Chlorophyll-a concentration (Chl-a) is used as a proxy for phytoplankton biomass to track blooms and monitor the state of the ecosystem. This dataset contains daily satellite Chl-a from July 2002 to July 2023 (further updates will be provided periodically). The data is at a 4.64-km^2 spatial resolution covering an area of the Northwest Atlantic from 39-82°N, 42-95°W. Chl-a was generated by applying the POLY4 algorithm (Clay et al., 2019, see resources) on level-3 binned remote sensing reflectance (Rrs) data retrieved by the Moderate Resolution Imaging Spectroradiometer sensor (MODIS) onboard the Aqua satellite, freely downloaded courtesy of NASA's Ocean Biology Processing Group (OBPG, https://oceancolor.gsfc.nasa.gov/). Regionally-tuned coefficients for POLY4 were generated by matching level-2 MODIS-Aqua pixels (i.e. individual satellite passes) to in situ HPLC (High Performance Liquid Chromatography) Chl-a samples within the region of interest according to the procedure described in Clay et al. POLY4 Chl-a was then optimized by simultaneously forcing it to the 1:1 line against in situ Chl-a to reduce the negative bias observed in higher concentrations. The Chl-a data were then projected onto an Equidistant Cylindrical grid (4.64-km^2) using a weighted mean for overlapping bins. This model and the Northwest Atlantic coefficients are available in the R package oceancolouR (https://github.com/BIO-RSG/oceancolouR).
