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

Abstract

<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>