Physical mechanisms for biological carbon uptake during the onset of the spring phytoplankton bloom in the northwestern Mediterranean Sea (BOUSSOLE site)

Merlivat, Liliane, Hemming, Michael, Boutin, Jacqueline, Antoine, David, Vellucci, Vincenzo, Golbol, Melek, Lee, Gareth A. ORCID: and Beaumont, Laurence (2022) Physical mechanisms for biological carbon uptake during the onset of the spring phytoplankton bloom in the northwestern Mediterranean Sea (BOUSSOLE site). Biogeosciences, 19 (16). pp. 3911-3920. ISSN 1726-4189

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Several trigger mechanisms have been proposed for the onset of the phytoplankton spring bloom. Among these is that phytoplankton cells begin to bloom when they experience higher average light levels in shallower mixed layers, a result of the surface net heat fluxes becoming positive and wind strength decreasing. We study the impact of these two forcings in the northwestern Mediterranean Sea. We take advantage of hourly measurements of oceanic and atmospheric parameters collected at two neighbouring moorings during the months of March and April in the years 2016 to 2019, combined with glider data in 2016. We identify the onset of the surface phytoplankton growth as concomitant with the start of significant biological activity detected by a sudden decrease in dissolved inorganic carbon derived from measurements in the upper 10 m of the water column. A rapid reduction in wind stress following high-wind events is observed at the same time. A resulting shallow mixing layer favours carbon uptake by phytoplankton lasting a few days. Simultaneously, the air–sea net heat flux switches from negative to positive, linked to changes in the latent air–sea heat flux, which is proportional to the wind speed. This results in an increased thermal stratification of the ocean's surface layers. In 2016, glider data show that the mixing layer is significantly shallower than the mixed layer at the onset of the surface phytoplankton bloom. We conclude that decreases in the mixing- and mixed-layer depths lead to the onset of the phytoplankton growth due to the relaxation of wind speed following storms. We estimate net daily community production in the mixing layer over periods of 3 d between 2016 and 2019 as between 38 and 191 mmol C m−2. These results have important implications, as biological processes play a major role in the seasonal evolution of surface pCO2 and thereby the rate of reduction in atmospheric CO2 by exchange at the air–sea interface.

Item Type: Article
Additional Information: Financial support: The BOUSSOLE time series project is funded by the European Space Agency (ESA; ESRIN contract nos. 4000102992/11/I-NB and 4000111801/14/I-NB) and the Centre National d’Etudes Spatiales (CNES, France). It is also supported by the Centre National de la Recherche Scientifique (CNRS), the Institut National des Sciences de l'Univers (INSU), Sorbonne Université and the Institut de la Mer de Villefranche (IMEV). The measurements of the carbonate system were specifically funded through the BIOCAREX project (Agence Nationale de la Recherche, ANR, Paris). Michael Hemming was a PhD student at the University of East Anglia and Sorbonne University under the supervision of Jan Kaiser, Karen J. Heywood, Dorothee Bakker and Jacqueline Boutin and was funded by the Defence Science and Technology Laboratory (contract no. DSTLX1000092277) in cooperation with the Direction Générale de l'Armement (DGA).
Faculty \ School: Faculty of Science > School of Environmental Sciences
Faculty of Science
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Depositing User: LivePure Connector
Date Deposited: 03 Sep 2022 00:22
Last Modified: 15 Sep 2022 19:06
DOI: 10.5194/bg-19-3911-2022

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