For the 21 years of the study, an examination of trends in chlorophyll concentration revealed a general decline throughout the Gulf over the production period. These trends, extracted from dynamic linear model, also allowed this decline to be quantified. Expressed as a percentage, a large part of the area below the 50 m bathymetric line showed a decrease of at least 10% over the period, corresponding to a value of at least 0.1 µg.l-1. However, the spatial distribution reveals some more local phenomena. In southern Brittany, from Quimper to Vannes, a particular feature appears, with an upward trend over several kilometres along the coast, followed by a pronounced gradient along the coast. This gradient includes a zone where a continuous monotonic increasing trend is observed, then a zone where the trend becomes not significant and finally, about 15 km from the coast, a new zone where a significant continuous monotonic decreasing trend is observed. The increase in chlorophyll a concentration in the very coastal part is greater than 0.1 µg.l-1 over the period. Another peculiarity concerns the central part, located at the edge of the plateau at Cap Ferrat and Pente Aquitaine, where an increase in chlorophyll a was observed, but the variations remained small, being less than 0.1 µg.l-1. About a hundred kilometres south-west of Saint Nazaire, an area of about 40 by 50 km shows a decrease in chlorophyll a of more than 20%, quantified as more than 0.1 µg.l-1 over the period.

This In Situ delayed mode product integrates the best available version of in situ oxygen, chlorophyll / fluorescence and nutrients data. The latest version of Copernicus delayed-mode BGC (bio-geo-chemical) product is also distributed from Copernicus Marine catalogue.

The ODATIS Ocean Color MR product provides optical reflectance measurements as well as related physical, subsurface and biogeochemical parameters at 300 m spatial resolution along the entire French metropolitan coastal zone, according to the criteria defined by the ODATIS Scientific Expert Consortium (CES) dedicated to ocean color : https://www.odatis-ocean.fr/activites/consortium-dexpertise-scientifique/ces-couleur-de-locean. Product processing is performed from Level 1 to Level 3, and is reprojected on a regular square grid format. Data are temporally aggregated and provided as daily, 8 day and monthly products. The "Basic" version of the ODATIS MR product includes data from the MODIS sensor processed with the "NIR/SWIR" atmospheric correction method (Wang and Shi, 2007), as well as data from the MERIS and OLCI-A/B sensors processed with the Polymer atmospheric correction (Hygeos, https://www.hygeos.com/polymer). List of available parameters for each sensor: • MODIS : NRRS555, CHL-OC5, SPM-G, CDOM, T-FNU, SST-NIGHT • OLCI-A/B / MERIS : NRRS560, CHL-OC5, SPM-G, CDOM, T-FNU