrient availability, may be extremely variable inside eddies, with W-MLD varying from 90.5 m at the western station to 396.5 m at the eastern station. W-MLDs are always deeper inside the eddies than outside where they are in keeping with climatological averages. AOU was low inside the eddies; this together with the near-identical export measured at 230 and 460 m seems to indicate that eddy cores are areas where low mineralisation of particulate organic matter occurs. "In" and "out" AOU comparisons indicate lower mineralisation inside the eddies suggesting a higher efficiency for CO2 sequestration via sedimentation of particulate organic matter. The three eddies are enriched in dissolved organic carbon (DOC). Sequestration of CO2 by vertical export of accumulated DOC therefore seems to be higher inside eddies. The relative importance of DOC transport in the biological pump is probably one of the main characteristics of low-P low chlorophyll (LPLC) areas, and it is likely to be reinforced inside anticyclonic eddies. The numerous anticyclonic eddies in the MS are likely to influence the water masses and their dispersion, and therefore have a strong impact on the biogeochemical properties at the scale of the MS. © Author(s) 2012.

Doble M.J.,CNRS Oceanography Laboratory of Villefranche | Bidlot J.-R.,European Center for Medium Range Weather Forecasts
Ocean Modelling | Year: 2013

The breakup of pack ice in the Weddell Sea is examined with respect to a single wave buoy, frozen into the pack ice six months earlier, and the ECMWF WAM model. The pack ice broke up around the buoy on 14th September 2000 as large amplitude storm waves approached the ice edge at the buoy's location. The WAM model is modified to allow waves to propagate into the ice cover, in contrast to the operational scheme which sets wave energy to zero at ice concentrations over 30%. A simple, lookup-table-based, wave scattering attenuation scheme is then added and is combined with a sea ice drag attenuation parameterisation. WAM results at the location of the buoy are compared to the observations over a two-month period straddling the breakup. The modified WAM scheme generally reproduces the significant wave height, wave period and spectral characteristics measured by the buoy, though the model does not yet have any concept of floe breaking and re-freezing, assuming only that the ice cover is broken if the concentration is less than 80%. The simplistic nature of these modifications is designed to allow operational implementation, to eventually provide a global assessment of the wave-influenced ice zone. © 2013 Elsevier Ltd.

Dolan J.R.,CNRS Oceanography Laboratory of Villefranche | Landry M.R.,University of California at San Diego | Ritchie M.E.,Syracuse University
ISME Journal | Year: 2013

Many microbial taxa in the marine plankton appear super-saturated in species richness. Here, we provide a partial explanation by analyzing how species are organized, species packing, in terms of both taxonomy and morphology. We focused on a well-studied group, tintinnid ciliates of the microzooplankton, in which feeding ecology is closely linked to morphology. Populations in three distinct systems were examined: an Eastern Mediterranean Gyre, a Western Mediterranean Gyre and the California Current. We found that species abundance distributions exhibited the long-tailed, log distributions typical of most natural assemblages of microbial and other organisms. In contrast, grouping in oral size-classes, which corresponds with prey-size exploited, revealed a geometric distribution consistent with a dominant role of a single resource in structuring an assemblage. The number of species found in a particular oral size-class increases with the numerical importance of the size-class in the overall population. We suggest that high species diversity reflects the fact that accompanying each dominant species are many ecologically similar species, presumably able to replace the dominant species, at least with regard to the size of prey exploited. Such redundancy suggests that species diversity greatly exceeds ecological diversity in the plankton. © 2013 International Society for Microbial Ecology.

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