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https://doi.org/10.5194/bg-2020-295
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-295
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  02 Sep 2020

02 Sep 2020

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This preprint is currently under review for the journal BG.

Dynamics of the Deep Chlorophyll Maximum in the Black Sea as depicted by BGC-Argo floats

Florian Ricour1,2,, Arthur Capet1,, Fabrizio D'Ortenzio2, Bruno Delille1, and Marilaure Grégoire1 Florian Ricour et al.
  • 1Freshwater and OCeanic science Unit of reSearch (FOCUS), University of Liege, Belgium
  • 2Laboratoire d’Océanographie de Villefranche, Sorbonne Universités, Villefranche-sur-Mer, France
  • These authors contributed equally to this work.

Abstract. The Deep Chlorophyll Maximum (DCM) is a well known feature of the global ocean. However, its description and the study of its formation are a challenge, especially in the peculiar Black Sea environment. The retrieval of Chlorophyll a (Chla) from fluorescence (Fluo) profiles recorded by Biogeochemical-Argo (BGC-Argo) floats is not trivial in the Black Sea, due to the very high content of Colored Dissolved Organic Matter (CDOM) which contributes to the fluorescence signal and produces an apparent increase of the Chla concentration with depth.

Here we revised Fluo correction protocols for the Black Sea context using co-located in-situ High-Performance Liquid Chromatography (HPLC) and BGC-Argo measurements. The processed set of Argo Chla data (2014–2019) is then used to provide a systematic description of the seasonal DCM dynamics in the Black Sea, and to explore different hypotheses concerning the mechanisms underlying its development.

Our results show that the corrections applied to Chla profiles are consistent with HPLC data. In the Black Sea, the DCM is initiated in March, throughout the basin, at a pycnal level set by the previous winter mixed layer. The DCM then remains attached to this particular layer until the end of September. The spatial homogeneity of this feature suggests a self-sustaining DCM structure, locally influencing environmental conditions rather than adapting instantaneously to external factors.

In summer, the DCM concentrates around 50 to 65 % of the total chlorophyll content around a depth of 30 m, where light conditions ranged from 0.5 to 4.5 % of surface incoming irradiance.

In October, as the DCM structure is gradually eroded, a longitudinal gradient appears in the DCM pycnal depth, indicating that autumnal mixing induces a relocation of the DCM which is this time driven by regional factors, such as nutrients lateral loads and turbidity.

Florian Ricour et al.

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Release of data and scripts used in the frame of the paper entitled "Dynamics of the Deep Chlorophyll Maximum in the Black Sea as depicted by BGC-Argo floats" Florian Ricour and Arthur Capet https://doi.org/10.5281/zenodo.3966002

Florian Ricour et al.

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Short summary
This paper addresses the phenology of the Deep Chlorophyll Maximum (DCM) in the Black Sea (BS). Our results show that the DCM is initiated in March at a density level corresponding to the depth of the maximum mixing of the water column in winter. It remains attached to this layer until the end of September before it starts to be eroded in October. The DCM seems to be a self-sustaining structure rather than being shaped by external factors and is important for further ecosystem studies in the BS.
This paper addresses the phenology of the Deep Chlorophyll Maximum (DCM) in the Black Sea (BS)....
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