LAUSR

Abstract The active hydrodynamics and complex circulation patterns in the northern Gulf of Mexico (nGoM) provide a dynamic scenario to investigate mesoscale responses of the size structure of phytoplankton communities to environmental variability. In this study, a large in-situ dataset acquired in inland, estuarine and coastal waters of the nGoM and U.S. East Coast were utilized to achieve a regional parameterization of the red-NIR ratio-based chlorophyll a (Chl a) algorithm for the high-spatial and spectral resolution Sentinel 3-OLCI ocean color sensor. Using an adaptive scheme, the algorithm was used in combination with other standard Chl a algorithms such as the Neural Network (C2RCC) and OC4ME to optimally extract Chl a in water types ranging from turbid estuarine to clear oceanic waters in the nGoM. This adaptive methodology (Chl a_AD) showed better performance in estimating OLCI-Chl a (R2=0.84, N = 178) compared to any single algorithm in the estuarine-coastal-ocean waters of the nGoM. Next, OLCI-derived phytoplankton spectral absorption coefficients (OLCI-aphy) were obtained for comparison, using a regionally-tuned 3rd order function of OLCI-Chl a based on Multi-regression (MR), and an estuarine-to-ocean adaptive Quasi-Analytical Algorithm (QAA-AD). Finally, an Empirical Orthogonal Function (EOF) algorithm was applied to OLCI-aphy to retrieve phytoplankton size fractions (PSFs) for the river-dominated nGoM region. Results indicate relatively better performance of MR-EOF in estimating fmicro (R2=0.87), fnano(R2=0.76) and fpico(R2=0.64) compared to QAA-AD-EOF. Chl a time-series (Oct 2016 – Jan 2020) obtained in the estuaries and shelf waters of the nGoM revealed seasonality (peak/lows in spring/fall) linked to river discharge, where hydrodynamics and wind driven variability due to storms further influenced the phytoplankton biomass spatiotemporal distribution. Microphytoplankton dominated in estuaries and the nGOM inner shelf waters, including the midshelf region during the spring peak river discharge period. PSF dynamics in the outer shelf is strongly influenced by Loop Current eddies where warm offshore waters contribute to the dominance of the picophytoplankton fraction. Floodwater discharge and strong winds due to Hurricane Barry (2019) transported estuarine microphytoplankton into the outer shelf and also increased nanophytoplankton fraction. Such fluctuations in phytoplankton size structure after hurricanes or storms can modify the pelagic food web dynamics in coastal systems.