North Brazil Current (NBC) rings are believed to play a key role in the Atlantic Ocean circulation and climate. Here, we use a new collection of high-resolution in-situ observations acquired during the EUREC${}^4$A-OA field experiment together with satellite altimetry to define, with unprecedented detail, the structure and evolution of these eddies. In-situ observations reveal a more complex structure than previously documented. In particular, we highlight a measurable impact of the Amazon outflow in creating a barrier layer over a large portion of the eddies. We show that this unprecedented data set allows us to estimate the accuracy of satellite altimetry gridded fields. The geostrophic velocities derived from satellite altimetry turn out to be considerably lower (up to 50% in amplitude) than the values measured by current meters. However, eddy properties as detected by TOEddies, a newly developed algorithm show to be relatively precise. For example, the eddy center and maximum azimuthal velocity contour fall within 25 ± 5 km and 16 ± 9 km, respectively, from the in-situ observed values. We apply TOEddies to 27 years of satellite altimetry to investigate the generic NBC rings behavior. We found a mean generation rate of 4.5 ± 1.1 rings per year, and a strong seasonal cycle in all eddy properties.

Remote sensing of suspended particulate matter (SPM) is crucial for water-quality monitoring, as it influences turbidity, light availability, or nutrient transport. This study aims to provide a comprehensive evaluation of twelve common and well-used SPM models for the Ocean and Land Color Instrument (OLCI) on-board Sentinel-3 satellite, based on different methods and assumptions, including estimation from water-leaving reflectance or proxies, a combination of semi-analytical equations, and machine learning algorithms. The models are tested in three stages: 1) performance assessment on in-situ measurements, 2) matchup exercise with OLCI and 3) visual assessment of satellite SPM products. The models are first tested on the GLORIA dataset (n = 767, 0.21 g.m⁻³ <SPM <2,626.82 g.m⁻³). The matchup analysis is then conducted in French coastal waters using the SOMLIT dataset (n = 71, 0.2 g.m⁻³ <SPM <722 g.m⁻³), based on the standard OLCI L2 remote sensing reflectance product. Finally, the visual assessment of the SPM maps provided by the twelve models is conducted for two French coastal sites. Results show that the algorithms proposed by Jiang et al. [ Remote. Sens. Environ. 258, 112386 (2021)10.1016/j.rse.2021.112386 ] and Novoa et al. [ Remote. Sens. 9, 61 (2017)10.3390/rs9010061 ] exhibit the highest score and the most accurate retrievals when compared to in-situ measurements. However, the matchup exercise shows that the method from Jiang et al. demonstrates more overall accurate SPM retrievals (Error = 49.85%, Bias = 0.55%, RMSLE = 0.35, Slope = 1.06). The visual assessment of SPM maps reveals that this model displays a larger dynamic range, making it suitable for applications in regions with a wide range of SPM concentrations. The sensitivity of these models to the atmospheric correction procedure is further explored. When all OLCI spectra are taken into account for the matchup exercise, the performance of the algorithms from Han et al. [ Remote. Sens. 8, 211 (2016)10.3390/rs8030211 ] improve, relative to the other one. Finally, the standard OLCI SPM product is evaluated, and the advantages of using the OLCI standard product over the MODIS one for studying coastal waters are discussed.

Suspended Particulate Matter (SPM) plays a central role in coastal marine ecosystems, influencing light availability, primary production, and transport of sediments, nutrients, and pollutants. Monitoring its variability is essential for understanding the impacts of both natural and anthropogenic pressures on coastal regions. In this study, nine years (2016–2025) of data from the Ocean and Land Colour Instrument (OLCI) aboard Sentinel‐3 were used to analyze SPM variability and trends across European and Mediterranean coastal waters at 300 m resolution. Monthly SPM composites were generated using a semi‐analytical model, and processed using time series decomposition (Census X‐11) and non‐parametric trend analysis. A continental‐scale climatology was generated, and high temporal variability was observed along the coasts, and in shelf seas such as the Irish Sea and the North Sea, where seasonal river discharge and resuspension events occur. Statistically significant trends were detected in several regions: declines in the Severn Estuary (−3.27%yea) and English Channel (−3.6%yea), and increases in the Northern North Sea (+3.51%yea), Gulf of Gabes (+6.52%yea), Abu Qir port, Egypt (+8.20%yea), and northwestern Black Sea (+4.01%yea). Case studies linked these trends to potential drivers including dredging and land reclamation (Abu Qir port), warfare‐related disturbances (Kakhovka Dam breach), and regional hydrodynamic or anthropogenic pressures. These findings are relevant to European marine policy frameworks, supporting efforts to establish robust indicators of coastal water quality. This study highlights the value of satellite observations for environmental monitoring and coastal management across European and Mediterranean waters.

Chlorophyll-a (Chla) and suspended particulate matter (SPM) are key indicators of water quality, playing critical roles in understanding marine biogeochemical processes and ecosystem health. Although satellite data from the Chinese Ocean Color and Temperature Scanner (COCTS) onboard the Haiyang-1C/D satellites is freely available, there has been limited validation of its standard Chla and SPM products. This study is a first step to address this gap by evaluating COCTS-derived Chla and SPM products against in situ measurements in French coastal waters. The matchup analysis showed robust performance for the Chla product, with a median symmetric accuracy (MSA) of 50.46% over a dynamic range of 0.13–4.31 mg·m⁻³ (n = 24, Bias = 41.11%, Slope = 0.93). In contrast, the SPM product showed significant limitations, particularly in turbid waters, despite a reasonable performance in the matchup exercise, with an MSA of 45.86% within a range of 0.18–10.52 g·m⁻³ (n = 23, Bias = −14.59%, Slope = 2.29). A comparison with another SPM model and Moderate Resolution Imaging Spectroradiometer (MODIS) products showed that the COCTS standard algorithm tends to overestimate SPM and suggests that the issue does not originate from the input radiometric data. This study provides the first regional assessment of COCTS Chla and SPM products in European coastal waters. The findings highlight the need for algorithm refinement to improve the reliability of COCTS SPM products, while the Chla product demonstrates suitability for water quality monitoring in low to moderate Chla concentrations. Future studies should focus on the validation of COCTS ocean color products in more diverse waters.

Les zones côtières, situées à l’interface entre la terre et l’océan, figurent parmi les milieux les plus dynamiques, productifs et vulnérables de la planète. Elles jouent un rôle écologique, économique et social majeur, mais sont soumises à de fortes pressions naturelles et anthropiques, amplifiées par le changement climatique. Dans ce contexte, le suivi de la qualité des eaux côtières est devenu un enjeu scientifique et sociétal important. Parmi les variables clés, la matière particulaire en suspension (SPM) constitue un indicateur essentiel. Sa variabilité spatiale et temporelle dépend à la fois de processus hydrodynamiques naturels et de perturbations d’origine anthropique. Cette thèse vise à améliorer la compréhension de la variabilité de la SPM, et sa quantification, dans les eaux côtières européennes et méditerranéennes, à partir d'images satellitaires. Dans un premier temps, douze algorithmes d’estimation de la SPM ont été comparés à partir de mesures in situ dans les eaux côtières françaises, et d’images du capteur Sentinel-3/OLCI, afin d’identifier les modèles les plus robustes dans des environnements optiquement complexes. Les produits standards de chlorophylle-a (Chla) et de SPM du capteur chinois COCTS ont également été évalués, révélant une bonne qualité pour la Chla, mais des limites importantes pour la SPM. Dans un second temps, un des algorithmes SPM les plus robuste a été appliqué à l’ensemble de l’archive OLCI (2016–2025) à 300 m de résolution, permettant d’établir une climatologie de la SPM à l’échelle européenne, de décomposer les séries temporelles mensuelles pour les analyser, et de caractériser les tendances régionales. Enfin, une approche basée sur de l'apprentissage automatisé a été développée pour classifier la zone d'étude en sédimento-régions, et estimer les contributions relatives des forçages environnementaux (vagues, vent, température, salinité, profondeur de la couche de mélange) à la variabilité de la SPM. Ces travaux mettent en avant des approches intégrées, combinant télédétection, mesures in situ et modélisation, pour un suivi durable et global des écosystèmes marins.