“Living on the edge” Fine-scale observations reveal distinct frontal phytoplankton communities

Phytoplankton community composition plays a key role in oceanic productivity and in the biological carbon pump. A few in situ surveys have shown the structuring role of fine-scale physical processes (1-100 km, day-weeks), in particular fronts, on phytoplankton communities. Most of these studies were conducted in highly productive and dynamic regions, while oligotrophic and moderate energy conditions have been less explored, though very common in the global ocean. Guided by novel SWOT altimetry, we used an adaptive and multidisciplinary strategy to conduct high-resolution sampling of the North-Balearic Front in the oligotrophic Mediterranean Sea. An unprecedented 24-hour continuous sampling within the front was crucial to unveiling a distinct frontal community, where the relative contribution of non-dominant phytoplankton groups increased relative to adjacent water masses. This highlights the pivotal role of fine-scale features in increasing phytoplankton heterogeneity, thus contributing to community diversity in oligotrophic and moderately energetic regions. These findings represent a key step toward understanding the ecological response to fine-scale structuring on a global scale.