Chlorophyll-a concentration climatology, phenology, and trends in the optically complex waters of the St. Lawrence Estuary and Gulf

The spatiotemporal distribution of phytoplankton biomass drives the marine ecosystem. Chlorophyll-a concentration (Chla) is a biomass index for microalgae in seawater that is commonly used to study phytoplankton by means of satellite remote sensing. The St. Lawrence Estuary and Gulf (SLEG) in Eastern Canada is a highly dynamic subpolar environment characterized by complex marine optical properties that make it difficult to distinguish Chla from the background signal caused by a strong freshwater discharge. In this study, we implement an inverse model based on a set of in situ Chla measurements analyzed by principal component analysis, making it specifically designed for local marine optical conditions. We used this model to convert a multi-mission remote sensing reflectance dataset to daily Chla between 1998 and 2019 at a 4 km spatial resolution. From the resulting Chla time series, we computed the climatology, phenology, and trends over the SLEG. The Chla climatology reveals relatively high Chla in the Gaspé Current, along the Gulf's North Shore, and in areas of strong tidal mixing. Substancial differences in phytoplankton phenology between the various subregions are found, with a prevailing shift towards earlier spring blooms of larger intensities. Finally, we found a positive mean Chla increase of 1.1% y−1 over the SLEG, with strong positive trends in the Magdalen Shallows and west of Anticosti Island. This description of the surface Chla in the SLEG provides important baseline information for the marine ecosystem.