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https://doi.org/10.5194/bg-2020-210
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-210
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  16 Jun 2020

16 Jun 2020

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This discussion paper is a preprint. It has been under review for the journal Biogeosciences (BG). The manuscript was not accepted for further review after discussion.

Warmer winter causes deepening and intensification of summer subsurface bloom in the Black Sea: the role of convection and self-shading mechanism

Elena A. Kubryakova and Arseny A. Kubryakov Elena A. Kubryakova and Arseny A. Kubryakov
  • Marine Hydrophysical Institute, Russian Academy of Sciences, Sevastopol, Russian Federation

Abstract. Large differences in the vertical distribution of chlorophyll-a concentration (Chl) in a year with cold and warm winter are observed in the Black Sea on the base of Bio-Argo data. Stronger winter nutrient flux from deeper isopycnal layer in cold 2017 caused an increase of Chl in the upper 40-meter layer observed throughout the whole year – from February to October, with a maximum exceeding 1.3 mg/m3 in February-May of 2017. In warm 2016 with weaker winter convection maximum of Chl during winter-spring in this layer was only about 0.8–0.9 mg/m3. However, the increase of Chl in 2017 led to strong light attenuation in the upper layer and a decrease of euphotic layer depth due to the self-shading mechanism. In 2016 with weaker bloom irradiance penetrated to a 40–70 m layer, below the maximum winter mixed layer depth (40–50 m) and reached the upper layer of nitroclyne, which was not affected by winter mixing. As a result, in warm 2016 the subsurface chlorophyll maximum deepens and Chl in deeper layers was on 0.2–0.6 mg/m3 higher than in 2017. The maximum difference (0.6 mg/m3) was observed during a summer seasonal peak of irradiance due to the largest increase of light attenuation in 2017. As a result, the column-averaged yearly values of Chl in warm 2016 and cold 2017 were comparable. These results demonstrate that the effect of self-shading largely compensates the role of winter convective entrainment of nutrients and causes the deepening of Chl subsurface maximum in warmer years.

Elena A. Kubryakova and Arseny A. Kubryakov

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Interactive discussion

Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement

Elena A. Kubryakova and Arseny A. Kubryakov

Elena A. Kubryakova and Arseny A. Kubryakov

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Latest update: 23 Nov 2020
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Short summary
In a year with cold winter, a larger amount of nutrients is convectively entrained in the upper layer, which increases the growth of phytoplankton in the upper layer and causes self-shading of deeper layers. In years with warm winter convective nutrient fluxes are low, the amount of phytoplankton and light attenuation decreases and light penetrates to the layer of nitrate maximum which causes intense summer deep bloom. The yearly-averaged concentration of chlorophyll in both years is comparable.
In a year with cold winter, a larger amount of nutrients is convectively entrained in the upper...
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