Quantification of the vertical distribution of chlorophyll a (Chl-a), as one of characteristics of primary production (PP), is critically important to estimate annual PP in the water column (IPP) using models and remote sensing data. IPP estimation in optically complex and highly variable waters such as the Kara Sea is not a trivial task. In the present study, based on the data obtained during three multidisciplinary cruises to the eastern regions of the Kara Sea in August-October, the differences in the vertical distribution of Chl-a and PP under and without the influence of the river plume (Case II and Case I water types, respectively) are established. In Case I waters in August 2014, under the low values of the diffuse attenuation coefficient (Kd) of downwelling photosynthetically available radiation (PAR) (the median value (Me) K-d = 0.158 m(-1)) and the deep euphotic layer (Z(eu)) (Me = 30 m), the deep chlorophyll maximum (DCM) was well-pronounced. At the end of September 2015 in Case II waters influenced by the river runoff, when Me K-d increased 1.7-fold and Z(eu) decreased 1.3-fold, the DCM was absent. Also, the DCM was not manifested at the end of the growing season, under conditions of extremely low underwater PAR (Me = 0.35 mol quanta m(-2) d(-1)). In the sampling period, the PP maxima were observed at the surface and the DCM did not influence the vertical PP distribution. The depth of the nitracline was directly associated with the distribution of riverine waters and determined the depth and degree of DCM manifestation. The outcomes of the presented study suggest that a decrease in subsurface PAR influenced by the impact of riverine waters and the total decline of incident radiation from August to October determine the vertical distribution of PP and Chl-a.

Glaciers retreat linked with rapid Arctic warming changes bioproductivity in the bays and coastal areas of the Arctic Ocean (AO). The impact of different types of glaciers shrinking on water column primary production (IPP) is widely discussed. This work studied the influence of the land-terminating glacier on IPP and chlorophyll a concentration (Chlph) in Blagopoluchiya Bay (Novaya Zemlya archipelago). It is known that meltwater runoff from land-terminating glaciers enriches coastal waters with inorganic suspended matter and nutrients. The aim of the present study was to establish the main environmental factor determining IPP in the bay influenced by the glacier. During summer and autumn, IPP in the bay was low. The medians of IPP were 69 and 26 mgC m−2 day−1, respectively, which was 1.7- and 1.9-fold lower than in open waters of the Kara Sea in the vicinity of the bay. Generally, IPP increased from the inner part, close to the glacier, to the outer part of the bay. In summer, IPP and Chlph depend on both nutrient concentrations in the photosynthetic layer and the parameters of water transparency. On the contrary, in autumn, IPP and Chlph depend only on the optical properties of the subsurface layer. It can be concluded that low IPP in Blagopoluchiya Bay is determined mainly by unfavorable conditions of underwater PAR formed by meltwater discharge from the glacier. Thus, the fast melting of the land-terminating glaciers linked with climate warming can lead to a decrease in the biological productivity of the AO bays.

Climate-induced variability of phytoplankton size structure influences primary productivity, marine food web dynamics, biosedimentation and exchange of CO 2 between the atmosphere and ocean. Investigation of phytoplankton size structure in the Arctic Ocean is important due to rapid changes in its ecosystems related to increasing temperature and declining sea ice cover. We estimated the contribution of surface micro-, nano- and picophytoplankton to the total carbon biomass, chlorophyll a concentration and primary production in the Kara and Laptev Seas and investigated the relationships of these phytoplankton size groups with environmental factors which determine their spatial variability. Additionally, we compared chlorophyll specific carbon fixation rate, specific growth rate and carbon to chlorophyll ratios among different phytoplankton size groups. The investigation was carried out from August to September 2018. Generally, picophytoplankton was dominant in terms of chlorophyll a and primary production in the whole study area. The spatial variability of phytoplankton size classes was influenced by river discharge and relied mainly on water temperature, salinity and dissolved silicon concentration. Microphytoplankton prevailed across the river runoff region under conditions of low salinity and relatively high water temperature, while picophytoplankton was predominant under conditions of high salinity and low water temperature. Our study is the first to characterize size-fractionated phytoplankton abundance in the Kara and Laptev Seas, and provides a baseline for future assessment of the response of Kara and Laptev Sea ecosystems to climate-induced processes using phytoplankton size structure.