Seasonal changes in phytoplankton community of the Straits of Florida near the Florida Keys

Traditional phytoplankton characterization based on morphology is often labor-intensive and requires specialized taxonomic expertise. CHEMTAX, a chemotaxonomic identification method, offers a more efficient process for assessing phytoplankton composition, providing a better alternative for long-term sampling studies. Hence, this study describes seasonal phytoplankton communities within the Florida Keys National Marine Sanctuary (FKNMS) from 2016 to 2021, using CHEMTAX. Phytoplankton biomass and community composition were analyzed across four seasons at nearshore and offshore locations using high-performance liquid chromatography to quantify the concentration of chlorophyll-a and other pigments. Results showed small but significant seasonal variability in phytoplankton biomass with no clear inter-annual trends. Total biomass and community composition differed between nearshore and offshore sites. Cyanobacteria-2 (i.e. Synechococcus) dominated across all sites (27% - 40% of Tchl a), while diatom-1 (e.g. Chaetoceros), dinoflagellates, and prasinophytes were consistently low (2% - 6%). Dinoflagellates and prasinophytes showed the least seasonal variability with standard deviations less than 5%. The nearshore sites had higher proportions of diatom-1 (11.9 ± 10.7%), diatom-2 (e.g. Pseudonitzschia, 13.7 ± 6.6%), and dinoflagellate (7.4 ± 4.3%), whereas offshore sites had higher proportions of cyanobacteria-4 (i.e. Prochlorococcus, 17.7 ± 9.0%) and haptophyte-8 (i.e. Phaeocystis, 11.0 ± 4.2%). Nearshore vs offshore components influenced community composition three times more than seasonality. Our findings establish a baseline for seasonal abundance and distribution patterns of phytoplankton pigment groups derived from CHEMTAX analysis within the FKNMS.