A Lagrangian strategy for in situ sampling the physical-biological coupling at fine scale : the PROTEVSMED-SWOT 2018 cruise
Authors: Tzortzis Roxane , Doglioli Andrea M. , Barrillon Stéphanie , Petrenko Anne A. , d'Ovidio Francesco , Izard Lloyd , Thyssen Melilotus , Pascual Ananda , Cyr Frédéric , Dumas Franck , Gregori Gérald 
Published: March 2020

&lt;p&gt;&amp;#160;&amp;#160;&amp;#160; The term &quot;fine scales&quot; is generally used to refer to the ocean processes occuring on horizontal scales smaller than 10 km and&lt;br&gt;characterized by a short lifetime (days/weeks). Fine scales have been predominantly studied with numerical simulations and&lt;br&gt;satellite observations which have highlighted their significant role on biological processes. Indeed, their short time scale is the&lt;br&gt;same as a lot of important processes in phytoplankton dynamics. Model simulations have shown that fine scales such as fronts&lt;br&gt;and filaments strongly influence the distribution of phytoplankton species. Nowadays, the combination of in situ measurements,&lt;br&gt;satellite observations and model simulations is a necessity to better understand these mechanisms. However these processes&lt;br&gt;are particularly challenging to sample in situ because of their size and their ephemeral nature.&lt;/p&gt;&lt;p&gt;&amp;#160;&amp;#160;&amp;#160; The PROTEVSMED-SWOT cruise was performed in the Western Mediterranean Sea, in the southern region of the Balearic&lt;br&gt;Islands, onboard BHO Beautemps-Beaupr&amp;#233;, between April 30&lt;sup&gt; th&lt;/sup&gt; and May 14 &lt;sup&gt;th&lt;/sup&gt; , 2018. In order to study the influence of fine&lt;br&gt;scales on the distribution of phytoplankton species, a satellite-based adaptive Lagrangian sampling strategy has been deployed&lt;br&gt;in order to i) identify a fine scale structure of interest, ii) sample it at high spatial resolution the phytoplankton community, and&lt;br&gt;iii) follow the evolution of this structure and the related distribution of phytoplankton. The SPASSO software package uses&lt;br&gt;satellite altimetry, SST and surface Chl a concentration data to generate and provide near-real time daily maps of the dynamical&lt;br&gt;and biogeochemical structures present in the area. The sampling strategy was defined in order to cross a frontal zone separating&lt;br&gt;different types of water. Multidisciplinary in situ sensors (hull-mounted ADCP, a Seasoar towed fish and an automated flow&lt;br&gt;cytometer installed on the seawater supply of the Thermosalinograph) were used to sample at high spatial resolution physical&lt;br&gt;and biological variables. A particular attention was put in adapting the temporal sampling in different water masses to the&lt;br&gt;biological time scales in order to reconstruct the phytoplankton diurnal cycle.&lt;/p&gt;&lt;p&gt;&amp;#160;&amp;#160;&amp;#160; Such a strategy was successful in sampling two different water masses separated by a narrow front and characterized by&lt;br&gt;different aboundances of several phytoplankton species and functional groups. Consequently, our results highlight the role of&lt;br&gt;the front on the physical and biological coupling confirming previous modelling and remote-sensing studies.&lt;/p&gt;&lt;p&gt;&amp;#160;&amp;#160;&amp;#160; The new generation of altimetric satellite, SWOT, will provide a 2D sea surface height at an unprecedented resolution and&lt;br&gt;it will be a unique opportunity to better observe fine scale structures in the global ocean. Our methodology paves the way to&lt;br&gt;future in situ experiments that are planned in 2022 during the SWOT fast-sampling phase, few months after its launch.&lt;/p&gt;