Vertical distribution of chlorophyll a in the western part of the Kara Sea in autumn 2020

The aim of the work was to study the distribution of chlorophyll a (Chl-a) in the deepest part of the western Kara Sea — the Novaya Zemlya Trough and in St. Anna Trough. Materials : field studies of Chl-a, macronutrients, dissolved oxygen and physical parameters were held in September 2020 during the 81st cruise of R/V «Akademik Mstislav Keldysh». The results showed that Chl-a concentration within the upper 100 m layer ranged from 0.01 to 1.37 mg/m 3 (median value of 0.23 mg/m3). The maximum concentrations of Chl-a in the Novaya Zemlya Trough (0.39–1.18 mg/m 3 ) were observed at depths of 10–35 m, in the St. Anna Trough (0.25–1.37 mg/m3) — in the upper 15-meter layer. In the vertical distribution of Chl-a the subsurface chlorophyll maximum (SCM) was noted at depths of 10–42 m. In the southern part of the St. Anna Trough, the maximum concentrations of Chl-a were observed in the surface horizon. SCM existence depended on the runoff water distribution: well-pronounced SCM was observed at stations with high surface salinity which were characterized by a deeper euphotic depth, and a low concentration of Chl-a at the surface. In regions affected by river runoff, which had low salinity and high nutrient content in the upper layer as well as shallow depth of the photic layer, the maximum concentrations of Chl-a were observed at the surface. The share of pheophytin varied from 18 to 73% (median value of 41%) with minimum values in the layer of chlorophyll maximum. Integrated Chl-a concentration ranged from 5.75 to 24.54 mg/m 2 (median value of 9.51 mg/m 2 ), the highest values were observed in regions with deeper euphotic layer and pronounced SCM. Practical value : the data on the vertical distribution of Chl-a concentration allow to expand understanding of the conditions of organic matter formation and productivity of the Kara Sea.