Impact of moderately energetic fine-scale dynamics on the phytoplankton community structure in the western Mediterranean Sea

Abstract

Abstract. 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 PROTEVSMED-SWOT 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 near-real-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 fine-scale 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 may have global significance when extrapolated.

The contrasted phytoplankton dynamics across a frontal system in the southwestern Mediterranean Sea

“…The PROTEVSMED-SWOT cruise followed an adaptive and Lagrangian strategy, measuring at high resolution several physical and biological variables with in situ sensors installed both on board (Acoustic Doppler Current Profiler (ADCP), thermosalinograph (TSG), flow cytometer installed on the seawater supply of the TSG) and on a towed vehicle (Seasoar). Tzortzis et al (2021) provides the detailed description of all the in situ measurements performed during the cruise. In the present study, as we focus on the biological aspects in two contrasted water masses separated by a front, the following description is focused on the sampling strategy as well as the measurements performed by the TSG and the flow cytometer.…”

“…1b). This front has also been detected using the in situ horizontal velocities, temperature and salinity, as described by Tzortzis et al (2021). Once the front localized, repeated transects were performed across the two water masses separated by the front adapting the temporal sampling to the biological time scales in order to reconstruct the phytoplankton 24-hours diurnal cycle for each water mass (Fig.…”

“…The sea surface temperature and salinity have been measured by a thermosalinograph (TSG) SeaBird Electronics (SBE) 45, which is an underway sensor able to continuously pump seawater at 3 m (more details about the TSG sensor in Tzortzis et al (2021)). These two physical variables have been converted into conservative temperature (Θ) and absolute salinity (S A ) using the TEOS-10 standards of McDougall et al (2012).…”

“…This approach consists in using remote sensing products (such as sea surface salinity (SSS), temperature (SST), chlorophyll a concentration ([chla]) and altimetry) to determine the position of oceanic features and guide in quasi-real time the route of the ship and the sampling strategy across these structures of interest. Thanks to this strategy it was possible to identify a moderately energetic front separating two distinct water masses and to highlight its influence on the distribution of the phytoplankton abundances (Tzortzis et al, 2021). As these two water masses had a signature in temperature and salinity, Tzortzis et al (2021) identified two types of Atlantic Water (AW) at different stage of mixing, separated by the front.…”

“…Thanks to this strategy it was possible to identify a moderately energetic front separating two distinct water masses and to highlight its influence on the distribution of the phytoplankton abundances (Tzortzis et al, 2021). As these two water masses had a signature in temperature and salinity, Tzortzis et al (2021) identified two types of Atlantic Water (AW) at different stage of mixing, separated by the front. The first AW located South of the front, characterized by absolute salinity (S A ) between 37 g kg −1 and 37.5 g kg −1 is named "younger AW" and corresponds to AW more recently entered into the Mediterranean Sea.…”