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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 cm3. 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.
Highlights
Phytoplankton plays a major role in marine ecosystems as it is the main primary producer in the euphotic layer
Abundances and optical properties of phytoplankton communities were monitored in situ at the single cell level, at the hourly scale over nearly 2 months in the northwestern Mediterranean Sea using a totally autonomous and remotely controlled facility
The flow cytometer used in this study is specially designed for phytoplankton analysis and offers an appropriate means to undertake in near real time the in situ study of phytoplankton functional types and their dynamics
Summary
Phytoplankton plays a major role in marine ecosystems as it is the main primary producer in the euphotic layer. Phytoplankton communities are highly diverse and were shown to respond to environmental changes at the scale of the hour (Jacquet et al, 2002; Thyssen et al, 2008b; Lefort and Gasol, 2013) This fast response capacity depends principally on the growth rate of some pico and nanophytoplankton species. They display daily cyclic variations of abundance due to the combination of synchronized cell cycles and losses (grazing, viral lysis, sinking), though some very high increases in abundance have been observed after intense and sporadic environmental changes (Thyssen et al, 2008b; Dugenne et al, this issue). Depending on the sampling strategy, sampling at one time or another may completely change the interpretation of the phytoplankton community structural changes (Dubelaar et al, 2004), thereby leading www.frontiersin.org
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