Relationships between phytoplankton assemblages and bio-optical variability in a coastal environment of the Gulf of St. Lawrence

Recent advances in Earth Observation technologies are promising in bring traditional ocean-color capabilities to coastal zones. However, the biogeochemical and optical complexity that are usually found in such environments commonly hinders the retrieval of geophysical variables using generic inversion algorithms. This is particularly critical for phytoplankton functional types and biomass studies. In this work, we have characterized the seasonal (spring to autumn) variability of surface phytoplankton assemblages and the corresponding water bio-optical properties in a coastal area of the St. Lawrence Gulf, (Bay of Sept-Îles, Canada). Concomitantly inherent optical properties (spectral absorption and backscattering at 6 wavelengths) and radiometry measurements with water sampling were taken over eleven campaigns, conducted from August 2016 to June 2019, with spatially distributed stations (n = 121). Water samples were analyzed for eukaryotic and cyanobacteria phytoplankton and heterotrophically bacteria cell count by class size (flow cytometry), phytoplankton pigments (HPLC), SPM, DOC and nutrients concentrations, as well as for the spectral absorption of CDOM and particulate matter (non-algal particles and phytoplankton). The resulting bio-optical variability are analyzed as a function of the main phytoplankton assemblages, as obtained by Hierarchical Cluster Analysis, using some diagnostic pigment concentrations examined together with size classes information. Through the computation of apparent optical properties (namely the remote sensing reflectance and attenuation coefficient), the implications for phytoplankton functional type and biomass retrieval from Earth Observation platforms are also discussed.