”Living on the edge”: Fine-scale observations of frontal phytoplankton communities

Phytoplankton communities, shaped by complex water dynamics, are vital to ecosystem structure and play a key role in oceanic productivity and the biological carbon pump. Previous studies suggest that fine-scale O(1-100km, day-week) physical features significantly influence phytoplankton production, distribution and diversity in highly productive and dynamic regions. However, in oligotrophic and moderate energy conditions, representing a significant part of the global ocean, how fine-scale dynamics impact phytoplankton dynamics and diversity remains a key open question. Observations of fine-scale fronts are particularly challenging due to the difficulties in tracking their spatial and temporal evolution. Using a multidisciplinary, adaptive Lagrangian approach that integrated novel SWOT altimetry data with high-resolution in situ observations, we conducted fine-scale physical and biological sampling of the North-Balearic Front in the oligotrophic Mediterranean Sea (BioSWOT-Med, doi.org/10.17600/18002392). We found that specific biomass proportions of phytoplankton functional types were associated with distinct water masses separated by the front. Furthermore, we performed high-resolution sampling within the front itself to demonstrate that the front hosts a distinct community, where dominant phytoplankton groups display intermediate or decreased biomass proportions relative to water masses on either side but non-dominant phytoplankton groups display increased biomass proportions. Overall, these results suggest that frontal systems drive biological heterogeneity by promoting the existence of a distinct frontal community. This highlights the crucial role of fine-scale features in maintaining community diversity in oligotrophic and moderate energy regions and represents an initial step toward understanding the global ecological response to fine-scale structuring.