Andrea Michelangelo Doglioli

Phytoplankton community composition plays a key role in oceanic productivity and in the biological carbon pump. A few in situ surveys documented the structuring role of fine-scale physical structures (1-100 km, day-weeks), such as fronts, on phytoplankton communities. These studies were primarily conducted in highly productive and dynamic regions rather than in oligotrophic and moderate energy conditions commonly found in the global ocean, partly due to challenges in tracking and sampling weaker fronts at high-resolution. Guided by novel SWOT (Surface Water Ocean Topography) altimetry, we used an adaptive and multidisciplinary strategy to conduct high-resolution sampling of a fine-scale 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. Our results statistically demonstrate that fine-scale features can enhance phytoplankton heterogeneity and community diversity in oligotrophic, moderately energetic regions.

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.

Near‐inertial waves (NIWs) are an important source of turbulence for the ocean interior. Mesoscale anticyclonic eddies are known to facilitate their propagation at depth while trapping them. However, in situ observations have so far focused on large ( km radius), energetic eddies, whereas most of the ocean is populated by smaller, moderately energetic fine‐scale structures. Are these smaller structures efficient to trap NIWs and enhance turbulence? Here, we present in situ observations from the BioSWOT‐Med 2023 cruise addressing this issue by surveying a fine‐scale frontal area of the North Balearic front in the Mediterranean Sea, assisted by the first high‐resolution Sea Surface Height images of the new Surface Water and Ocean Topography (SWOT) satellite mission during its Calibration/Validation phase. We explore how fine scales modulate the evolution of turbulence below the mixed layer after experiencing two consecutive strong wind events. We show that turbulence remains low in the front and its cyclonic side, while being greatly enhanced in the anticyclonic side. The latter side is dominated by a fine‐scale anticyclone (12.8 km of radius, Rossby number of 0.5) that trapped NIWs, increasing turbulent dissipation level to several W . The NIW‐induced vertical kinetic energy flux reach up to 5.1 mW below the pycnocline and represent % of the wind power input into inertial motions, higher or similar to previous estimations outside and inside mesoscale anticyclones. Future work is needed to investigate whether these results extend to fine scales elsewhere in the world ocean, especially in regions with larger baroclinic Rossby radius of deformation.

Fine-scale oceanic fronts are ubiquitous and ephemeral physical features that separate contrasting water masses, creating significant heterogeneity in the physical seascape and plankton distributions. Because phytoplankton community composition (PCC) is a key driver of marine ecosystem functioning, understanding the extent to which fine-scale fronts influence PCC is a critical challenge. However, studying PCC across and within fronts is particularly difficult due to data scarcity and high biophysical variability. We developed a tailored statistical model to characterize PCC within an oceanic front we studied in the Mediterranean Sea. We modeled the frontal community as a finite mixture model with three components: two communities of adjacent water masses and a potential front-adapted community. Each component was further considered as a discrete mixture of an unknown number of multivariate Gaussian sub-components. First, we used an Expectation–Maximization algorithm to estimate the Gaussian parameters and determine the optimal number of sub-components based on in situ datasets of the PCC within a frontal zone and its adjacent water masses. Second, a hierarchical Bayesian approach was applied to estimate the weight of all components within the frontal dataset. Our analysis suggests that within the front a new community component, distinct from those in adjacent water masses, accounts for 70 % of the frontal community, indicating that a specific phytoplankton community can emerge in fine-scale oceanic fronts. Despite the limited number of frontal observations, our Bayesian modelling approach provides statistical evidence of the front's influence on phytoplankton community composition, effectively overcoming data scarcity and high variability.

Observations, models and theory have suggested that ocean fronts are ecological hotspots, generally associated with higher diversity and biomass across many trophic levels. Nutrient injections are often associated with higher chlorophyll concentrations at fronts, but the response of the zooplankton community is still insufficiently understood. The present study investigates mesozooplankton stocks and composition during late spring, northeast of Menorca, along two north-south transects that crossed the North Balearic Front separating central waters of the Northwestern Mediterranean Sea gyre from peripheral waters originating from the Algerian basin. During the BioSWOT-Med campaign, vertical triple-net tows with 200 and 500 µm meshes were carried out at three depths (100, 200, and 400 m), and the samples were processed with ZooScan to classify organisms into eight taxonomic groups. Zooplankton distributions were analyzed for the surface layer (0–100 m), a mid-depth layer (100–200 m), and a deeper layer (200–400 m). The results did not show a significant increase in biomass in the front in any layers. The NBF appears to act as a boundary between communities rather than a pronounced area of active or passive zooplankton accumulation. Analyses of stratified vertical distributions of zooplankton highlighted distinct taxonomic compositions in the three layers, and a progressive homogenization of community structure with depth, reflecting a weaker impact of hydrological processes on deeper communities. The clearest impact of the front was within the upper 100 m, where the mesozooplanktonic taxonomic composition differed between the front and adjacent water masses, with a decrease in all taxonomic groups except Cnidaria, which increased dramatically. In the two deeper layers, the front also influenced community composition, although to a lesser extent, with marked increases in Foraminifera and Cnidaria. Moreover, the northern water mass and the front were dominated by large copepods, while the southern water mass exhibited higher zooplankton diversity and smaller-sized copepods. The results of this study highlight the complexity of processes shaping planktonic communities over time and space in the NBF zone and its adjacent waters. These processes include zooplankton stock reduction in the transitional post-bloom period, marked effect of diel variation linked to vertical migrations, and potentially the impact of storm-related mixing in the surface layer that can disrupt established ecological patterns.

Observations, models and theory have suggested that ocean fronts are ecological hotspots, generally associated with higher diversity and biomass across many trophic levels. Nutrient injections are often associated with higher chlorophyll concentrations at fronts, but the response of the zooplankton community is largely unknown. The present study investigates mesozooplankton stocks and composition during late spring, northeast of Menorca along two north-south transects that crossed the North Balearic Front (NBF) separating central water of the Northwestern Mediterranean Sea (NWMS) gyre from peripheral waters originating from the Algerian basin. During the BioSWOT-Med campaign, samples were collected using vertical triple net tows at three depths (100 m, 200 m, 400 m) with 200 μm and 500 μm mesh nets, processed with ZooScan, and the organisms assigned to eight taxonomic groups. Zooplankton distributions were analysed for the surface (0–100 m), intermediate (100–200 m), and deeper (200–400 m) layers. The results showed no significant biomass increase at the front across all vertical layers. The NBF seems to act more like a boundary between communities rather than a pronounced area of active or passive zooplankton accumulation. Analysis of stratified vertical distributions of zooplankton highlighted distinct taxonomic compositions in the surface, intermediate, and deeper layers, and a progressive homogenization of community structure with depth, reflecting a weaker impact of hydrological processes on deeper communities. The front’s clearest impact was within the upper 100 meters, where the taxonomic composition showed differences between the front and the adjacent water masses, with a decrease in Eu-malacostraca and Foraminifera, while Cnidaria increased sharply. In the 100–200 m layer, the front also influenced community composition, although to a lesser extent, with a marked increase in Foraminifera and a strong decline in Cnidaria. Moreover, the northern water mass and the front were dominated by large copepods, while the southern water mass exhibited higher diversity of zooplankton and a community of smaller-sized copepods. The results of this study highlight the complexity of processes shaping planktonic communities over time and space in the NBF zone and its adjacent waters. These processes include zooplankton stock reduction in the transitional post-bloom period, marked effect of diel variation linked to vertical migrations, and potentially the impact of storm-related mixing in the surface layer that can disrupt established ecological patterns.

Observations, models and theory have suggested that ocean fronts are ecological hotspots, generally associated with higher diversity and biomass across many trophic levels. Nutrient injections are often associated with higher chlorophyll concentrations at fronts, but the response of the zooplankton community is largely unknown. The present study investigates mesozooplankton stocks and composition during late spring, northeast of Menorca along two north-south transects that crossed the North Balearic Front (NBF) separating central water of the Northwestern Mediterranean Sea (NWMS) gyre from peripheral waters originating from the Algerian basin. During the BioSWOT-Med campaign, samples were collected using vertical triple net tows at three depths (100 m, 200 m, 400 m) with 200 μm and 500 μm mesh nets, processed with ZooScan, and the organisms assigned to eight taxonomic groups. Zooplankton distributions were analysed for the surface (0–100 m), intermediate (100–200 m), and deeper (200–400 m) layers. The results showed no significant biomass increase at the front across all vertical layers. The NBF seems to act more like a boundary between communities rather than a pronounced area of active or passive zooplankton accumulation. Analysis of stratified vertical distributions of zooplankton highlighted distinct taxonomic compositions in the surface, intermediate, and deeper layers, and a progressive homogenization of community structure with depth, reflecting a weaker impact of hydrological processes on deeper communities. The front’s clearest impact was within the upper 100 meters, where the taxonomic composition showed differences between the front and the adjacent water masses, with a decrease in Eu-malacostraca and Foraminifera, while Cnidaria increased sharply. In the 100–200 m layer, the front also influenced community composition, although to a lesser extent, with a marked increase in Foraminifera and a strong decline in Cnidaria. Moreover, the northern water mass and the front were dominated by large copepods, while the southern water mass exhibited higher diversity of zooplankton and a community of smaller-sized copepods. The results of this study highlight the complexity of processes shaping planktonic communities over time and space in the NBF zone and its adjacent waters. These processes include zooplankton stock reduction in the transitional post-bloom period, marked effect of diel variation linked to vertical migrations, and potentially the impact of storm-related mixing in the surface layer that can disrupt established ecological patterns.

Observations, models and theory have suggested that ocean fronts are ecological hotspots, generally associated with higher diversity and biomass across many trophic levels. Nutrient injections are often associated with higher chlorophyll concentrations at fronts, but the response of the zooplankton community is largely unknown. The present study investigates mesozooplankton stocks and composition during late spring, northeast of Menorca along two north-south transects that crossed the North Balearic Front (NBF) separating central water of the Northwestern Mediterranean Sea (NWMS) gyre from peripheral waters originating from the Algerian basin. During the BioSWOT-Med campaign, samples were collected using vertical triple net tows at three depths (100 m, 200 m, 400 m) with 200 μm and 500 μm mesh nets, processed with ZooScan, and the organisms assigned to eight taxonomic groups. Zooplankton distributions were analysed for the surface (0–100 m), intermediate (100–200 m), and deeper (200–400 m) layers. The results showed no significant biomass increase at the front across all vertical layers. The NBF seems to act more like a boundary between communities rather than a pronounced area of active or passive zooplankton accumulation. Analysis of stratified vertical distributions of zooplankton highlighted distinct taxonomic compositions in the surface, intermediate, and deeper layers, and a progressive homogenization of community structure with depth, reflecting a weaker impact of hydrological processes on deeper communities. The front’s clearest impact was within the upper 100 meters, where the taxonomic composition showed differences between the front and the adjacent water masses, with a decrease in Eu-malacostraca and Foraminifera, while Cnidaria increased sharply. In the 100–200 m layer, the front also influenced community composition, although to a lesser extent, with a marked increase in Foraminifera and a strong decline in Cnidaria. Moreover, the northern water mass and the front were dominated by large copepods, while the southern water mass exhibited higher diversity of zooplankton and a community of smaller-sized copepods. The results of this study highlight the complexity of processes shaping planktonic communities over time and space in the NBF zone and its adjacent waters. These processes include zooplankton stock reduction in the transitional post-bloom period, marked effect of diel variation linked to vertical migrations, and potentially the impact of storm-related mixing in the surface layer that can disrupt established ecological patterns.

Observations, models and theory have suggested that ocean fronts are ecological hotspots, generally associated with higher diversity and biomass across many trophic levels. Nutrient injections are often associated with higher chlorophyll concentrations at fronts, but the response of the zooplankton community is largely unknown. The present study investigates mesozooplankton stocks and composition during late spring, northeast of Menorca along two north-south transects that crossed the North Balearic Front (NBF) separating central water of the Northwestern Mediterranean Sea (NWMS) gyre from peripheral waters originating from the Algerian basin. During the BioSWOT-Med campaign, samples were collected using vertical triple net tows at three depths (100 m, 200 m, 400 m) with 200 μm and 500 μm mesh nets, processed with ZooScan, and the organisms assigned to eight taxonomic groups. Zooplankton distributions were analysed for the surface (0–100 m), intermediate (100–200 m), and deeper (200–400 m) layers. The results showed no significant biomass increase at the front across all vertical layers. The NBF seems to act more like a boundary between communities rather than a pronounced area of active or passive zooplankton accumulation. Analysis of stratified vertical distributions of zooplankton highlighted distinct taxonomic compositions in the surface, intermediate, and deeper layers, and a progressive homogenization of community structure with depth, reflecting a weaker impact of hydrological processes on deeper communities. The front’s clearest impact was within the upper 100 meters, where the taxonomic composition showed differences between the front and the adjacent water masses, with a decrease in Eu-malacostraca and Foraminifera, while Cnidaria increased sharply. In the 100–200 m layer, the front also influenced community composition, although to a lesser extent, with a marked increase in Foraminifera and a strong decline in Cnidaria. Moreover, the northern water mass and the front were dominated by large copepods, while the southern water mass exhibited higher diversity of zooplankton and a community of smaller-sized copepods. The results of this study highlight the complexity of processes shaping planktonic communities over time and space in the NBF zone and its adjacent waters. These processes include zooplankton stock reduction in the transitional post-bloom period, marked effect of diel variation linked to vertical migrations, and potentially the impact of storm-related mixing in the surface layer that can disrupt established ecological patterns.

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.