Seasonal patterns and multidecadal trends in phytoplankton functional groups in the Benguela upwelling system off...

Understanding the response of phytoplankton to climate change is crucial for predicting shifts in marine ecosystems. Despite the Benguela being the world's most productive eastern boundary upwelling system, the distribution and susceptibility of its phytoplankton functional groups (PFGs) to climate change remain poorly understood. Here, we use 20 years (2003-2022) of daily MODIS-Aqua satellite data to uncover distinct spatial, seasonal and multidecadal trends in key PFGs (diatoms, dinoflagellates, flagellates, coccolithophores). PFGs exhibit strong regional and seasonal variability in their Spatial Extent and Frequency of dominance, with evidence of niche conditions favoring particular PFGs. Multidecadal trends reveal strong spatial variability in PFG adjustments, which are closely aligned with major upwelling cells. Some cells show increasing diatom dominance coupled with declines in coccolithophores, dinoflagellates and flagellates, while others exhibit the opposite trend. Seasonal transitions are also evident, with diatom-to-dinoflagellates shifts in Summer and enhanced diatom dominance in Autumn. Overall, PFGs increased in Spatial Extent and Frequency of dominance at distinct seasonal timings, except for declining coccolithophores. The link between alongshore equatorward winds and PFG trends strongly suggests an altered nutrient supply-driven response. Diatoms, significant carbon exporters, exhibit pronounced adjustments, highlighting their sensitivity to environmental changes. These findings are indicative of potential impacts on biogeochemical cycling and higher trophic levels, affecting carbon export and marine services. These insights provide a critical foundation for identifying climate-sensitive regions and seasonal windows of ecological vulnerability, supporting the development of early warning systems for adaptive conservation efforts.