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

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/m in February-May of 2017. In warm 2016 with weaker winter convection maximum of Chl during 10 winter-spring in this layer was only about 0.8-0.9 mg/m. 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/m higher than in 2017. The maximum difference (0.6 mg/m) was observed during a summer 15 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.

https://doi.org/10.5194/bg-2020-210 Preprint. Discussion started: 16 June 2020 c Author(s) 2020. CC BY 4.0 License. reduces the transparency of its water. As a result, the diffuse attenuation coefficient in the sea is large (Organelli et al., 2017;Churilova et al., 2019) and the euphotic layer is relatively shallow, about 50 m (Vedernikov & Demidov, 1993). Due to the strong haline stratification the mixed layer depth also generally does not exceed 40-50 m in the central part of the sea and 50-70 m on its periphery and in anticyclonic eddies (Kubryakov, Belokopytov, et al., 2019). Due to these features, the upper border of the nitroclyne is relatively shallow. It is situated approximately at 40-50 m depth and tightly coupled to isopycnals (isohalines) positons 85 (Konovalov et al., 2005).
The results of this study are based on a comparative analysis of the bio-optical characteristics of the Black Sea in 2016 and 2017, which were characterized by different winter thermal conditions. In 2016, the winter was significantly warmer than in cold 2017 https://doi.org/10.5194/bg-2020-210 Preprint. Discussion started: 16 June 2020 c Author(s) 2020. CC BY 4.0 License. (Fig. 1a, b). The temperature in the upper 70-meter layer from January to March 2016 did not fall below 8.5°C (Fig. 2a), and its column-averaged value varied from 8.5 to 9.0°C (Fig. 3, solid red line). Due to such warm conditions in winter of 2016 the cold 90 intermediate layera characteristic feature of the thermohaline structure of the Black Sea with a temperature of less than 8°Cwas absent this year (Fig. 2a). In 2017, the temperature in the upper 50-meter layer was on 1℃ lower (Fig. 2b, Fig. 3     In Fig. 4a, b the black line marks the border of the photic zone is related to the isolume Ed=3 μmol of photons m -2 s -1 . It can be seen that in 2017, this border was higher by 10 m in April, and by 20 m in June compared to 2016. This indicates that in 2017, the growth of phytoplankton in the deep layer was limited by the low light conditions. The reason for this was an increase in the light 10-30 m layer, the values of PAR difference during the early spring bloom in February-April and autumn bloom in October-November reached 20 µmol photons m -2 s -1 (Fig. 6c). In summer, these differences were greatest and exceeded 200 µmol photons 150 m -2 s -1 in the 0-25 m layer and were more than 10 µmol photons m -2 s -1 in the 40-meter layer. Taking the value of compensational irradiance in the Black Sea as 3 µmol photons m -2 s -1 such PAR increase caused a significant widening of the euphotic zone. convective entrainment of nutrients in winter was compensated by its increase in the summer period due to the lack of self-shading effect and the development of a deep Chl maximum directly in the nitroclyne. Because of this, the yearly-averaged integral Chl in the euphotic layer in 2016 and 2017 became comparable (Fig. 3).

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The reasons for the variability of the characteristics of the deep chlorophyll maximum is one of the important and actively investigated oceanographic tasks (Cullen, 2015;Leach et al., 2018;Barbieux et al., 2019). Variability of its position and strength are related in different studies to the vertical distribution of nutrients (Hartman et al., 2014;Barbieux et al., 2019), optical characteristics of water and light availability (Morel, 1991;Mignot et al., 2014;Leach et al., 2018), density stratification (Navarro & Ruiz, 2013). Our study approves that all these factors are important and provide a link between their impact on the vertical 180 distribution of Chl on the example of the Black Sea based on continuous Bio-Argo measurements (see scheme in Fig. 7). Nitroclyne in the highly-stratified waters of the Black Sea is closely related to the isopycnals position (Konovalov et al., 2005). The increase of density of the upper mixed layer in winter leads to the convective mixing reaching the isopycnals with the same density. Deep isopycnal layer with a high amount of nutrients mixes with surface waters and defines the concentration of nutrients in the winter mixed layer (0-40 m in the Black Sea) (Kubryakova et al., 2018;Mikaelyan et al., 2018;Silkin et al., 2019). Further thermal 185 stratification stabilizes the water column but keeps the same concentration of nutrients. Rise of the irradiance causes the following spring growth of phytoplankton, which utilizes these nutrients. Part of them regenerates and another part sinks out to the nitroclyne.
The regenerated nutrients further are used by the summer population of phytoplankton. For example, in the Black Sea, the strong coccolithophore bloom emerges after strong spring bloom of diatoms and their magnitude depends on the winter sea surface temperature (Silkin et al., 2014;Mikaelyan et al., 2015). Thus, the phytoplankton concentration in the upper layer through the year 190 will depend on the density of the preceding winter mixed layer.
https://doi.org/10.5194/bg-2020-210 Preprint. Discussion started: 16 June 2020 c Author(s) 2020. CC BY 4.0 License. Figure 7: a scheme of the impact of convection and self-shading on the vertical distribution of chlorophyll-a. (a) In a year with cold winter, the larger amount of nutrients (grey color) is convectively entrained in the upper layer, which increases the growth of phytoplankton in the upper layer and causes self-shading of deeper layer. Therefore, DCM moves to the upper layer. (b) In years with warm winter convective nutrient fluxes 195 are low, the amount of phytoplankton and light attenuation decreases. In the summer period with the increase of PAR, light penetrates the upper layer of nitroclyne and causes intense and deep summer subsurface bloom. Therefore, the total amount of nutrients used by the phytoplankton in both years is comparable.