Estimation of the Contribution of Size-Fractionated Phytoplankton in the Kara Sea to Primary Production and Chlorophyll a during Different Seasons

Abstract Based on data obtained during seven Kara Sea expeditions (2017–2023), seasonal variation in the contribution of phytoplankton size groups to the total values of primary production (PP) and chlorophyll a (chl a ) are examined for the first time. Micro- and nanophytoplankton (MPh + NPh) (>3 µm) dominated in the community composition during the entire ice-free period (June–October). Its predominance was especially noticeable during the spring bloom immediately after first-year sea-ice retreat (up to 97% for PP and up to 93% for chl a ). The role of picophytoplankton (PPh) (<3 µm) increased in summer (July, August) (up to 50% for PP and up to 44% for chl a ) and decreased by the end of the growing season (September, October). Seasonal variation in the size composition of phytoplankton during the growing season was determined mainly by variability in water temperature and incoming solar radiation. The contribution of PPh to the total chl a increased (up to 51%) at depths of the deep chlorophyll maximum in July and August. The assimilation activity of PPh was higher than that of MPh + NPh in July–September, with an increase in its contribution to the total PP and chl a . For the first time, annual PP of Kara Sea phytoplankton size groups was assessed: 8 ТgС (65%) for MPh + NPh and 5 ТgС (35%) for PPh.