Vertical distribution of primary production and chlorophyll a in the eastern Kara Sea: Relations with river plume effects in late summer and autumn

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 (K_d) 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.