Fine-scale oceanic structures (1-100 km), though ephemeral (days-weeks), play a key role in shaping marine physical, chemical, and biological dynamics. Despite their ubiquity, their influence on plankton remains poorly documented due to the difficulty of targeting them in situ, particularly in the low-energy and oligotrophic regions that constitute the majority of the ocean. The new SWOT satellite makes it possible to observe the fine-scale component of ocean dynamics even in such environments. In spring 2023, the BioSWOT-Med cruise (northwestern Mediterranean Sea) employed an adaptive Lagrangian strategy to target three distinct dynamical features: a cyclonic circulation to the north, an anticyclonic eddy to the south, too small to be clearly resolved by conventional altimetry, and the front itself.

Heterotrophic prokaryote (HP) communities are the main drivers of organic matter remineralization in the ocean. By integrating hydrographic and cytometric data on HP abundance and diversity across the water column (0–500 m), we show that water masses act as ecological filters, shaping HP assemblages with distinct compositions, abundances, and functional traits. Analyses revealed that the HP community at the front differed markedly from those in adjacent waters, with higher surface concentrations of small- to medium-sized cells, suggesting that this nutrient-rich structure promotes HP growth. The cyclonic circulation was characterized by larger cells, whereas the anticyclonic eddy exhibited lower abundances and a deeper distribution of HP populations within the water column.

These results highlight the need for multi-scale and multidisciplinary observations to better understand the biological seascape and its broader implications for biogeochemical cycles and marine ecosystem functioning.