Abstract.Ocean 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.

Phytoplankton plays an important role in the global carbon cycle via the fixation of inorganic carbon during photosynthesis. However, the efficiency of this “biological pump of carbon” strongly depends on the nature of the phytoplankton. Monitoring spatial and temporal variations of the distribution of dominant phytoplankton groups at the global scale is thus of critical importance. Recently, an algorithm has been developed to detect the major dominant phytoplankton groups from anomalies of the marine signal measured by ocean color satellites. This method, called PHYSAT, allows to identify nanoeucaryotes, Prochlorococcus, Synechococcus and diatoms. In this paper, PHYSAT has been improved to detect an additional group, named phaeocystis‐like, by analyzing specific signal anomalies in the Southern Ocean during winter months. This new version of PHYSAT was then used to process daily global SeaWiFS GAC data between 1998 and 2006. The global distribution of major phytoplankton groups is presented in this study as a monthly climatology of the most frequent phytoplankton group. The contribution of nanoeucaryotes‐dominated waters to the global ocean varies from 45 to 70% depending on the season, whereas both diatoms and phaeocystis‐like contributions exhibit a stronger seasonal variability mostly due to the large blooms that occur during winter in the Southern Ocean. Three regions of particular interest are also studied in more details: the Southern Ocean, the North Atlantic, and the Equatorial Pacific. The North Atlantic diatom bloom shows a large interannual variability. Large blooms of both diatoms and phaeocystis‐like are observed during winter in the Southern Ocean, with a larger contribution from diatoms. Their respective geographical distribution is shown to be tightly related to the depth of the mixed‐layer, with diatoms prevailing in stratified waters. Synechococcus and Prochloroccocus prevail in the Equatorial Pacific, but our data show also sporadic diatoms contributions in this region during La Niña. The observed seasonal cycle and interannual variability of phytoplankton groups in the global ocean suggest that the PHYSAT archive is suitable to study the impact of climate variability on the structure of marine ecosystems.

The biogeochemical role of phytoplanktonic organisms strongly varies from one plankton type to another, and their relative abundance and distribution have fundamental consequences at the global and climatological scales. In situ observations find dominant types often associated to specific physical and chemical water properties. However, the mechanisms and spatiotemporal scales by which marine ecosystems are organized are largely not known. Here we investigate the spatiotemporal organization of phytoplankton communities by combining multisatellite data, notably high-resolution ocean-color maps of dominant types and altimetryderived Lagrangian diagnostics of the surface transport. We find that the phytoplanktonic landscape is organized in (sub-)mesoscale patches (10-100 km) of dominant types separated by physical fronts induced by horizontal stirring. These physical fronts delimit niches supported by water masses of similar history and whose lifetimes are comparable with the timescale of the bloom onset (few weeks). The resonance between biological activity and physical processes suggest that the spatiotemporal (sub-)mesoscales associated to stirring are determinant in the observation and modeling of marine ecosystems.biogeochemical cycles | chaotic stirring | marine biogeography | marine ecology | phytoplankton functional types P hytoplankton play several key roles in the ocean and climate systems, supporting the marine food chain and taking part in chemical cycles, including CO 2 recycling (1, 2). The specific biogeochemical role of planktonic organisms and their response to environmental changes strongly depend on the functional type they belong to: For instance, sinking diatoms efficiently export carbon to the deep ocean; coccolithophorids' biocalcification reduces sea water alkalinity and is critically affected by ocean acidification; Phaeocystis produces dimethyl sulphide, influencing the Earth's climate through sulphate aerosol formation. A widespread characteristic of planktonic populations is that a huge number of species are present at any location, but only a few dominate the biomass (3). At the basin-scale, the dominance of a specific type is generally understood in terms of adaptation to the local water properties (4-7) and a climatological emergence of plankton biogeography has been documented in both models and field studies (3,6,8). At a smaller scale, remotesensing and in situ observations of tracers such as sea surface temperature and chlorophyll concentration evidence strong contrasts in the upper ocean layer. Here ∼100 km-large and ∼month-lasting mesoscale turbulence structures organize biogeochemical tracers in filaments of mesoscale length but of thinner width (∼10 km) and shorter lifetimes (days/weeks) (9-14). In the following, we will refer to this spatiotemporal domain as the (sub-) mesoscale. How this variability is reflected by the biogeography of planktonic communities is not known, but is of key concern for fishery management, conservation ecology, tuning next-generation high-resoluti...

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The JGOFS Report Series is published by SCOR and includes the following:No.

A field experiment in the southwesternIndian Ocean provides new insights into ocean-atmosphere interactions in a key climatic region. W hile easterly trade winds blow year-round over the southern Indian Ocean, surface winds experience a striking reversal north of 10°S. During boreal summer, the low-level easterly flow penetrates northward, is deflected when crossing the equator, and forms the strong Indian monsoon jet. During boreal winter, northeasterly winds also bend while crossing the equator southward and form a weak low-level westerly jet between the equator and 10°S (Fig. la)

Monthly averaged level-3 SeaWiFS chlorophyll concentration data from 1998 to 2001 are globally analyzed using Fourier's analysis to determine the main patterns of temporal variability in all parts of the world ocean. In most regions, seasonal variability dominates over interannual variability, and the timing of the yearly bloom can generally be explained by the local cycle of solar energy. The studied period was influenced by the late consequences of the very strong El Niño of 1997-98. After this major event, the recovery to normal conditions followed different patterns at different locations. Right at the equator, chlorophyll concentration was abnormally high in 1998, and then decreased, while aside from the equator, it was low in 1998, and increased later when equatorial upwelled waters spread poleward.

Abstract. A persistent phytoplankton bloom was observed during August-December 1998 around the Marquesas Islands in the South Pacific using a spaceborne ocean color sensor. The enhancement of the phytoplankton production is attributed to the island mass effect. The effect consists of a combination of turbulent mixing and advection from the south equatorial current flowing through and around the islands, and iron-enriched waters originating from land drainage and hydrothermal fluxes through old volcanic formations. The enhanced phytoplankton production effects are noticeable a large distance downstream from the island (500 to 1000 km). This island mass effect is an important contributor to the productivity of the region and therefore potentially important for fisheries in the area.