To better understand the potential responses of phytoplankton blooms to global warming, an in situ mesocosm experiment was carried out in the coastal Mediterranean (Thau Lagoon, south France) in April 2018. Four treatments were applied: 1) natural planktonic community with ambient water temperature (C); 2) natural planktonic community at +3 °C elevated temperature (T); 3) exclusion of larger zooplankton (> 200 µm) with ambient water temperature (MicroZ); and 4) exclusion of larger zooplankton at +3 °C elevated temperature (TMicroZ). Warming strongly depressed the phytoplankton bloom amplitude as the chlorophyll a concentrations was two times lower in the T treatment, most likely due to the increase of the top-down control by zooplankton. This was supported when the larger zooplankton fraction was removed (MicroZ vs TMicroZ), as in this weakened top-down control conditions, the phytoplankton biomass increased under warming. Warming also modified phytoplankton composition by enhancing smaller green flagellates and dinoflagellates at the expense of diatoms and prymnesiophytes. Finally, warming modified phytoplankton successions, promoting an early bloom of picophytoplankton, mainly small green flagellates, and a late bloom of diatoms. Our findings suggest potentially weaker spring blooms, dominated by smaller phytoplankton and dinoflagellates in the future in coastal waters and point at the role of the top-down pressure intensification on its changes, with potential consequences for coastal food web dynamics and trophic transfer efficiencies under future ocean conditions.

Primary Presenter: Thomas Trombetta, University of Amsterdam (t.trombetta@uva.nl)

Authors:

Thomas Trombetta, MARBEC (Marine Biodiversity, Exploitation and Conservation)  (t.trombetta@uva.nl)

Behzad Mostajir, MARBEC (Marine Biodiversity, Exploitation and Conservation)  (behzad.mostajir@cnrs.fr)

Justine Courboulès, MARBEC (Marine Biodiversity, Exploitation and Conservation)  (justine.courboules@hotmail.fr)

Maria Protopapa, HCMR (Hellenic Centre for Marine Research)  (mariaprot@hcmr.gr)

Katharina Bading, NTNU (Norwegian University of Science and Technology)  (katharina.bading@ntnu.no)

Sébastien Mas, MEDIMEER (Mediterranean Platform for Marine Ecosystems Experimental Research)  (sebastien.mas1@umontpellier.fr)

Nicole Aberle, NTNU (Norwegian University of Science and Technology)  (nicole.aberle-malzahn@ntnu.no)

Francesca Vidussi, MARBEC (Marine Biodiversity, Exploitation and Conservation)  (francesca.vidussi@cnrs.fr)