Response of the deep chlorophyll maximum to fluctuations in vertical mixing intensity

• Published in: Progress in Oceanography
• Main Authors: Liccardo, Antonella, Fierro, Annalisa, Iudicone, Daniele, Bouruet-Aubertot, Pascale, Dubroca, Laurent
• Other Authors: Stazione Zoologica Anton Dohrn (SZN), Échanges dans la couche de surface : des pôles aux tropiques (SURF), Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques (LOCEAN), Muséum national d'Histoire naturelle (MNHN)-Institut de Recherche pour le Développement (IRD)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Institut Pierre-Simon-Laplace (IPSL (FR_636)), École normale supérieure - Paris (ENS-PSL), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-École normale supérieure - Paris (ENS-PSL), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Muséum national d'Histoire naturelle (MNHN)-Institut de Recherche pour le Développement (IRD)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Institut Pierre-Simon-Laplace (IPSL (FR_636)), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), Centre d'Études Biologiques de Chizé (CEBC), Institut National de la Recherche Agronomique (INRA)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2013
• Subjects: [PHYS.PHYS.PHYS-GEO-PH]Physics [physics]/Physics [physics]/Geophysics [physics.geo-ph]; [SDU.STU.GP]Sciences of the Universe [physics]/Earth Sciences/Geophysics [physics.geo-ph]; [SDE.MCG]Environmental Sciences/Global Changes; North East Atlantic
• Online Access: https://hal.science/hal-00815124; https://doi.org/10.1016/J.POCEAN.2012.09.004

International audience Extreme atmospheric events, such as storms, can temporarily increase the vertical diffusivity in the upper ocean by several orders of magnitude, causing variability in the deep chlorophyll maximum (DCM). In this study, we investigate the impact of intermittent vertical mixing on the deep chlorophyll maximum. For this purpose, we analyze a one-dimensional phytoplankton-nutrient model exposed to short-term fluctuations in vertical diffusivity generated by internal waves. As a first step, we use singular perturbations to analyze the impact of a single burst of turbulence on the DCM characteristics. The simulations show that the DCM exhibits a long transient response, even after a single short perturbation. In particular, a single burst of turbulence (i) spreads out the DCM, thereby reducing the local phytoplankton maximum, and (ii) gives rise to a diffusion-driven enhancement of nutrient availability, producing a temporary increase in the phytoplankton growth rates at and above the DCM. Subsequently, we introduce a realistic annual sequence of fluctuations in vertical diffusivity in our model, and we also incorporate the seasonal cycle of light conditions. Fluctuations in vertical diffusivity were derived from high-resolution measurements from the POMME experiment in the north-east Atlantic Ocean, where the contribution of internal waves to the variation in vertical mixing has been characterized for more than one year. The annual light cycle, in combination with intermittent vertical mixing, leads to the superposition of a series of complex dynamical responses. At seasonal and shorter time scales, the vertically integrated productivity is temporally augmented by periods of medium/high diffusivity. Seasonal variation in light availability modifies the DCM response to mixing variability.