Phytocenoses of the Ob Estuary and Kara Sea Shelf in the Late Spring Season

Abstract

Based on the material obtained in the spring-summer season of 2016, the composition and quantitative distribution of phytoplankton in the Ob estuary and over the Kara Sea shelf has been estimated. The latitudinal extent of the region exceeded 5° (~560 km), encompassing areas with different river discharge impact, which formed latitudinal zoning in the distribution of hydrophysical and hydrochemical conditions. These environmental properties determined the zoning in the spatial structure of phytoplankton communities. The estuarine biotope with a salinity <1 PSU and high concentrations of nitrate, phosphate, and silica was inhabited by a freshwater phytocenosis stable in species composition with high-quantitative characteristics and dominated by Aulacoseira species. The most spatial and seasonal variability of phytoplankton is characteristic of the area of intense interaction of fresh and marine waters in the outer part of the Ob estuary and over the adjacent shallow shelf, where freshwater algae species are replaced by marine species. A sharp biotopic boundary separates the inner shallow shelf with depths of approximately 30-35 m and outer shelf of the depth from 150 to 200 m. In the area of the outer shelf the lower boundary of the euphotic zone descends to 25-35 m. At theses depths plankton algae actively vegetate, and form subsurface phytoplankton maximum under conditions of sufficient nutrient concentration.

Water Exchange Between the Gulf of Ob and the Kara Sea During Ice-Free Seasons: The Roles of River Discharge and Wind Forcing

“…In accordance with the abovementioned successional cycle, the spring bloom after ice melting occurs due to the development of diatoms, as was observed at the end of July 2016 (Sukhanova et al, 2018). Blooming occurs due to diatoms of all ecological groups, namely, freshwater, marine, and brackish.…”

“…The spring bloom, which is formed mainly by diatoms shortly after ice melting, shifts to a steady decrease of abundance and biomass till the ice formation period (Makarevich, 2008). Generally, three zones with different phytoplankton communities are distinguished, namely, the freshwater zone with maximal productivity, the frontal zone with a distinct two-layered structure and bottom salinities increasing from 5-6 to 18-20 characterized by an abrupt decrease of productivity, and the saline zone with highly variable phytoplankton characteristics (Druzhkov and Makarevich, 1996;Makarevich et al, 2003;Sukhanova et al, 2010Sukhanova et al, , 2018. However, the field surveys, which are reported in these papers, were performed during different seasons and were limited to certain zones within the gulf, i.e., not covering all the three zones within one survey.…”

Picophytoplankton distribution along Khatanga Bay-shelf-continental slope environment gradients in the western Laptev Sea

“…The SCM was observed in areas with a stratified water column and nitrate-poor surface waters at the northernmost shelf station and above the continental slope. The SCM occurred in some ice-free Arctic waters during late summer and early fall ( Martin et al., 2010 ; Arrigo et al., 2011 ; Ardyna et al., 2013 ; Brown et al., 2015 ; Demidov et al., 2018 ; Sukhanova et al., 2018 ). The SCM is located within the nutricline close to the lower boundary of the euphotic zone.…”