Synergistic use of OLCI and TROPOMI measurements to determine phytoplankton functional types

Phytoplankton play an important role in the aquatic biogeochemical cycling such as for the formation of organic matter by photosynthetic processes through the fixation of carbon dioxide, and assimilation of macro- and micronutrients depending on their metabolic needs. These processes are common to all phytoplankton; however, some phytoplankton groups have specific needs and thus play different functional roles in the biogeochemical cycle. Information on the phytoplankton groups (PFTs) can be obtained from satellite observations such as the Ocean and Land Colour Instrument (OLCI) on board of Sentinel-3 as well as the TROPOspheric Monitoring Instrument (TROPOMI) on board the Copernicus Sentinel-5 Precursor satellite. PFTs global ocean abundance from multispectral satellites can be estimated based on the OC-PFT algorithm which is based on the assumption that a marker pigment for a specific PFT varies in dependence to the chlorophyll-a concentration. While PFTs from hyperspectral satellite measurements, as from TROPOMI, can be estimated by Differential Optical Absorption Spectroscopy (DOAS) method. In this study, chlorophyll-a concentration for three main phytoplankton functional types (diatoms, coccolithophores and cyanobacteria) are derived by combining retrievals from space-borne measurements at a high spatial resolution by the empirical OC-PFT algorithm applied to OLCI data with data retrieved from TROPOMI measurements with high spectral resolution by analytical method (Phyto-DOAS). A previous algorithm and data set based on OC-PFT retrievals applied to OC-CCI Chlorophyll-a product and Phyto-DOAS retrievals from SCIAMACHY data have shown the validity and high quality of the synergistic PFT product (Losa et al. 2017). Here, a first evaluation of the synergy of Sentinel-3 and Sentinel-5P PFT retrievals by combining OC-PFT and Phyto-DOAS is evaluated and compared to field measurements of PFT sampled during the RV Polarstern expedition PS113 in the Atlantic Ocean from May to June 2018. In addition, the adaptation of the method to enlarge the capabilities of PFT data in inland and coastal waters analytically retrieved from high spectral and high spatial data such as DESIS, EnMAP or PRISMA by synergistic use with OLCI OC-PFT data sets is discussed