Recent progress in the study of fine-scale physical-biological coupling in the Mediterranean Sea.

<p>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).</p><p><br>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 performed in the Strait of Gibraltar in October 2020. This region of study is characterized by an important exchange of Mediterranean and Atlantic waters, and also by an intense circulation that generates energetic processes, which make it a favorable place for the formation of fine scale structures.</p><p><br>The new knowledge acquired with these studies paves the way to the future BIOSWOT-Med campaign planned for 2022 in the western Mediterranean Sea under the future SWOT satellite crossover tracks.</p>

A Lagrangian strategy for in situ sampling the physical-biological coupling at fine scale : the PROTEVSMED-SWOT 2018 cruise

<p>    The term "fine scales" is generally used to refer to the ocean processes occuring on horizontal scales smaller than 10 km and<br>characterized by a short lifetime (days/weeks). Fine scales have been predominantly studied with numerical simulations and<br>satellite observations which have highlighted their significant role on biological processes. Indeed, their short time scale is the<br>same as a lot of important processes in phytoplankton dynamics. Model simulations have shown that fine scales such as fronts<br>and filaments strongly influence the distribution of phytoplankton species. Nowadays, the combination of in situ measurements,<br>satellite observations and model simulations is a necessity to better understand these mechanisms. However these processes<br>are particularly challenging to sample in situ because of their size and their ephemeral nature.</p><p>    The PROTEVSMED-SWOT cruise was performed in the Western Mediterranean Sea, in the southern region of the Balearic<br>Islands, onboard BHO Beautemps-Beaupré, between April 30<sup> th</sup> and May 14 <sup>th</sup> , 2018. In order to study the influence of fine<br>scales on the distribution of phytoplankton species, a satellite-based adaptive Lagrangian sampling strategy has been deployed<br>in order to i) identify a fine scale structure of interest, ii) sample it at high spatial resolution the phytoplankton community, and<br>iii) follow the evolution of this structure and the related distribution of phytoplankton. The SPASSO software package uses<br>satellite altimetry, SST and surface Chl a concentration data to generate and provide near-real time daily maps of the dynamical<br>and biogeochemical structures present in the area. The sampling strategy was defined in order to cross a frontal zone separating<br>different types of water. Multidisciplinary in situ sensors (hull-mounted ADCP, a Seasoar towed fish and an automated flow<br>cytometer installed on the seawater supply of the Thermosalinograph) were used to sample at high spatial resolution physical<br>and biological variables. A particular attention was put in adapting the temporal sampling in different water masses to the<br>biological time scales in order to reconstruct the phytoplankton diurnal cycle.</p><p>    Such a strategy was successful in sampling two different water masses separated by a narrow front and characterized by<br>different aboundances of several phytoplankton species and functional groups. Consequently, our results highlight the role of<br>the front on the physical and biological coupling confirming previous modelling and remote-sensing studies.</p><p>    The new generation of altimetric satellite, SWOT, will provide a 2D sea surface height at an unprecedented resolution and<br>it will be a unique opportunity to better observe fine scale structures in the global ocean. Our methodology paves the way to<br>future in situ experiments that are planned in 2022 during the SWOT fast-sampling phase, few months after its launch.</p>

Turbulence induces clustering and segregation of non-motile, buoyancy-regulating phytoplankton

Turbulence plays a major role in shaping marine community structure as it affects organism dispersal and guides fundamental ecological interactions. Below oceanographic mesoscale dynamics, turbulence also impinges on subtle physical–biological coupling at the single cell level, setting a sea of chemical gradients and determining microbial interactions with profound effects on scales much larger than the organisms themselves. It has been only recently that we have started to disentangle details of this coupling for swimming microorganisms. However, for non-motile species, which comprise some of the most abundant phytoplankton groups on Earth, a similar level of mechanistic understanding is still missing. Here, we explore by means of extensive numerical simulations the interplay between buoyancy regulation in non-motile phytoplankton and cellular responses to turbulent mechanical cues. Using a minimal mechano-response model, we show how such a mechanism would contribute to spatial heterogeneity and affect vertical fluxes and trigger community segregation.

SATOUMI APPROACH FOR REALIZING SUSTAINABLE COASTAL USE IN A RIASTYPE BAY: A CASE OF SHIZUGAWA BAY IN SANRIKU COAST HIT BY THE HUGE TSUNAMI ON 11 MARCH 2011

Rias-type bays are one of the most common coasts in Japan where aquacultures have been active due to sheltered geological shape with a deep bottom. The huge tsunami hit Sanriku Coast consisting of open rias-type bays near the epicenter facing Pacific Ocean on 11 March 2011. For recovering Sanriku Coast, it is important to include sustainability in its program. Satoumi is defined as the human use and management of coastal seas for high productivity while maintaining high biodiversity. Therefore, we proposed Satoumi approach to an open rias-type bay, Shizugawa Bay, in southern Sanriku Coast. We conducted scientific researches on mapping of coastal habitats and aquaculture facilities, hydrography, and material flows of nutrients, a minor element (Fe) and organic matters in the bay including those from the rivers and from the offshore waters. At the same time, Committee for Shizugawa Bay Management of Fishermen’s Cooperative of Miyagi Prefecture decided to decrease in aquaculture facilities for sustainable development of aquaculture. Based on these data, a physical-biological coupling model was used for calculating the number of aquaculture facilities that are suitable not only for yields but also for environments. These researches were established on strong collaborations among a fishermen’s’ cooperative, local governments and scientists. Results of this practice may help to realize sustainable coastal use of a rias-type bay.