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The study of extreme weather events and their impact on ocean physics and biogeochemistry is challenging due to the difficulty of collecting in situ data. Yet, recent research pointed out the major influence of such physical forcing events on microbiological organisms. Moreover, such violent event occurrences may rise in the future in the context of global change. In May 2019, an intense storm occurred in the Ligurian Sea (north-western Mediterranean Sea) and was captured during the FUMSECK cruise. In situ multi-platform measurements (vessel-mounted ADCP, thermosaligraph, fluorometer, flow cytometer, a Moving Vessel Profiler equipped with a multi-sensor towed vehicle, and glider) along with satellite data and a 3D atmospherical model were used to characterise the fine-scale dynamics occurring in the impacted oceanic zone. The most affected area was marked by a lower water temperature (1 °C colder), and an increase by a factor two in surface chlorophyll-a and seven in nitrate concentrations, exhibiting strong gradients with respect to the surrounding waters. Our results show that this storm led to a deepening of the mixed layer depth from 15 to 50 m and a dilution of the deep chlorophyll maximum. As a result, the surface phytoplankton biomass of most groups identified by automated flow cytometry increased by up to a factor of two. Conversely, the phytoplankton carbon-chlorophyll ratio of most groups dropped down by a factor of two, evidencing significant changes in the phytoplankton cell compositions. These results suggest that the role of storms on the biogeochemistry and ecology of the Mediterranean open sea may be underestimated and highlight the need for high-resolution measurements coupling physics and biology during these events.

Phytoplankton plays a major role in the ocean, being the basis of the marine food web and controlling the biogeochemical cycles. Numerical simulation have shown that finescale structures such as fronts are often suitable places for the generation of vertical velocities, transporting subsurface nutrients to the euphotic zone and thus modulating phytoplankton abundance and community structure. Since several years, observations have concentrated on nutrient fluxes along these structures. Instead, direct in situ estimations of the phytoplankton growth rates are much less numerous; although difficult to obtain, they provide a precious information on the ecosystem functioning. Here, we consider the case of a front separating two water masses characterized by several phytoplankton groups with different abundances, in the southwestern Mediterranean Sea. In order to estimate possible differences in growing rates, we used an adaptive and Lagrangian sampling strategy to measure the phytoplankton diurnal cycle in these two water masses. The use of a size-structured population model was then applied to these data to estimate the growth and division rates for each phytoplankton groups identified by flow cytometry, showing that these two population parameters are significantly different on the two sides of the front, and consistent with the relative abundances. Our results introduce a general method for estimating growth rates at frontal systems, paving the way to in situ exploration of finescale biophysical scenarios.

Phytoplankton plays a major role in the ocean, being the basis of the marine food web and controlling the biogeochemical cycles. Numerical simulation have shown that finescale structures such as fronts are often suitable places for the generation of vertical velocities, transporting subsurface nutrients to the euphotic zone and thus modulating phytoplankton abundance and community structure. Since several years, observations have concentrated on nutrient fluxes along these structures. Instead, direct in situ estimations of the phytoplankton growth rates are much less numerous; although difficult to obtain, they provide a precious information on the ecosystem functioning. Here, we consider the case of a front separating two water masses characterized by several phytoplankton groups with different abundances, in the southwestern Mediterranean Sea. In order to estimate possible differences in growing rates, we used an adaptive and Lagrangian sampling strategy to measure the phytoplankton diurnal cycle in these two water masses. The use of a size-structured population model was then applied to these data to estimate the growth and division rates for each phytoplankton groups identified by flow cytometry, showing that these two population parameters are significantly different on the two sides of the front, and consistent with the relative abundances. Our results introduce a general method for estimating growth rates at frontal systems, paving the way to in situ exploration of finescale biophysical scenarios.

Vertical velocities knowledge is essential to study fine-scale dynamics in the surface layers of the ocean and to understand their impact on biological production mechanisms. However, these vertical velocities have long been neglected, simply parameterized, or considered as not measurable, due mainly to their order of magnitude (< mm s−1 up to cm s−1), generally much lower than the one of the horizontal velocities (cm s−1 to dm s−1), hence the challenge of their in situ measurement. In this paper, we present an upgraded method for direct in situ measurement of vertical velocities using data from different Acoustic Doppler Current Profilers (ADCPs) associated with CTD probes, and we perform a comparative analysis of the results obtained by this method. The analyzed data were collected during the FUMSECK cruise, from three ADCPs: two Workhorse (conventional ADCPs), one lowered on a carousel and the other deployed in free-fall mode, and one Sentinel V (a new generation ADCP with four classical beams and a fifth vertical beam), also lowered on a carousel. Our analyses provide profiles of vertical velocities on the order of mm s−1, as expected, with standard deviations of a few mm s−1. While the fifth beam of the Sentinel V exhibits a better accuracy than conventional ADCPs, the free-fall technique provides a more accurate measurement compared to the carousel technique. Finally, this innovative study opens up the possibility to perform simple and direct in situ measurements of vertical velocities, coupling the free-fall technique with a five-beam ADCP.

Model simulations and remote sensing observations show that ocean dynamics at fine scales (1–100 km in space, day–weeks in time) strongly influence the distribution of phytoplankton. However, only few in situ samplings have been performed and most of them in boundary currents which may not be representative of less energetic regions. The PROTEVSMED-SWOT cruise took place in the moderately energetic waters of the western Mediterranean Sea, in the southern region of the Balearic Islands. Taking advantage of near-real time satellite information, a sampling strategy was defined in order to cross a frontal zone separating different water masses. Multi-parametric in situ sensors mounted on the vessel, on a towed fish and on an ocean glider were used to sample at high spatial resolution both physical and biogeochemical variables. A particular attention was put in adapting the sampling route, in order to also estimate the vertical velocities in the frontal area. Such a strategy was successful in sampling quasi-synoptically an oceanic area characterized by the presence of a narrow front with an associated vertical circulation. A multiparametric statistical analysis of the collected data identifies two water masses characterized by different abundances of several phytoplankton cytometric functional groups, as well as different contents in chlorophyll a and O2. Our study shows that the Lagrangian fronts induced by the fine-scale circulation, even if much weaker than the fronts occurring in boundary current systems, maintain a strong structuring effect on phytoplankton community by segregating different taxa at the surface.

The fine scales are defined here as oceanic dynamical features (eddies, fronts and filaments) generally induced by mesoscale interactions and frontogenesis, and often associated with intense vertical exchanges. These processes are characterized by horizontal scales of 1–10 km with a relatively short lifetime of days/weeks to months. This temporal scale is similar to that of many biological processes, such as, phytoplankton growth, suggesting a physical and biological coupling. Numerical simulations and satellite observations have allowed the characterization of this regime highlighting the role played by these fine scales on structuring the phytoplankton community. To better understand this coupling mechanism, physical and biological in situ measurements are necessary. However, the observations of fine scales remains challenging due to the difficulties of sampling at high spatio-temporal frequency (~km ~daily). Over the past few years, the Mediterranean Sea has become a lab for developing fine scale in situ strategies. Indeed, a series of campaigns using a satellite based adaptative and Lagrangian strategy coupled with a high-resolution physical-biological sampling, have been performed in order to follow and describe fine scale structures. Following this strategy, the PROTEVSMED-SWOT 2018 cruise has been leaded in the South of the Balearic Islands, with a particular attention to correlate the Lagrangian sampling with the temporal phytoplankton growth, in order to reconstruct the phytoplankton diurnal cycle. Multidisciplinary in situ sensors have allowed to identify a frontal area with a dynamic vertical circulation. Furthermore, the presence of two Atlantic waters, at different stages of mixing associated with various abundances of several phytoplankton groups, corroborated that fine scales must be dynamical barriers to transport, as previous modeling studies have proposed. In order to better understand fine scale mechanisms, the Protevs Gibraltar cruise was perform

Model simulations and remote sensing observations show that ocean dynamics at fine scales (1–100 km in space, day–weeks in time) strongly influence the distribution of phytoplankton. However, only a few in situ-based studies at fine scales have been performed, and most of them concern western boundary currents which may not be representative of less energetic regions. The PROTEVSMEDSWOT cruise took place in the moderately energetic waters of the western Mediterranean Sea (WMS), in the region south of the Balearic Islands. Taking advantage of nearreal-time satellite information, we defined a sampling strategy in order to cross a frontal zone separating different water masses. Multi-parametric in situ sensors mounted on the research vessel, on a towed vehicle and on an ocean glider were used to sample physical and biogeochemical variables at a high spatial resolution. Particular attention was given to adapting the sampling route in order to estimate the vertical velocities in the frontal area also. This strategy was successful in sampling quasi-synoptically an oceanic area characterized by the presence of a narrow front with an associated vertical circulation. A multiparametric statistical analysis of the collected data identifies two water masses characterized by different abundances of several phytoplankton cytometric functional groups, as well as different concentrations of chlorophyll a and O2. Here, we focus on moderately energetic fronts induced by fine-scale circulation. Moreover, we explore physical–biological coupling in an oligotrophic region. Our results show that the fronts induced by the finescale circulation, even if weaker than the fronts occurring in energetic and nutrient-rich boundary current systems, maintain nevertheless a strong structuring effect on the phytoplankton community by segregating different groups at the surface. Since oligotrophic and moderately energetic regions are representative of a very large part of the world ocean, our results

The FUMSECK cruise was a one-week technological cruise, which took place in spring 2019, in the gulf of Genoa (NW Mediterranean Sea), onboard the R/V T´ethys II. Its aim was to perform several technological tests of some instruments used for the study of the fine-scale processes and dynamics. It was conducted in preparation of the BioSWOT-Med cruise in the SW Mediterranean Sea in 2022, planned as part of the “Adopt a Cross Over” initiative organising simultaneous oceanographic cruises around the world during the SWOT fast sampling phase. During FUMSECK we tested various technological innovations for the study of fine-scale dynamics and their coupling with biogeochemistry.