Structure of phytoplankton communities in the Yenisei estuary and over the adjacent Kara Sea shelf

Material was collected in the Yenisei estuary and over the adjacent Kara Sea shelf at a quasimeridional transect from 71A degrees 49'70aEuro(3) to 75A degrees 59'93aEuro(3) N in September 2011. The structural characteristics of the phytoplankton community were determined by latitudinal zonality of environmental conditions. Two well-distinguished phytocenoses-freshwater and marine-were found in this region. Phytoplankton in the freshwater part of the estuary was composed solely of the freshwater algae species and was distinguished by the highest numbers (up to 2 x 10(6) cell/L) and biomass (up to 1.4 mg/L). The marine phytocenoses over the Yenisei shoal was composed of marine neritic species; the abundance and biomass of phytoplankton in this area were significantly lower (0.2 x 10(6) cell/L and 0.4 mg/L, respectively). The area of intensive interaction of riverine and marine waters-the estuarine frontal zone, with similar to 130 km latitudinal extension (from 72A degrees to 74A degrees N)-was characterized by a sharp halocline, which separated the desalinated upper layer from the underlying marine water. Freshwater algal species predominated above the halocline, whereas marine species predominated below. The lower border of the euphotic layer was located 8 to 15 m below the halocline. The niche between the halocline and the lower border of the euphotic layer was characterized by high nutrient concentrations, which together with sufficient illumination determined the intensive development of phytoplankton and high values of primary production.