Chlorophyll-a concentration (Chla)

Chlorophyll-a products are based on optical satellite data from the Sentinel-2 constellation. It is generated in near real-time (NRT) once a week for the entire France and overseas area and derived from the Remote Sensing Reflectances (Rrs) products. Since chlorophyll a is found in phytoplankton, its concentration in lake water serves as an indirect indicator of the biomass of phytoplankton present in the water column. Thus, the concentration of chlorophyll a is linked to the amount of phosphorus present in lake water. The higher the phosphorus content of the water, the more nutrients the phytoplankton has to reproduce, and the greater the quantity of chlorophyll a present.

Retrieval methodology

Products are derived from a single Sentinel-2 L2A that has been corrected for atmospheric contribution (see section about Remote Sensing Reflectance (Rrs)). Algorithms (NASA - O’Reilly 1998 “clear waters”) are then applied to retrieve the estimations of Chlorophyll-a concentration from bottom of atmosphere Rrs derived from the atmospheric corrections.

Product limitations

As Level-2A optical Rrs estimates are the input data used to derive Chla products, uncertainties in Rrs are consequently propagated to Level-2B data. Moreover, no data can be produced when clouds are present in the atmosphere. Clouds can be present over the scenes but the masks are not applied directly over the level 2B produced. Therefore, the user needs to specify (mask==0) to eliminate land and clouds to obtain usable data. Considering the difficulty to retrieve the band 2 data over inland waters especially with the caveats in aerosols modeling (Pahlevan et al., 2021), this band is often overcorrected which leads to negative values of the reflectances (physically impossible). Therefore, the algorithm can suffer from unresolved pixels and or a lack of sensitivity in the blue-green region preventing it from accurately retrieving Chla from dark waters. On inland waters, there is an important contrast between land reflecting high NIR (Near-Infra-Red) signal, and the very low NIR reflected by water for which absorption of NIR by water is very high.