On the possibility of recognising different phytoplankton functional groups and size classes in optically complex waters

Knowledge of the phytoplankton functional types (PFTs) and size classes (PSCs) is critical for many purposes, from early warning of potentially harmful blooms to carbon cycle studies. The feasibility of retrieving this information has been demonstrated in clear natural waters where phytoplankton is the main optically active substance. We studied the possibility of retrieving PFTs and PSCs in optically complex waters, namely in the Baltic Sea, using multiple methods: (1) recognising the group-specific marker pigments in water reflectance and/or phytoplankton absorption coefficient spectra, (2) linking particle backscattering to particle size classes, (3) using look-up-tables and (4) applying empirical algorithms linking biomass to percentage presence of PFTs and PSCs. Most of the marker pigments characteristic of a certain functional type absorb light at the same wavelengths in the blue part (400-500 nm) of the spectrum. These pigments are optically indistinguishable from one another, even when using pure pigments and hyperspectral sensors - let alone when trying to detect them within natural phytoplankton assemblages using sensors such as OLCI on Sentinel-3 or MSI on Sentinel-2. In the Baltic Sea, PSCs did not correlate with the backscattering coefficient, because most of the particulate material was mineral particles rather than organic particulate matter, and not all organic particulate matter was phytoplankton. The biomass-based approach, which retrieves all PFTs and PSCs simultaneously (e.g., as used by the Copernicus Marine Service), does not work in the Baltic Sea.Not only because the remote sensing estimates of biomass (Chl-a), from which the PFTs and PSCs are calculated, are poor. The median concentrations of the optically active substances (phytoplankton, CDOM, SPM) in the Baltic Sea are similar to the medians in the largest coastal and inland water bio-optical database, GLORIA. Consequently, the results of this study – showing that PFT/PSC retrieval from water reflectance is not possible in waters whose optical properties are not determined by phytoplankton - apply to most optically complex waters worldwide.