Vertical Variability of Primary Production and Chlorophyll a in the Kara Sea in the Middle of Summer: Contribution of Subsurface Maxima to the Water Column Values

Abstract—Spatial and vertical variability of primary production (PP) and Chl a were studied in the framework of the 76th cruise of R/V Akademik Mstislav Keldysh to the Kara Sea from July 7 to August 1, 2019. In the middle of the summer the subsurface chlorophyll maximum (SCM) was well pronounced in the regions outside of river run-off influence with surface salinity >25. The water column PP (IPP) values depended on the vertical PP and Chl a distribution and were meaningfully (4.5 times) higher in the regions with well pronounced SCM. The contribution of SCM to the IPP in the middle of the summer was 55% on average, and approximately four times higher than in the autumn according to previous studies. A statistically significant link between the contribution of SCM to IPP and the contribution of SCM to the photosynthetic layer integrated Chl a was established. The absolute values of PP formed in the SCM did not depend on Chl a concentration and were affected by assimilation activity of phytoplankton, which in turn depended on underwater irradiance at those depths. Our findings suggest that the development of regionally and seasonally adopted production models is relevant for the estimation of the annual Kara Sea IPP.