Étude de la distribution des principaux groupes de<br /> phytoplancton par télédétection satellitaire: <br />Développement de la méthode PHYSAT à partir des données GeP&CO et application à l'archive SEAWIFS entre 1998 et 2004.

Phytoplankton plays an important role in the carbon cycle in the ocean. Ocean color sensors allow for more than 20 years a quasi-daily monitoring of the surface concentration in phytoplankton. These data are used to quantify the “ocean biological pump” of carbon. However, the efficiency of this « biological pump » strongly depends on the phytoplankton group present in surface waters. Distinguishing dominant phytoplankton groups from remote sensing was thus one of the major challenges of ocean color at the beginning of my PhD.During my PhD, I used an empirical approach to derive relationship between SeaWiFS spectral measurements and dominant phytoplankton groups by using a large set of quantitative inventories of phytoplankton pigments collected during GeP&CO cruises between France and New Caledonia (10/1999 - 07/2002). I have shown that four major phytoplankton groups, ie. Haptophytes, Prochlorococcus, Synechococcus-like cyanobacteria and diatoms can be detected from their spectral signatures and that three additional ones can also likely be detected (Trichodesmiums, Phaeocystis, Coccolithophorids). This classification, named PHYSAT, has been applied to all daily SeaWiFS data from 1997 to 2004. These results bring for the first time information about the space and time distribution of main phytoplankton groups at the global scale.Le phytoplancton joue un rôle important dans le cycle océanique du carbone. Les capteurs spatiaux de « couleur de l'océan » permettent, depuis plus de vingt ans, un suivi quasi-quotidien de la concentration en phytoplancton dans les eaux de surface du globe. Ces données sont utilisées pour quantifier la « pompe biologique marine » de carbone. Cependant, l'efficacité de cette « pompe biologique » varie fortement en fonction du groupe de phytoplancton présent dans les eaux de surface. Distinguer les principaux groupes depuis l'espace était donc un enjeu majeur des études de la couleur de l'océan au début de ma thèse.Durant ma thèse, j'ai utilisé une approche empirique visant à mettre en évidence des relations entre les mesures spectrales effectuées par le capteur spatial SeaWiFS et la présence de groupes dominants de phytoplancton identifiés à l'aide d'inventaires pigmentaires collectés lors des campagnes GeP&CO (10/1999 - 07/2002) entre la France et la Nouvelle-Calédonie. J'ai montré que quatre grands groupes de phytoplancton peuvent être identifiés de façon fiable (Haptophytes, Prochlorococcus, Cyanobacterie et Diatomées) et que trois groupes supplémentaires sont probablement détectables (Trichodesmiums, Phaeocystis, Coccolithophoridés). J'ai appliqué cette classification, appelée PHYSAT, à l'ensemble des données journalières du capteur SeaWIFS de 1997 à 2004. Les résultats obtenus apportent, pour la première fois, une information sur la distribution spatio-temporelle des principaux groupes de phytoplancton à l'échelle du globe

En quoi, dans le cadre d’interactions entre élèves et chercheurs, les conditions installées pour et par les mises en récit peuvent-elles favoriser les apprentissages scientifiques.

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En quoi, dans le cadre d’interactions entre élèves et chercheurs, les conditions installées pour et par des mises en récit peuvent-elles favoriser les apprentissages scientifiques ?

Si l’usage du récit apparaît fréquemment dans les activités scientifiques, de nombreux auteurs signalent à son propos la nécessité d’en questionner l’intérêt et les limites. En s’interrogeant précisément sur les rapports qu’entretiennent sciences et récits, des travaux en didactique en Sciences de la vie et de la Terre ont montré la tendance des élèves à utiliser un type de récit particulier, des « petites histoires » racontées sur le mode de l’anecdote, pour expliquer le fonctionnement d’un ..

Remote sensing of phytoplankton groups in case 1 waters from global SeaWiFS imagery

International audienceOcean color sensors enable a quasi-permanent monitoring of the chlorophyll a concentration, Chl a, in surface waters. This ubiquitous photosynthetic pigment cannot, however, be used to distinguish between phytoplankton species. Distinguishing phytoplankton groups from space is nevertheless necessary to better study some biochemical processes such as carbon fixation at the global scale, and is thus one of the major challenges of ocean color research. In situ data have shown that the water-leaving radiances (nLw), measured by ocean color sensors at different wavelengths in the visible spectrum, vary significantly for a given Chl a. This natural variability is due partly to differences in optical properties of phytoplankton species. Here we derive relationships between nLw and phytoplankton species by using a large set of quantitative inventories of phytoplankton pigments collected during nine cruises from Le Havre (France) to Nouméa (New Caledonia) in the framework of the GeP&CO program. Coincident SeaWiFS nLw data between 412 and 555 nm are extracted and normalized to remove the effect of Chl a. These normalized spectra vary significantly with in-situ pigment composition, so that four major phytoplankton groups, i.e., haptophytes, Prochlorococcus, Synechococcus-like cyanobacteria and diatoms, can be distinguished. This classification (PHYSAT) is applied to the global SeaWiFS dataset for year 2001, and global maps of phytoplankton groups are 1 presented. Haptophytes and diatoms are found mostly in high latitudes and in eutrophic regions. Diatoms show a strong seasonal cycle with large-scale blooms during spring and summer. These results, obtained with only five channels in the visible spectrum, demonstrate that ocean color measurements can be used to discriminate between dominant phytoplankton groups provided that sufficient data are available to establish the necessary empirical relationships

Fluid dynamical niches of phytoplankton types

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Response of diatoms distribution to global warming and potential implications: A global model study

Using a global model of ocean biogeochemistry coupled to a climate model, we explore the effect of climate change on the distribution of diatoms, a key phytoplankton functional group. Our model results suggest that climate change leads to more nutrient-depleted conditions in the surface ocean and that it favors small phytoplankton at the expense of diatoms. At 4xCO 2 , diatoms relative abundance is reduced by more than 10% at the global scale and by up to 60% in the North Atlantic and in the subantarctic Pacific. This simulated change in the ecosystem structure impacts oceanic carbon uptake by reducing the efficiency of the biological pump, thus contributing to the positive feedback between climate change and the ocean carbon cycle. However, our model simulations do not identify this biological mechanism as a first-order process in the response of ocean carbon uptake to climate chang

A species-dependent bio-optical model of case I waters for global ocean color processing

International audienceThe PHYSAT method, which enables identification of four different phytoplankton groups from their impact on the normalized water-leaving radiance (nLw) spectra, is applied to coincident in situ measurements of both chlorophyll a concentration (Chl a) and nLw. Observations show that measurements acquired in waters dominated by haptophytes, diatoms and Synechococcus-like cyanobacteria have optical properties that deviate significantly from the mean OC4V4 bio-optical model, which is currently used as the standard for global ocean color processing. A specific OC4v4-like relationship, i.e., Chl a as a 4th-order polynomial of the "maximum band ratio", was fitted for each dominant phytoplankton group identified by PHYSAT. The resulting OC4-SD bio-optical model thus starts from a classification of the dominant phytoplankton group, followed by a species-dependent estimate of Chl a. It has been applied to global daily SEAWIFS data of the year 2001. Monthly mean maps of Chl a derived from OC4-SD or OC4v4 show large regional differences that can reach 50% at high latitudes.The new algorithm leads to lower concentrations in regions where the standard model retrievals are known to be too high, such as the Mediterranean Sea. Conversely, higher concentrations are retrieved in regions dominated by diatom blooms, such as the northern North Atlantic in summer, where previous studies have demonstrated a low bias in standard SEAWIFS Chl a