title: Onset of the spring bloom in the northwestern Mediterranean Sea: influence of environmental pulse events on the in situ hourly-scale dynamics of the phytoplankton community structure.
abstract: Most of phytoplankton influence is barely understood at the sub meso scale and daily scale because of the lack of means to simultaneously assess phytoplankton functionality, dynamics and community structure. For a few years now, it has been possible to address this objective with an automated in situ high frequency sampling strategy. In order to study the influence of environmental short-term events (nutrients, wind speed, precipitation, solar radiation, temperature, and salinity) on the onset of the phytoplankton bloom in the oligotrophic Bay of Villefranche-sur-Mer (NW Mediterranean Sea), a fully remotely controlled automated flow cytometer (CytoSense) was deployed on a solar-powered platform (EOL buoy, CNRS-Mobilis). The CytoSense carried out single-cell analyses on particles (1-800 μm in width, up to several mm in length), recording optical pulse shapes when analyzing several cm(3). Samples were taken every 2 h in the surface waters during 2 months. Up to 6 phytoplankton clusters were resolved based on their optical properties (PicoFLO, Picoeukaryotes, Nanophytoplankton, Microphytoplankton, HighSWS, HighFLO). Three main abundance pulses involving the 6 phytoplankton groups monitored indicated that the spring bloom not only depends on light and water column stability, but also on short-term events such as wind events and precipitation followed by nutrient pulses. Wind and precipitation were also determinant in the collapse of the clusters' abundances. These events occurred within a couple of days, and phytoplankton abundance reacted within days. The third abundance pulse could be considered as the spring bloom commonly observed in the area. The high frequency data-set made it possible to study the phytoplankton cell cycle based on daily cycles of forward scatter and abundance. The combination of daily cell cycle, abundance trends and environmental pulses will open the way to the study of phytoplankton short-term reactivity to environmental conditions.
authors.affiliation: CNRS/INSU, IRD, Mediterranean Institute of Oceanography, Aix Marseille Université Marseille, France.
authors.forename: Melilotus
authors.initials: M
authors.lastname: Thyssen
authors.affiliation: CNRS/INSU, IRD, Mediterranean Institute of Oceanography, Aix Marseille Université Marseille, France.
authors.forename: Gerald J
authors.initials: GJ
authors.lastname: Grégori
authors.affiliation: Sorbonne Universités, UPMC Univ. Paris 06, UMR 7093, LOV, Observatoire Océanologique Villefranche-sur-Mer, France ; CNRS, UMR 7093, LOV, Observatoire Océanologique Villefranche-sur-Mer, France.
authors.forename: Jean-Michel
authors.initials: JM
authors.lastname: Grisoni
authors.affiliation: Sorbonne Universités, UPMC Univ. Paris 06, UMR 7093, LOV, Observatoire Océanologique Villefranche-sur-Mer, France ; CNRS, UMR 7093, LOV, Observatoire Océanologique Villefranche-sur-Mer, France.
authors.forename: Maria Luiza
authors.initials: ML
authors.lastname: Pedrotti
authors.affiliation: Sorbonne Universités, UPMC Univ. Paris 06, UMR 7093, LOV, Observatoire Océanologique Villefranche-sur-Mer, France ; CNRS, UMR 7093, LOV, Observatoire Océanologique Villefranche-sur-Mer, France.
authors.forename: Laure
authors.initials: L
authors.lastname: Mousseau
authors.affiliation: Laboratoire d'Océanologie et Géosciences, Univ. du Littoral Côte d'Opale, CNRS, UMR8187 Wimereux, France.
authors.forename: Luis F
authors.initials: LF
authors.lastname: Artigas
authors.affiliation: Sorbonne Universités, UPMC Univ. Paris 06, UMR 7093, LOV, Observatoire Océanologique Villefranche-sur-Mer, France ; CNRS, UMR 7093, LOV, Observatoire Océanologique Villefranche-sur-Mer, France.
authors.forename: Sophie
authors.initials: S
authors.lastname: Marro
authors.affiliation: CNRS/INSU, IRD, Mediterranean Institute of Oceanography, Aix Marseille Université Marseille, France.
authors.forename: Nicole
authors.initials: N
authors.lastname: Garcia
authors.affiliation: Sorbonne Universités, UPMC Univ. Paris 06, UMR 7093, LOV, Observatoire Océanologique Villefranche-sur-Mer, France ; CNRS, UMR 7093, LOV, Observatoire Océanologique Villefranche-sur-Mer, France.
authors.forename: Ornella
authors.initials: O
authors.lastname: Passafiume
authors.affiliation: CNRS/INSU, IRD, Mediterranean Institute of Oceanography, Aix Marseille Université Marseille, France.
authors.forename: Michel J
authors.initials: MJ
authors.lastname: Denis
keywords: Coastal Mediterranean Sea; cell cycle; diel variations; in situ; phytoplankton; remotely controlled flow cytometry; spring bloom