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26 pages, 15 figures, 13 tables, 1 annex, The Ocean and Land Color Instruments (OLCI) operated onboard the Copernicus Sentinel-3 satellites are providing globally distributed Ocean Color Radiometry (OCR) data products of relevance for environmental and climate applications. This work summarizes results on the assessment of fundamental OCR data from the Operational Baseline 3 Collection OL_L2M.003.01 of OLCI-A and OLCI-B onboard Sentinel-3A and Sentinel-3B, respectively. Evaluated products are the satellite derived normalized water-leaving radiance L(λ), aerosol optical depth at 865 nm τ(865) and Ångström exponent α determined in the near-infrared spectral region. The analyses were performed relying on in situ reference data from the Ocean Color component of the Aerosol Robotic Network (AERONET-OC) from sites representative of diverse water types. The comparison of OLCI-A and OLCI-B with AERONET-OC L(λ) for oligotrophic/mesotrophic waters shows cross-mission consistent spectral median percent differences (i.e., biases) varying within ±6% at the blue-green center-wavelengths. The analysis of data from regions characterized by optically complex waters, however, displays systematic negative biases for both OLCI-A and OLCI-B further increasing for waters dominated by chromophoric dissolved organic matter, thus suggesting a dependence of the atmospheric correction on water type. The direct inter-comparison of OLCI-B and OLCI-A L(λ) from the Tandem Phase characterized by Sentinel-3B and Sentinel-3A flying 30 s apart on the same orbit, shows spectral median percent differences lower than ±1% in the 412–560 nm interval, of approximately +5% at 620 and 665 nm, and −7% at 400 nm. However, outside the Tandem Phase, the inter-comparison of OLCI-B and OLCI-A data products indicates large and systematic differences explained by a notable dependence on the viewing angle. The evaluation of τ(865) and α across different geographic regions exhibits overestimated values between +48 and + 79% for the former and underestimated values between −28% and − 41% for the latter. A complementary evaluation of OCR data products from the Visible Infrared Imager Radiometer Suite on board the Suomi National Polar-orbiting Partnership (VIIRS-S), proposed as a further indirect term of reference for OLCI-A and OLCI-B data, shows large underestimates of L(λ) with respect to the in situ reference data in the various water types at 410 nm. Nevertheless, opposite to OLCI-A and OLCI-B data products, absolute differences between VIIRS-S and in situ reference data do not reveal any large or systematic dependence on water type and satellite viewing angle. Overall results suggest the need for further developing the OLCI-A and OLCI-B atmospheric correction, possibly improving the capability to identify aerosol types and to model scattering processes, This work benefitted of support from the Joint Research Centre through the COLORS project, from the European Metrology Programme for Innovation and Research (EMPIR) through the METEOC-4 project (grant 19ENV07), and from the European Commission Directorate-General for Defence Industry and Space through the Copernicus Programme. The additional support of the ‵Severo Ochoa Centre of Excellence' accreditation (CEX2019-000928-S) is also acknowledged

International audience; We present a three-step inverse model (3SAA) for estimating the inherent optical properties (IOPs) of surface waters from the remote sensing reflectance spectra, Rrs(λ). The derived IOPs include the total (a(λ)), phytoplankton (aphy(λ)), and colored detrital matter (acdm(λ)), absorption coefficients, and the total (bb(λ)) and particulate (bbp(λ)) backscattering coefficients. The first step uses an improved neural network approach to estimate the diffuse attenuation coefficient of downwelling irradiance from Rrs. a(λ) and bbp(λ) are then estimated using the LS2 model (Loisel et al., 2018), which does not require spectral assumptions on IOPs and hence can assess a(λ) and bb(λ) at any wavelength at which Rrs(λ) is measured. Then, an inverse optimization algorithm is combined with an optical water class (OWC) approach to assess aphy(λ) and acdm(λ) from anw(λ).The proposed model is evaluated using an in situ dataset collected in open oceanic, coastal, and inland waters. Comparisons with other standard semi-analytical algorithms (QAA and GSM), as well as match-up exercises, have also been performed. The applicability of the algorithm on OLCI observations was assessed through the analysis of global IOPs spatial patterns derived from 3SAA and GSM. The good performance of 3SAA is manifested by median absolute percentage differences (MAPD) of 13%, 23%, 34% and 34% for bbp(443), anw(443), aphy(443) and acdm(443), respectively for oceanic waters. Due to the absence of spectral constraints on IOPs in the inversion of total IOPs, and the adoption of an OWC-based approach, the performance of 3SAA is only slightly degraded in bio-optical complex inland waters.

The proxy for phytoplankton biomass, Chlorophyll a (Chl a), is an important variable to assess the health and state of the oceans which are under increasing anthropogenic pressures. Prior to the operational use of satellite ocean-colour Chl a to monitor the oceans, rigorous assessments of algorithm performance are necessary to select the most suitable products. Due to their inaccessibility, the oligotrophic open-ocean gyres are under-sampled and therefore under-represented in global in situ data sets. The Atlantic Meridional Transect (AMT) campaigns fill the sampling gap in Atlantic oligotrophic waters. In-water underway spectrophotometric data were collected on three AMT field campaigns in 2016, 2017 and 2018 to assess the performance of Sentinel-3A (S3-A) and Sentinel-3B (S3-B) Ocean and Land Colour Instrument (OLCI) products. Three Chl a algorithms for OLCI were compared: Processing baseline (pb) 2, which uses the ocean colour 4 band ratio algorithm (OC4Me); pb 3 (OL_L2M.003.00) which uses OC4Me and a colour index (CI); and POLYMER v4.8 which models atmosphere and water reflectance and retrieves Chl a as a part of its spectral matching inversion. The POLYMER Chl a for S-3A OLCI performed best. The S-3A OLCI pb 2 tended to under-estimate Chl a especially at low concentrations, while the updated OL_L2M.003.00 provided significant improvements at low concentrations. OLCI data were also compared to MODIS-Aqua (R2018 processing) and Suomi-NPP VIIRS standard products. MODIS-Aqua exhibited good performance similar to OLCI POLYMER whereas Suomi-NPP VIIRS exhibited a slight under-estimate at higher Chl a values. The reasons for the differences were that S-3A OLCI pb 2 Rrs were over-estimated at blue bands which caused the under-estimate in Chl a. There were also some artefacts in the Rrs spectral shape of VIIRS which caused Chl a to be under-estimated at values >0.1 mg m-3. In addition, using in situ Rrs to compute Chl a with OC4Me we found a bias of 25% for these waters, related to the implementation of the OC4ME algorithm for S-3A OLCI. By comparison, the updated OLCI processor OL_L2M.003.00 significantly improved the Chl a retrievals at lower concentrations corresponding to the AMT measurements. S-3A and S-3B OLCI Chl a products were also compared during the Sentinel-3 mission tandem phase (the period when S-3A and S-3B were flying 30 sec apart along the same orbit). Both S-3A and S-3B OLCI pb 2 under-estimated Chl a especially at low values and the trend was greater for S-3A compared to S-3B. The performance of OLCI was improved by using either OL_L2M.003.00 or POLYMER Chl a. Analysis of coincident satellite images for S-3A OLCI, MODIS-Aqua and VIIRS as composites and over large areas illustrated that OLCI POLYMER gave the highest Chl a concentrations and percentage (%) coverage over the north and south Atlantic gyres, and OLCI pb 2 produced the lowest Chl a and % coverage.

The bulk backscattering ratio ($\tilde{b_{bp}}$) is commonly used as a descriptor of the bulk real refractive index of the particulate assemblage in natural waters. Based on numerical simulations, we analyze the impact of heterogeneity of phytoplankton cells on $\tilde{b_{bp}}$. $\tilde{b_{bp}}$ is modeled considering viruses, heterotrophic bacteria, phytoplankton, detritus, and minerals. Three study cases are defined according to the relative abundance of these different components. Two study cases represent typical situations in open ocean, outside (No-B/No-M) and inside bloom (B/No-M). The third study case is typical of coastal waters with the presence of minerals. Phytoplankton cells are modeled by a two-layered spherical geometry representing a chloroplast surrounding the cytoplasm. The $\tilde{b_{bp}}$ values are higher when heterogeneity is considered because the contribution of coated spheres to backscattering is higher than homogeneous spheres. The impact of heterogeneity is however strongly conditioned by the hyperbolic slope $\xi$ of the particle size distribution. Even if the relative concentration of phytoplankton is small (

Over the past years, a large number of new approaches in the domain of ocean-color have been developed, leading to a variety of innovative descriptors for phytoplankton communities. One of these methods, named PHYSAT, currently allows for the qualitative detection of five main phytoplankton groups from ocean-color measurements. Even though PHYSAT products are widely used in various applications and projects, the approach is limited by the fact it identifies only dominant phytoplankton groups. This current limitation is due to the use of biomarker pigment ratios for establishing empirical relationships between \textit{in-situ} information and specific ocean-color radiance anomalies in open ocean waters. However, theoretical explanations of PHYSAT suggests that it could be possible to detect more than dominance cases but move more towards phytoplanktonic assemblage detection. Thus, to evaluate the potential of PHYSAT for the detection of phytoplankton assemblages, we took advantage of the Continuous Plankton Recorder (CPR) survey, collected in both the English Channel and the North Sea. The available CPR dataset contains information on diatom abundance in two large areas of the North Sea for the period 1998-2010. {Using this unique dataset, recurrent diatom assemblages were retrieved based on classification of CPR samples}. Six diatom assemblages were identified \textit{in-situ}, \textcolor{blue}{each having indicators taxa or species}. Once this first step was completed, the \textit{in-situ} analysis was used to empirically associate the diatom assemblages with specific PHYSAT spectral anomalies. This step was facilitated by the use of previous classifications of regional radiance anomalies in terms of shape and amplitude, coupled with phenological tools. Through a matchup exercise, three CPR assemblages were associated \textcolor{blue}{with} specific radiance anomalies. The maps of detection of these specific radiances anomalies are in close agreement with current \textit{in-situ} ecological knowledge.

International audience; PurposeMarine mangrove sediments in the Manche-à-Eau lagoon (Guadeloupe, Caribbean Sea) harbor locally extensive, white microbial mats. These mats cover the surface of reduced sediments near the roots of red mangrove trees, Rhizophora mangle, and are mainly composed of sulfur-oxidizing bacteria belonging to the Beggiatoaceae family, with some filamentous cyanobacteria. The goal of this study was to investigate the possible influence of sediment characteristics on the presence of these microbial mats.Materials and methodsFour push cores were collected in April 2013, two from zones with microbial mats and two from zones without mats. Sediment characteristics (grain-size distribution, mineralogy, total organic carbon (TOC) and total nitrogen (TN) contents, atomic TOC/TN ratios, and organic matter (OM) δ13C values) were compared for all four cores.Results and discussionSignificant differences were observed between sediments below microbial mats and those without mats. Sediments with microbial mats contained greater amounts of clay, and higher TOC, TN, and TOC/TN ratios, with lower total carbonate content and δ13C values. The higher clay content most likely results from lower fluid flow velocity near to mangrove roots, while higher TOC/TN ratios and lower δ13C values indicate higher inputs of OM from mangrove trees. These results are consistent with the fact that microbial mats were observed near the roots of mangrove trees, which trap OM from terrestrial vegetation and fine sediments.ConclusionsThe grain-size distribution of sediment particles, the total carbonate content, and the δ13C values are the main parameters discriminating between zones with microbial mats and those without mats. Variations in total carbonate content, which is mainly of biogenic origin, result from conditions that are more favorable for benthic organisms in zones without microbial mats. Variations of the TOC/TN ratios are controlled by the presence of a non-negligible amount of inorganic nitrogen bound to surface clay mineral particles and/or by microbial processes.

International audience; From 1998 to 2011, the effects of environmental conditions on the spatial and temporal trends of sardine and sardinella catch rates in the Mauritanian waters were investigated using generalized additive models. Two models were used: a global model and an oceanographic model. The global models explained more of the variability in catch rates (60.4% for sardine and 40% for sardinella) than the oceanographic models (42% for sardine and 32.4% for sardinella). Both species showed clear and inverse seasonal variations in abundances corresponding to their main spawning activities and the hydrologic seasons off the Mauritanian waters. Sardine prefer colder waters and seem to occupy the ‘gap’ in the northern part of the Mauritanian waters as soon as sardinella has left the area because of to lower water temperatures. Unlike sardinella, sardine showed a gradual southward extension between 2002 and 2009. The oceanographic model revealed that a high proportion of catch variability for the two species could be explained by environmental variables. The main environmental parameters explaining the variability are sea surface temperature (SST) and the upwelling index for sardinella, and the chlorophyll-a (Chl-a) concentration, the upwelling index and SST for sardine. The sardinella spatio-temporal variations off Mauritania seem to be more controlled by thermal than productivity gradients, probably linked to the species physiological constraints (thermal tolerance) whereas sardine seems to be more controlled by an ‘optimal upwelling and temperature’ windows. The results presented herein may be useful for understanding the movement of these species along the Mauritanian coast and hence their management under a changing climate.

Spatio-temporal patterns of the chlorophyll- a concentration, Chla , have been assessed from the MEdium Resolution Imaging Spectrometer (MERIS) over the whole Vietnamese coastal waters from 2002 to 2012. For that purpose, six bio-optical algorithms already documented and based on different approaches have been tested over a large in situ data set collected at different seasons and locations along the Vietnamese coast. The OC5 algorithm (Gohin, Druon, and Lampert, 2002) presents the best performances and has been selected to assess Chla in the studied region. The notion of optimal bio-optical environment associated with the best performance of OC5 has been introduced. For suspended particulate matter concentration, SPM , lower than 100 g·m − 3 , and colored dissolved organic matter, a cdom (443), lower than 0.5 m − 1 , Chla is estimated with an uncertainty of 36% and a bias of 5%. A Chla climatology has been generated and the temporal patterns (seasonal variability, long term trend, and irregular component) have been described using the Census-X-11 time series decomposition method. Three-dimensional hydrodynamical numerical simulations have been used to analyze the spatio-temporal patterns of Chla . The seasonal contribution dominates the variance of the signal, in good relationship with the dynamic of the mixed layer depth as well as with the occurrence of a seasonal upwelling induced by the summer monsoon. The irregular variability of Chla which may reach up to 35% of the total variance in the central part of the Vietnamese coast, can be explained through the surface kinetic energy and its standard deviation which is associated with small scales processes. A long term monotonic trend from about 2 to > 5%·yr − 1 (that is 20 to > 50% from 2002 to 2012) have been noticed in different coastal areas where aquaculture activities exhibit a concurrence increase, ranging from 31% to 113% (in production weight) over the same time period. In situ measurements, and especially of nutrients, are however necessary to confirm the link between aquaculture activities and phytoplankton biomass long term evolutions.

Spatio-temporal patterns of suspended particulate matter, SPM, in coastal waters under the Mekong's influence are examined through remote sensing data collected from January 2003 to April 2012 by the MEdium Resolution Imaging Spectrometer (MERIS) at full spatial resolution (300 × 300 m2). The first SPM climatology over this region is provided and the SPM temporal variation schemes (irregular variability, seasonal variability, and long term trend) are described using the Census-X-11 time series decomposition method. The different spatio-temporal patterns are then analyzed with regard to regional oceanographic and hydrologic conditions. The origin of the processes controlling the seasonality of the Mekong Delta plume is characterized. The increase of turbidity observed from June to December, starts with the Mekong sediment inputs which are maximum during the summer monsoon (the water discharge reaches its maximum in September/October). While the Mekong water discharge decreases, the concentration of suspended sediment keeps increasing in coastal waters during the following two/three months (November to January). This increase is explained by resuspension effects occurring in the shallow coastal areas. Due to higher wave energy and oblique orientation of the waves breaking near the coast, the winter monsoon triggers a high level of agitation and high value of resuspended material concentration which are submitted to a longshore current directed towards the South–West. Deposition (in front of the Delta) and erosion (northern and southern areas of the delta) areas are identified in good agreement with recent results obtained from a prognostic model. While the temporal variability is strongly dominated by the seasonal component, a long term trend of about − 5% SPM concentration per year is observed in the pro-delta area and is attributed to the decrease of the Mekong river sediment output during the high flow season.

International audience; The composition of marine ecosystems is determined by spatial and temporal patterns of global biogeochemical cycles. Shifts in marine ecosystem composition driven by changes in climate can in turn affect biogeochemical cycles, especially through their impact on air-sea fluxes of CO2 and trace gas concentrations in the atmosphere. However, the response of marine ecosystems to climate is difficult to assess at global scale due to the scarcity of large-scale in-situ biological monitoring programmes. Here, we combine and analyse remote-sensing observations, in-situ observations, and a global ocean biogeochemistry model to gain insight into interactions between marine ecosystem composition and climate variability over the last decade, in the North Atlantic and in the Southern Ocean. Our results show large-scale shifts in the dominance of diatoms, with mean anomalies 63.3% higher during extreme positive phases of North Atlantic Oscillation index compared to its extreme negative phases and with mean anomalies 134% higher during extreme positive phases of Southern Annular Mode in the Southern Ocean over the period 1998-2008. Significant changes in the frequency of diatom dominance are detected concurrently in the three data sources studied. Model outputs indicate that this increase is driven by nutrient supply from deep waters during windier and more turbulent climate conditions.

Remote sensing of suspended particulate matter, SPM, from space has long been used to assess its spatio-temporal variability in various coastal areas. The associated algorithms were generally site specific or developed over a relatively narrow range of concentration, which make them inappropriate for global applications (or at least over broad SPM range). In the frame of the GlobCoast project, a large in situ data set of SPM and remote sensing reflectance, R-rs(lambda), has been built gathering together measurements from various coastal areas around Europe, French Guiana, North Canada, Vietnam, and China. This data set covers various contrasting coastal environments diversely affected by different biogeochemical and physical processes such as sediment resuspension, phytoplankton bloom events, and rivers discharges (Amazon, Mekong, Yellow river, MacKenzie, etc.). The SPM concentration spans about four orders of magnitude, from 0.15 to 2626 g center dot m(-3). Different empirical and semi-analytical approaches developed to assess SPM from R-rs(lambda) were tested over this in situ data set. As none of them provides satisfactory results over the whole SPM range, a generic semi-analytical approach has been developed. This algorithm is based on two standard semi-analytical equations calibrated for low-to-medium and highly turbid waters, respectively. A mixing law has also been developed for intermediate environments. Sources of uncertainties in SPM retrieval such as the bio-optical variability, atmospheric correction errors, and spectral bandwidth have been evaluated. The coefficients involved in these different algorithms have been calculated for ocean color (SeaWiFS, MODIS-A/T, MERIS/OLCI, VIIRS) and high spatial resolution (LandSat8-OLI, and Sentinel2-MSI) sensors. The performance of the proposed algorithm varies only slightly from one sensor to another demonstrating the great potential applicability of the proposed approach over global and contrasting coastal waters.

This study presents an application of the Cytosense flow cytometer (CytoBuoy b.v., NL) for the analysis of the optical properties of phytoplankton cells. For the first time, the forward, sideward and backward cross sections (σFSC, σSSC and σsubbsubb/sub/sub) were derived for two species morphologically different (Chlamydomonas concordia and Thalassiosira pseudonana). The objective of this work is to check the validity of the estimates before any applications in the frame of marine optics studies. Thus, estimates of σFSC and σSSC are tested with radiative transfer computations as no in situ measurements are available. A synthetic database is built considering homogeneous, multi-layered spheres, aggregates and cylinders. Scanning electron micrographs were performed to investigate the cell morphology to simulate particles as close as possible to the real cells. This set of numerical results represents a valuable database for many kinds of applications dealing with marine optics. Comparisons showed that the Cytosense estimates for the cultures are consistent with values predicted by the theory. In average, more than 92% of the Cytosense estimates were encompassed by predicted values. The backscattering cross section and the backscattering efficiency were compared with in situ measurements found in the literature wherever possible. Results showed that σsubbsubb/sub/suband Qsubbsubb/sub/subestimations fall within the range of the referenced values.

Increasing our knowledge on dissolved organic carbon (DOC) spatio-temporal distribution in the coastal ocean represents a crucial challenge for better understanding the role of these ecosystems in the global oceanic carbon cycle. The assessment of DOC concentration from the absorption properties of the colored part of the dissolved organic matter (a(cdom)) was investigated from an extensive data set covering a variety of coastal environments. Our results confirmed that variation in the acdom(412) to DOC ratio (a*(cdom)(412)) can be depicted from the CDOM spectral slope in the UV domain (S275-295). They also evidenced that regional first order variation in both a*(cdom)(412) and S275-295 are highly correlated to variation in acdom(412). From these observations, generalized relationships for estimating a*(cdom)(412) from S275-295 or a(cdom)(412) were parameterized from our development sites (N = 158; English Channel, French Guiana, Hai Phong Bay) and tested against an independent data set covering others coastal regions (N = 223; French Polynesia, Rhone River estuary, Gulf of Maine, Chesapeake Bay, Southern Middle Atlantic Bight) demonstrating the possibility to derive DOC estimates from in situ CDOM optical properties with an average accuracy of similar to 16% over very contrasted coastal environments (with DOC ranging from 50 to 250 mu mol.L-1). The applicability of these generalized approaches was evaluated in the context of ocean color remote sensing observation emphasizing the limits of S275-295-based formulations and the potential for a(cdom)-based approaches to represent a compelling alternative for assessing synoptic DOC distribution. (C) 2015 Optical Society of America

International audience; The European sardine, Sardina pilchardus, exhibits a complex population structure, which has produced conflicting results in previous genetic studies. Despite its importance in the fisheries industry, stock delineation for management and conservation purposes is still a matter of debate throughout the distribution range of the species. This study examines whether otolith shapes are more efficient than genetic markers to detect population structure in pelagic species with large population sizes. Sardines were analyzed from 15 sampling localities in the Northeast Atlantic and Mediterranean Sea covering almost the whole distribution range of the species. A combination of otolith shape indices and elliptic Fourier descriptors was investigated by multivariate statistical procedures. Within the studied area, three distinct groups were identified with an overall correct classification of 77%. Group A: northern Mediterranean Sea and Gulf of Gabès; group B: Atlantic Morocco–south Alboran–Algero-provençal coasts; and group C: European Atlantic coast. The Almeria–Oran front and the Gibraltar strait are not an efficient barrier for sardine population separation as there seems to be exchanges between populations of the south-western Mediterranean Sea and those of the Moroccan Atlantic Ocean coast or Gulf of Cadiz. The results are discussed in relation to environmental conditions, oceanographic features, and physical barriers to dispersal in the study area, and compared with those obtained by previous genetic, morphometric, and meristic data. For pelagic species with high gene flow, present results highlighted the need to take into account the identification of phenotypic stocks to ensure sustainable fishery benefits and efficient conservation as they may have unique demographic properties and responses to exploitation.

International audience; Phytoplankton observation in the ocean can be a challenge in oceanography. Accurate estimations of its biomass and dynamics will help to understand ocean ecosystems and refine global climate models. Relevant data sets of phytoplankton defined at a functional level and on a sub-meso-and daily scale are thus required. In order to achieve this, an automated, high-frequency, dedicated scanning flow cytometer (SFC, Cytobuoy b.v., the Netherlands) has been developed to cover the entire size range of phytoplankton cells whilst simultaneously taking pictures of the largest of them. This cytometer was directly connected to the water inlet of a PocketFerryBox during a cruise in the North Sea, 08–12 May 2011 (DYMAPHY project, INTERREG IV A " 2 Seas "), in order to identify the phytoplankton community structure of near surface waters (6 m) with a high spatial resolution basis (2.2 ± 1.8 km). Ten groups of cells, distinguished on the basis of their optical pulse shapes, were described (abundance, size estimate, red fluorescence per unit volume). Abundances varied depending on the hydrological status of the traversed waters, reflecting different stages of the North Sea blooming period. Comparisons between several techniques analysing chlorophyll a and the scanning flow cytometer, using the integrated red fluorescence emitted by each counted cell, showed significant correlations. For the first time, the community structure observed from the automated flow cytometry data set was compared with PHYSAT reflectance anomalies over a daily scale. The number of matchups observed between the SFC automated high-frequency in situ sampling and remote sensing was found to be more than 2 times better than when using traditional water sampling strategies. Significant differences in the phyto-plankton community structure within the 2 days for which matchups were available suggest that it is possible to label PHYSAT anomalies using automated flow cytometry to resolve not only dominant groups but also community structure .

Phytoplankton observation in the ocean can be a challenge in oceanography. Accurate estimations of their biomass and dynamics will help to understand ocean ecosystems and refine global climate models. This requires relevant datasets of phytoplankton at a functional level and on a daily and sub meso scale. In order to achieve this, an automated, high frequency, dedicated scanning flow cytometer (SFC, Cytobuoy, NL), has been developed to cover the entire size range of phytoplankton cells whilst simultaneously taking pictures of the largest of them. This cytometer was directly connected to the water inlet of a~pocket Ferry Box during a cruise in the North Sea, 8–12 May 2011 (DYMAPHY project, INTERREG IV A "2 Seas"), in order to identify the phytoplankton community structure of near surface waters (6 m) with a high resolution spacial basis (2.2 ± 1.8 km). Ten groups of cells, distinguished on the basis of their optical pulse shapes, were described (abundance, size estimate, red fluorescence per unit volume). Abundances varied depending on the hydrological status of the traversed waters, reflecting different stages of the North Sea blooming period. Comparisons between several techniques analyzing chlorophyll a and the scanning flow cytometer, using the integrated red fluorescence emitted by each counted cell, showed significant correlations. The community structure observed from the automated flow cytometry was compared with the PHYSAT reflectance anomalies over a daily scale. The number of matchups observed between the SFC automated high frequency in situ sampling and the remote sensing was found to be two to three times better than when using traditional water sampling strategies. Significant differences in the phytoplankton community structure within the two days for which matchups were available, suggest that it is possible to label PHYSAT anomalies not only with dominant groups, but at the level of the community structure.

International audience; Remote sensing of ocean color is a powerful tool for monitoring phytoplankton in the ocean with a high spatial and temporal resolution. Several methods were developed in the past years for detecting phytoplankton functional types from satellite observations. In this paper, we present an automatic classification method based on a neural network clustering algorithm in order to classify the anomalies of water leaving radiance spectra (Ra), introduced in the PHYSAT method by Alvain et al. (2005), and analyze their variability at the global scale. The use of an unsupervised classification aims at improving the characterization of the spectral variability of Ra in terms of shape and amplitude as well as the expansion of its potential use to larger in situ datasets for global phytoplankton remote sensing. The Self-Organizing Map algorithm (SOM, Kohonen, 1984) aggregates similar spectra into a reduced set of pertinent groups, allowing the characterization of the Ra variability, which is known to be linked with phytoplankton community composition (Alvain et al., 2012). Based on the same sample of Ra spectra, a comparison between the previous version of PHYSAT (Alvain et al., 2005, 2008) and the new one using SOM shows that it is now possible to take into consideration all the types of spectra. This was not possible with the previous approach, based on thresholds, defined in order to avoid overlaps between the spectral signatures of each phytoplankton group. The SOM-based method is relevant for characterizing a wide variety of Ra spectra through its ability to handle large amounts of data, in addition to its statistical reliability and processing speed compared to the previous PHYSAT. The former approach might have introduced potential biases and thus, its extension to larger databases was very restricted. This is not the case with the new statistical design presented in this work. In the second step, some new Ra spectra have been related to phytoplankton groups using collocated field pigment inventories from a large in situ database. Phytoplankton groups were identified based on biomarker pigment ratio thresholds taken from the literature. SOM was then applied to the global daily SeaWiFS (Sea-viewing Wide Field-of-view Sensor) imagery archive between 1997 and 2010. Global distributions of major phytoplankton groups were analyzed and validated against in situ data. Thanks to its ability to capture a wide range of spectra and to manage a larger in situ pigment dataset, the neural network tool allows to classify a much higher number of pixels (2 times more) than the previous PHYSAT method for the five phytoplankton groups taken into account in this study (Synechococcus-like-cyanobacteria, diatoms, Prochlorococcus, Nanoeucaryotes and Phaeocystis-like). In addition, different Ra spectral signatures have been associated to diatoms. These signatures are located in various environments where the inherent optical properties affecting the Ra spectra are likely to be significantly different. Local phenomena such as diatom blooms in the upwelling regions or during climatic events (i.e. La Niña) are more clearly visible with the new method.

International audience; Knowledge on absorption by colored dissolved organic matter, acdom, spatio-temporal variability in coastal areas is of fundamental importance in many field of researches related to biogeochemical cycles studies, coastal areas management, as well as land and water interactions in the coastal domain. A new method, based on the theoretical link between the vertical attenuation coefficient, Kd, and the absorption coefficient, has been developed to assess acdom. This method, confirmed from radiative transfer simulations and in situ measurements, and tested on an independent in situ data set (N = 126), allows acdom to be assessed with a Mean Relative Absolute Difference, MRAD, of 33% over two order of magnitude (from 0.01 to 1.16 m−1). In the frame of ocean color observation, Kd is not directly measured but estimated from the remote sensing reflectance, Rrs. Based on 109 satellite (SeaWiFS) and in situ coincident (i.e. match-up) data points acdom is retrieved with a MRAD value of 37%. This simple model generally presents slightly better performances than recently developed empirical or semi-analytical algorithms.

International audience; Particulate suspended matter concentration (SPM) were estimated over a 8 year time period (2002-2010) in the coastal waters of French Guiana from a regional algorithm applied to the MODIS monthly reflectance measurements. Comparison between SPOT images and MODIS-SPM maps revealed the strong spatio-temporal coupling between in water SPM and the dynamics of local (i.e. Kourou and Cayenne) mud banks. Highest MODIS SPM values (>13 g m−3 approximately) can be significantly associated with the subtidal part of the banks as well as to the related turbid plume. The migration of these mud banks induces strong interannual changes in SPM reaching up to 6% year−1 within increasing and decreasing patchy areas distributed alternatively along the coastline of French Guiana. Mud banks migration rates derived from MODIS SPM data reach in average about 2 km year−1 in agreement with previous studies. The MODIS time series have allowed for a detailed description of the seasonality and interannual variations in the in-water SPM loads. Seasonal changes in SPM are related to the onset of the trade wind season. Marked non-linear patterns including a sharp evolution in the SPM values around 2005 as well as additional high frequency modulations have been emphasized within the upward and downward SPM trend regions. Concurrent temporal variations in the frequency of northward swells (favoring mud banks migration and reworking) as well as interannual changes in the amount of sediment delivered by the Amazon River have been shown to play a major role in the SPM temporal patterns observed in the French Guiana coastal waters. Our results clearly demonstrate the advantage for ocean color data to describe mud banks dynamics through the assessment of in water SPM temporal variability.

[1] The diffuse attenuation coefficient, Kd(λ) is a fundamental radiometric parameter that is used to assess the light availability in the water column. A neural network approach is developed to assess Kd(λ) at any visible wavelengths from the remote sensing reflectances as measured by the SeaWiFS satellite sensor. The neural network (NN) inversion is trained using a combination of simulated and in-situ data sets covering a broad range ofKd(λ), between 0.0073 m−1 at 412 nm and 12.41 m−1at 510 nm. The performance of the retrieval is evaluated against two data sets, one consisting of mainly synthetic data while the other one contains in-situ data only and is compared to those obtained with previous published empirical (NASA, Morel and Maritorena (2001) and Zhang and Fell (2007)) and semi-analytical (Lee et al., 2005b) algorithms. On the in-situ data set from the COASTLOOC campaign, the retrieval accuracy of the present algorithm is quite similar to published algorithms for oligotrophic and mesotrophic ocean waters. But for Kd(490) > 0.25 m−1, the NN approach allows to retrieve Kd(490) with a much better accuracy than the four other methods. The results are consistent when compared with other SeaWiFS wavelengths. This new inversion is as suitable in the open ocean waters as in the turbid waters. The work here is straightforwardly applicable to the MERIS sensor and with few changes to the MODIS-AQUA sensor. The algorithm in matlab and C code is provided as auxiliary material.

International audience; Optical water types were identified from an in situ data set of concomitant biogeochemical and optical parameters collected in contrasted turbid coastal areas of the eastern English Channel, southern North Sea and French Guiana at different seasons (211 stations). Four optical classes have been defined using a clustering approach performed on the spectrally normalized reflectance spectra. Normalization of the reflectance spectra was carried out during the statistical analysis to emphasize the shape of the reflectances rather than their magnitude. Each optical water type is associated with a specific bio-optical environment, in agreement with previous works. Two classes present a very marked optical character, one being mostly determined by strong phytoplankton and dissolved material loads, and the other one by a high proportion of mineral particles. The two remaining classes are related to optically mixed situations although there are some differences in the relative proportion of particulate mineral material. Applying this optical typology to the SeaWiFS daily reflectance data, we emphasized the high representativeness of these 4 optical water types which allow to describe about two thirds of the reflectance spectra found within the development sites whatever the season. The adequacy of optical water type definition for monitoring the spatio-temporal variability of coastal water masses optical quality, which reflects the impact of biological and hydrodynamic processes occurring at different time scales (i.e. from high frequency to seasonal processes), has been demonstrated. The four optical classes' typology has been shown to remain highly representative at global scale. This underlines the effective optical vicinity of some parts of the coastal ocean during some periods of the year, in spite of the recognized high optical diversity of coastal waters. This further demonstrates the high pertinence of class-based approach for large scale coastal applications. Finally, the potential for class-based inversion algorithms for improving ocean color products retrieval, as well as the applicability of such class-specific algorithms with respect to satellite information have been illustrated from the estimation of the suspended matter concentration. This work provides very encouraging evidence of the potential and adequacy of class-based inversion methods for deriving bio-optical products in optically complex waters such as the coastal ocean.

International audience; Past years have seen the development of different approaches to detect phytoplankton groups from space. One of these methods, the PHYSAT one, is empirically based on reflectance anomalies. Despite observations in good agreement with in situ measurements, the underlying theoretical explanation of the method is still missing and needed by the ocean color community as it prevents improvements of the methods and characterization of uncertainties on the inversed products. In this study, radiative transfer simulations are used in addition to in situ measurements to understand the organization of the signals used in PHYSAT. Sensitivity analyses are performed to assess the impact of the variability of the following three parameters on the reflectance anomalies: specific phytoplankton absorption, colored dissolved organic matter absorption, and particles backscattering. While the later parameter explains the largest part of the anomalies variability, results show that each group is generally associated with a specific bio-optical environment which should be considered to improve methods of phytoplankton groups detection

The influences of physico-chemical and biological processes on dimethylsulfide (DMS) dynamics in the most oligotrophic subtropical zones of the global ocean were investigated. As metrics for the dynamics of DMS and the so-called ‘summer DMS paradox’ of elevated summer concentrations when surface chlorophyll a (Chl) and particulate organic carbon (POC) levels are lowest, we used the DMS-to-Chl and DMS-to-POC ratios in the context of three independent and complementary approaches. Firstly, field observations of environmental variables (such as the solar radiation dose, phosphorus limitation of phytoplankton and bacterial growth) were used alongside discrete DMS, Chl and POC estimates extracted from global climatologies (i.e., a ‘station based’ approach). We then used monthly climatological data for DMS, Chl, and POC averaged over the biogeographic province wherein a given oligotrophic subtropical zone resides (i.e., a ‘province based’ approach). Finally we employed sensitivity experiments with a new DMS module coupled to the ocean general circulation and biogeochemistry model PISCES to examine the influence of various processes in governing DMS dynamics in oligotrophic regions (i.e., a ‘model based’ approach). We find that the ‘station based’ and ‘province based’ approaches yield markedly different results. Interestingly, the ‘province based’ approach suggests the presence of a ‘summer DMS paradox’ in most all of the oligotrophic regions we studied. In contrast, the ‘station based’ approach suggests that the ‘summer DMS paradox’ is only present in the Sargasso Sea and eastern Mediterranean. Overall, we found the regional differences in the absolute and relative concentrations of DMS between 5 of the most oligotrophic regions of the world’s oceans were better accounted for by their nutrient dynamics (specifically phosphorus limitation) than by physical factors often invoked, e.g., the solar radiation dose. Our ‘model based’ experiments suggest that it is the limitation of phytoplankton/bacterial production and bacterial consumption of DMS by pervasive phosphorus limitation that is responsible for the ‘summer DMS paradox’.

Time series of chlorophyll-a concentration (chl a), backscattering coefficient at 490 nm (bbp(490)) and spectral slope of bbp (γ) derived from satellite imagery of ocean color were used to study the aquatic ecosystem of the Southern Yellow Sea during the time period of 1997–2007. Our study indicated that chl a increased in offshore waters by 0.02 mg m−3 y −1 (p < 0.05), in contrast to a decline observed in the middle and low-latitude global waters. bbp(490), a proxy of total suspended particulate matter concentration did not have any significant trend, while γ, a proxy of the relative proportion of small-sized and larger particles in the surface ocean, decreased significantly. Annual spring phytoplankton blooms occurred in nearshore and offshore waters of the central Southern Yellow Sea; while autumn blooms only occurred in nearshore waters.

The fine-scale study of the diffuse attenuation coefficient, K d (λ), of the spectral solar downward irradiance is only feasible by ocean color remote sensing. Several empirical and semi-analytical methods exist. However, most of these models are generally applicable for clear open ocean waters and estimate this coefficient only at 490 nm. They show limitations when applied to coastal waters. A new empirical method based on neural networks has been developed using a relationship between the remote-sensing reflectances between 412 and 670 nm and K d (λ) between 412 and 490 nm, for the SeaWiFS ocean color remote sensor. The first results concern the estimation of K d (490). The architecture of the neural network has been defined using synthetical and in situ dataset The comparison with the SeaWiFS empirical algorithms shows similar retrievals accuracies for low values of K d (490) (i.e.

The variability of inherent optical properties is investigated in the ultra-oligotrophic waters of the Mediterranean Sea sampled during the BOUM experiment performed during early summer 2008. Bio-optical relationships found for ultra-oligotrophic waters of the three anticyclonic gyres sampled significantly depart from the mean standard relationships provided for the global ocean, confirming the peculiar character of these Mediterranean waters. These optical anomalies are diversely related to the specific biological and environmental conditions occurring in the studied ecosystem. Specifically, the surface specific phytoplankton absorption coefficient exhibits values lower than those expected from the general relationships mainly in relation with a high contribution of relatively large sized phytoplankton. Conversely, the particulate backscattering coefficient, bbp, values are much higher than the mean standard values for a given chlorophyll-a concentration, TChl-a. This feature can presumably be related to the relevant influence of highly refractive submicrometer particles of Saharan origin in the surface layer of the water column. The present measurements also show that the Mediterranean Sea is greener than TChl-a alone indicates, as already stressed in previous studies. This color anomaly is partly explained by the estimated colored dissolved organic matter and submicrometer particles absorption coefficients, and to a greater extent by the high bbp/TChl-a values assuming that these particles backscatter light similarly in the green and blue parts of the visible spectrum. The diel variation of both the particulate matter attenuation and backscattering coefficients were also investigated specifically. Despite some differences in the timing and the magnitude of the daily oscillations found for these optical parameters, potential for the backscattering coefficient daily oscillation to be used, similarly to that for the attenuation coefficient, as a proxy for estimating carbon community production budget has been highlighted for the first time. This result is particularly relevant for present and future geostationary spatial ocean color missions.

The fine-scale study of the diffuse attenuation coefficient, K d (O ), of the spectral solar downward irradiance is only feasible by ocean color remote sensing. Several empirical and semi-analytical methods exist. However, most of tthese models are generally applicable for clear open ocean waters . They show limitations when applied to coastal waters. A new empirical method based on neural networks has been developed using a relationship between the remote-sensing reflectances between 412 and 670 nm and K d (490), for the SeaWiFS ocean color re mote sensor. The architecture of the neural network has been defined using synthetical and in situ dataset and the optimal design is a tow hidden layer neural network with 4 neurons of the first layer and three on the second layer. The comparis on with the SeaWiFS empirical algorithms shows similar retrievals accuracies for low values of K d (490) (i.e.

International audience; The impact of the inherent optical properties (IOP) variability on the chlorophyll, Chl, retrieval from ocean color remote sensing algorithms is analyzed from an in situ data set covering a large dynamic range. The effect of the variability of the specific phytoplankton absorption coefficient, a(phy)/Chl, specific particulate backscattering coefficient, b(bp)/Chl, and colored detrital matter absorption to non-water absorption ratio, a(cdm)/a(nw), on the performance of standard operational algorithms is examined. This study confirms that empirical algorithms are highly dependent on the specifics IOP values (especially b(bp)/Chl and a(cdm)/a(nw)): Chl is over-estimated in waters with specific IOP values higher than averaged values, and vice versa. These results clearly indicate the necessity to account for the influence of the specific IOP variability in Chl retrieval algorithms.

Knowledge of the oceanic particulate organic carbon (POC) pool and of its spatio-temporal variability is important for understanding the oceanic carbon cycle. Until now, POC estimates from space have been restricted to the surface layer. An empirical algorithm is developed to derive the POC content integrated over the euphotic layer ( POC Zeu ) from the near-surface POC concentration ( POC surf ). The relationship follows a power-law distribution, POC Zeu = A × POC surf B , with A and B depending on the type of waters (stratified or well-mixed) and on the near-surface chlorophyll concentration. A global climatology of POC surf and POC Zeu is generated from the SeaWiFS archive between 1998 and 2006. The global patterns of POC surf and POC Zeu follow the major gyre systems and other large-scale circulation features. High surface and integrated POC content, around 150 mg m − 3 and 6000 mg m − 2 , are encountered at high latitude, whereas low content, 50 mg m − 3 and 2000 mg m − 2 , are observed in subtropical gyres. The mean global values of POC surf and POC Zeu over the global ocean are 53 mg m − 3 and 3742 mg m − 2 . The standing stocks of POC are 3.92 × 10 14 and 1.19 × 10 15 g over the first penetration and euphotic depth, respectively.

International audience; A methodology is proposed to infer the altitude of aerosol plumes over the ocean from reflectance ratio measurements in the O2 absorption A-band (759 to 770 nm). The reflectance ratio is defined as the ratio of the reflectance in a first spectral band, strongly attenuated by O2 absorption, and the reflectance in a second spectral band, minimally attenuated. For a given surface reflectance, simple relations are established between the reflectance ratio and the altitude of an aerosol layer, as a function of atmospheric conditions and the geometry of observation. The expected accuracy for various aerosol loadings and models is first quantified using an accurate, high spectral resolution, radiative transfer model that fully accounts for interactions between scattering and absorption. The method is developed for POLDER and MERIS, satellite sensors with adequate spectral characteristics. The simulations show that the method is only accurate over dark surfaces when aerosol optical thickness at 765 nm is relatively large (> 0.3). In this case, the expected accuracy is on the order of ± 0.5 km or ± 0.2 km for POLDER or MERIS respectively. More accurate estimates are obtained with MERIS, since in this case the spectral reflectance ratio is more sensitive to aerosol altitude. However, a precise spectral calibration is needed for MERIS. The methodology is applied to MERIS and POLDER imagery acquired over marine surfaces. The estimated aerosol altitude is compared with in situ lidar profiles of backscattering coefficient measured during the AOPEX-2004 experiment for MERIS, or obtained with the space-borne lidar CALIOP for POLDER. The retrieved altitudes agree with lidar measurements in a manner consistent with theory. These comparisons demonstrate the potential of the differential absorption methodology for obtaining information on aerosol altitude over dark surfaces

International audience; A methodology is developed to derive the backscattering cross section of individual particles as measured with the CytoSense (CytoBuoy b.v., NL). This in situ flow cytometer detects light scatter in forward and sideward directions and fluorescence in various spectral bands for a wide range of particles. First, the weighting functions are determined for the forward and sideward detectors to take into account their instrumental response as a function of the scattering angle. The CytoSense values are converted into forward and sideward scattering cross sections. The CytoSense estimates of uniform polystyrene microspheres from 1 to 90 μm are compared with Mie computations. The mean absolute relative differences ΔE are around 33.7% and 23.9% for forward and sideward scattering, respectively. Then, a theoretical relationship is developed to convert sideward scattering into backscattering cross section, from a synthetic database of 495,900 simulations including homogeneous and multi-layered spheres. The relationship follows a power law with a coefficient of determination of 0.95. To test the methodology, a laboratory experiment is carried out on a suspension of silica beads to compare backscattering cross section as measured by the WET Labs ECO-BB9 and derived from CytoSense. Relative differences are between 35% and 60%. They are of the same order of magnitude as the instrumental variability. Differences can be partly explained by the fact that the two instruments do not measure exactly the same parameter: the cross section of individual particles for the CytoSense and the bulk cross section for the ECO-BB9.

A methodology is presented to estimate aerosol altitude from reflectance ratio measurements in the O2 absorption A-band. Previous studies have shown the impact of the vertical distribution of scatterers on the reflectance ratio. The reflectance ratio is defined as the ratio of the reflectance in a first spectral band, strongly attenuated by O2 absorption, to the reflectance in a second spectral band, minimally attenuated. First, a sensitivity study is performed to quantify the expected accuracy for various aerosol loadings and models. An accurate, high spectral resolution, radiative transfer model that fully accounts for interactions between scattering and absorption is used in the simulations. Due to their adequate spectral characteristics, POLDER and MERIS instruments are considered for simulations. For a moderately loaded atmosphere (i.e., aerosol optical thickness of 0.3 at 760 nm), the expected error on aerosol altitude is about 0.3 km for MERIS and 0.7 km for POLDER. More accurate estimates are obtained with MERIS, since the spectral reflectance ratio is more sensitive. Second, the methodology is applied to MERIS and POLDER imagery. Estimates of aerosol altitude are compared with lidar profiles of backscattering coefficient acquired during the AOPEX-2004 experiment. Retrievals are consistent with measurements and theory. These comparisons demonstrate the potential of the differential absorption methodology for obtaining information on aerosol vertical distribution.

The particulate backscattering coefficient and the degree of polarization provide a means to study and characterize the nature of suspended marine particles. Here, we show that such information may be retrieved from remote sensing.

[1] A methodology is presented and evaluated to retrieve vertically integrated water vapor content over the ocean in any viewing geometry from POLDER data. The methodology is based on differential absorption by water vapor in the near-infrared. Over the ocean, except in sun glint conditions, surface reflectance is small, and interaction between aerosol scattering and water vapor absorption is exploited to estimate total water vapor content. A sensitivity study performed with an accurate radiative transfer code (GAME) shows that a determination of total water vapor content is theoretically possible if the optical thickness and scale height of aerosols are known. An inaccuracy of 0.1 to 0.6 g cm−2 is expected, depending on water vapor content. The aerosol optical thickness δa at 865 nm is available from POLDER standard products. The aerosol scale height Ha can be estimated from the surface pressure derived from POLDER oxygen channels at 763 and 765 nm. A retrieval scheme is devised using parameterizations calculated using GAME for water vapor and surface pressure. The inversion scheme is applied to 20 POLDER orbits with observed water vapor contents ranging from 0.2 to 6 g cm−2. Comparisons with SSM/I estimates indicate a RMS error ranging from 0.21 to 0.58 g cm−2, depending on water vapor content, with a mean RMS error of 0.44 g cm−2. This result is slightly larger than deviations obtained between POLDER and SSM/I during the POLDER standard product validation over ocean in sun glint conditions (mean RMS error of 0.42 g cm−2).

The biogeochemical characterization of marine particles suspended in sea water, is of fundamental importance in many areas of ocean science. Previous studies based on theoretical calculations and field measurements have demonstrated the importance of the use of the polarized light field in the retrieval of the suspended marine particles properties. However, because of the weakness of the water leaving polarized signal and of the limited number of appropriate spatial sensors, such measurements have never been exploited from space. Here we show that the marine polarized remote sensing reflectance, as detected from the POLarization and Directionality of the Earth's Reflectances (POLDER) sensor, can be measured from space over bright waters and in absence of aerosols. This feasibility study is carried out over two oceanic areas characterized by different nature of the bulk particulate assemblage: the Barents sea during an intense coccolithophore bloom, and the Rio de la Plata estuary waters dominated by suspended sediments. The retrieved absolute values of the degree of polarization, P, its angular pattern, and its behavior with the scattering level are consistent with theory and field measurements. Radiative transfer simulations confirm the sensitivity of the POLDER-2 P values to the nature of the particulate assemblage. These preliminary results are very promising for the assessment of the bulk particle composition from remote sensing of the polarized signal, at least over highly scattering waters.

A methodology for water vapor retrieval over ocean, using a differential absorption technique from near-IR channels, is presented. Over ocean, surface reflectance is close to zero and aerosol scattering is used to estimate water vapor contents, from an accurate radiative transfer code to account for optical thickness and scale height of aerosols. This method has been applied to POLDER data and comparisons with ECMWF (European Centre for Medium-Range Weather Forecasts) data are presented over the Straits of Gibraltar.