Review of: ‘Dynamics of the Deep Chlorophyll Maximum in the Black Sea as depicted by BGC-Argo floats’ by Ricour et al.

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&#160; challenge, especially in the peculiar environment that is the Black Sea. 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 Chla data (2014&#8211;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 the Chla profiles are consistent with HPLC data. In the Black Sea, the DCM begins to form in March, throughout the basin, at a density level set by the previous winter mixed layer. During a first phase (April-May), the DCM remains attached to this particular layer. The spatial homogeneity of this feature suggests a hysteresis mechanism, i.e., that the DCM structure locally influences environmental conditions rather than adapting instantaneously to external factors.In a second phase (July-September), the DCM migrates upward, where there is higher irradiance, which suggests the interplay of biotic factors. Overall, the DCM concentrates around 45 to 65% of the total chlorophyll content within a 10 m layer centered around a depth of 30 to 40 m, which stresses the importance of considering DCM dynamics when evaluating phytoplankton productivity at basin scale.

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

10.5194/bg-2020-295 ◽

2020 ◽

Author(s):

Florian Ricour ◽

Arthur Capet ◽

Fabrizio D'Ortenzio ◽

Bruno Delille ◽

Marilaure Grégoire

Keyword(s):

Black Sea ◽

High Performance ◽

Global Ocean ◽

The Black Sea ◽

Fluorescence Signal ◽

Deep Chlorophyll Maximum ◽

Lateral Loads ◽

Spatial Homogeneity ◽

Argo Floats ◽

Chlorophyll Maximum

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.

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Dynamics of the deep chlorophyll maximum in the Black Sea as depicted by BGC-Argo floats

Biogeosciences ◽

10.5194/bg-18-755-2021 ◽

2021 ◽

Vol 18 (2) ◽

pp. 755-774

Author(s):

Florian Ricour ◽

Arthur Capet ◽

Fabrizio D'Ortenzio ◽

Bruno Delille ◽

Marilaure Grégoire

Keyword(s):

Black Sea ◽

Global Ocean ◽

The Black Sea ◽

Second Phase ◽

Deep Chlorophyll Maximum ◽

Phytoplankton Productivity ◽

Chl A ◽

Argo Floats ◽

Chlorophyll Maximum ◽

Basin Scale

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 environment that is the Black Sea. The retrieval of chlorophyll a (chl a) 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 coloured dissolved organic matter (CDOM) which contributes to the fluorescence signal and produces an apparent increase in the chl a 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 chl a 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 the chl a profiles are consistent with HPLC data. In the Black Sea, the DCM begins to form in March, throughout the basin, at a density level set by the previous winter mixed layer. During a first phase (April–May), the DCM remains attached to this particular layer. The spatial homogeneity of this feature suggests a hysteresis mechanism, i.e. that the DCM structure locally influences environmental conditions rather than adapting instantaneously to external factors. In a second phase (July–September), the DCM migrates upward, where there is higher irradiance, which suggests the interplay of biotic factors. Overall, the DCM concentrates around 45 % to 65 % of the total chlorophyll content within a 10 m layer centred around a depth of 30 to 40 m, which stresses the importance of considering DCM dynamics when evaluating phytoplankton productivity at basin scale.

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Phytoplankton light absorption in the deep chlorophyll maximum layer of the Black Sea

European Journal of Remote Sensing ◽

10.1080/22797254.2018.1533389 ◽

2018 ◽

Vol 52 (sup1) ◽

pp. 123-136 ◽

Cited By ~ 4

Author(s):

T. Churilova ◽

V. Suslin ◽

H.M. Sosik ◽

T. Efimova ◽

N. Moiseeva ◽

...

Keyword(s):

Black Sea ◽

Light Absorption ◽

The Black Sea ◽

Deep Chlorophyll Maximum ◽

Chlorophyll Maximum

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Warmer winters causes an increase of chlorophyll-a concentration in deeper layers: the opposite role of convection and self-shading on the example of the Black Sea

10.5194/bg-2020-366 ◽

2020 ◽

Author(s):

Elena A. Kubryakova ◽

Arseny A. Kubryakov

Keyword(s):

Chlorophyll A ◽

Black Sea ◽

Light Attenuation ◽

The Self ◽

Lower Amount ◽

The Black Sea ◽

Chlorophyll A Concentration ◽

Warm Winter ◽

Chlorophyll Maximum ◽

Subsurface Chlorophyll Maximum

Abstract. Winter vertical entrainment of deep waters determines not only the amount of nutrients in the upper layers, but also the light conditions in it, through the self-shading mechanism. In this paper, we use Bio-Argo data to demonstrate significant differences in the vertical distribution of chlorophyll-a concentration (Chl) in the Black Sea between a year with cold winter (2017) and a year with warm winter (2016). Stronger vertical entrainment of nutrient-rich waters from deeper isopycnal layers in cold 2017 caused an increase of Chl in winter up to 0.6–0.7 mg/m3 compared to a warm winter of 2016, when Chl was only 0.4–0.5 mg/m3. Further, during almost the whole year from February to October Chl in the upper 0–40 m layer of cold 2017 year was on 0.1–0.2 mg/m3 higher than in 2016. This rise of Chl in 2017 led to an increase in light attenuation due to the self-shading effect. In contrast, in warm 2016 with a lower amount of nutrients light attenuation decreased and the irradiance reached deeper isopycnals layers with a higher amount of nutrients. As a result, in warm 2016 the subsurface chlorophyll maximum deepens and the values of Chl in 40–60 m layers were significantly higher than in 2017. The maximum positive difference in this layer (0.5 mg/m3) was observed during a summer seasonal peak of irradiance due to the largest increase of light attenuation in the summer of 2017. As a result, the column-averaged yearly values of Chl in warm 2016 and cold 2017 were comparable. However, in the year with intense winter mixing upper layers are more productive, while in the year with low winter vertical mixing, subsurface chlorophyll maximum widens and reaches deeper layers. These results show that the observed long-term warming may lead to the continuous deepening of the subsurface chlorophyll maximum in the ocean.

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