Frontal plankton communities: How fine-scale oceanic features shape « bacterial » community structure

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 energetic and oligotrophic regions constituting the majority of the ocean. The new «SWOT satellite era» makes possible to observe the fine-scale component of ocean dynamics even in low energetic and oligotrophic regions. In 2023, the BioSWOT-Med cruise (northwestern Mediterranean) 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 detected by conventional altimetry, and the front itself.

Heterotrophic prokaryotic communities are the main drivers of oceanic mineralization of the organic matter. By integrating hydrographic and cytometric data for bacterial abundance and diversity across the water column (0-500 m), we show that water masses act as ecological filters, shaping bacterial assemblages with distinct compositions, abundances, and functional traits. Functional statistics revealed that the composition of the bacterial community at the front was not similar to that of the adjacent waters, with higher surface concentrations of medium- to small-sized cells, suggesting that this type of nutrient-rich structure promotes bacterial growth. The open water mass was characterized by larger cells, while the eddy was distinguished by lower abundances and deeper bacterial signatures in 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.