Assessing phytoplankton community composition using in-situ multispectral excitation fluorescence and potential for application to BGC-Argo profiling floats

Abstract. Phytoplankton community composition is a key determinant of ocean biogeochemical cycles, yet its observation from 20 autonomous platforms remains challenging. In this study, we assessed the potential of in situ multispectral excitation fluorescence (MXF) to discriminate phytoplankton assemblages in the Northwestern Mediterranean Sea, with a view toward applications on Biogeochemical-Argo (BGC-Argo) profiling floats. Laboratory measurements on ten phytoplankton strains confirmed that MXF ratios at 440, 470, and 532 nm provide taxon-specific signatures, especially for picocyanobacteria and green algae. Field observations of phytoplankton pigments were clustered into four ecologically distinct phytoplankton communities along the seasonal cycle. A machine learning model was then trained to classify these clusters using MXF and additional bio-optical indices. Results show that existing BGC-Argo configurations (single-wavelength fluorescence, particulate backscattering and beam attenuation coefficients) reliably distinguish broad community structures such as pico- versus microphytoplankton dominance, but resolving finer pigment-based differences requires the added spectral information of MXF. The different excitation channels contributed unequally: 440 and 470 nm provided robust pigment sensitivity across communities, while 532 nm was particularly informative for detecting phycoerythrin- and chlorophyll b–rich taxa. Overall, combining MXF with bio-optical proxies improved classification performance by integrating pigment-specific and size-structure information, demonstrating the potential of MXF to enhance autonomous monitoring of phytoplankton community dynamics and their role in ocean biogeochemical cycles.