The frequency of marine heatwaves (HWs) is projected to increase in the Mediterranean Sea in the next decades. It is still unclear how consecutive HWs affect coastal plankton communities and their functions within ecosystems. In this regard, an in situ mesocosm experiment was performed in a Mediterranean lagoon for 33 days. Three mesocosms were used as controls following the natural temperature of the lagoon, while in three others two HWs of +5°C compared to the controls were applied from experimental day (d) 1 to d5 (HW1) and from d11 to d15 (HW2), while their temperature was similar to the controls during non-HW periods. High-frequency (every 1 min) data of dissolved oxygen, chlorophyll-a (chl-a), temperature, salinity and light from sensors immersed in all mesocosms were used to calculate gross primary production (GPP), respiration (R) and phytoplankton growth (µ) and loss (L) rates. Nutrients, photochemical quantum yield and phytoplankton community structure from pigments were also sampled and analyzed. HW1 significantly increased by 7 to 38% GPP, R, chl-a, µ and L. HW2 shifted the system toward heterotrophy by only enhancing R. Normal phytoplankton succession from diatoms to haptophytes was altered by both HWs as cyanobacteria and chlorophytes were favored at the expense of haptophytes. These results indicate that HWs have important effects on Mediterranean plankton communities and that their resistance and resilience during and after a first HW attenuate the effects of a second HW on phytoplankton processes, but not on R which was strongly regulated by temperature.

Primary Presenter: Tanguy Soulié, CNRS (tanguy.soulie@gmail.com)

Authors:

Tanguy Soulié, MARBEC (MARine Biodiversity, Exploitation and Conservation), CNRS, Univ Montpellier, Ifremer, IRD  (tanguy.soulie@gmail.com)

Francesca Vidussi, MARBEC (MARine Biodiversity, Exploitation and Conservation), CNRS, Univ Montpellier, Ifremer, IRD  (francesca.vidussi@cnrs.fr)

Sébastien Mas, MEDIMEER (MEDIterranean platform for Marine Ecosystems Experimental Research), OSU OREME, CNRS, Univ Montpellier, IRD, INRAE)  (sebastien.mas1@umontpellier.fr)

Behzad Mostajir, MARBEC (MARine Biodiversity, Exploitation and Conservation), CNRS, Univ Montpellier, Ifremer, IRD  (behzad.mostajir@umontpellier.fr)