Deep Chlorophyll Maxima in the global ocean: occurrences, drivers and characteristics

Type Article
Acceptance Date 2021 IN PRESS
Language English
Author(s) Cornec M.ORCID1, Claustre H.1, Mignot A.2, Guidi L.ORCID1, Lacour L.ORCID3, 4, Poteau A.ORCID1, D'Ortenzio F.1, Gentili B.1, Schmechtig C.ORCID1
Affiliation(s) 1 : CNRS & Sorbonne Université Laboratoire d'Océanographie de Villefranche LOV Villefranche‐sur‐Mer ,France
2 : Mercator Océan International Ramonville‐Saint‐Agne, France
3 : Takuvik Joint International Laboratory, Laval University (Canada) - CNRS (France), Département de biologie et Québec-Océan, Université Laval, Québec, Canada
4 : Takuvik Joint International Laboratory, Laval University (Canada) - CNRS (France), Département de biologie et Québec-Océan, Université Laval, Québec, Canada
Source Global Biogeochemical Cycles (0886-6236) (American Geophysical Union (AGU)) In Press
DOI 10.1029/2020GB006759
Keyword(s) BGC‐Argo floats, open ocean, deep chlorophyll maximum, particle backscattering, nitracline, irradiance, stratification
Abstract

Stratified oceanic systems are characterized by the presence of a so‐called Deep Chlorophyll a Maximum (DCM) not detectable by ocean color satellites. A DCM can either be a phytoplankton (carbon) biomass maximum (Deep Biomass Maximum, DBM), or the consequence of photoacclimation processes (Deep photoAcclimation Maximum, DAM) resulting in the increase of chlorophyll a per phytoplankton carbon. Even though these DCM (further qualified as either DBMs or DAMs) have long been studied, no global‐scale assessment has yet been undertaken and large knowledge gaps still remain in relation to the environmental drivers responsible for their formation and maintenance. In order to investigate their spatial and temporal variability in the open ocean, we use a global dataset acquired by more than 500 Biogeochemical‐Argo floats given that DCMs can be detected from the comparative vertical distribution of chlorophyll a concentrations and particulate backscattering coefficients. Our findings show that the seasonal dynamics of the DCMs are clearly region‐dependent. High‐latitude environments are characterized by a low occurrence of intense DBMs, restricted to summer. Meanwhile, oligotrophic regions host permanent DAMs, occasionally replaced by DBMs in summer, while subequatorial waters are characterized by permanent DBMs benefiting from favorable conditions in terms of both light and nutrients. Overall, the appearance and depth of DCMs are primarily driven by light attenuation in the upper layer. Our present assessment of DCM occurrence and of environmental conditions prevailing in their development lay the basis for a better understanding and quantification of their role in carbon budgets (primary production and export).

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