Longitudinal variability in the zooplankton community of the upper Ob river

Abstract

Zooplankton is important component of aquatic ecosystems playing the essential role in organic matter circulation and self-purification of water bodies. Current data on zooplankton from one of the largest rivers in the world, i.e. the Ob river, is very fragmentary and cover small areas adjacent to large settlements. This paper presents the results of the zooplankton study of the 458-km stretch of the Ob river – from its source (Fominskoye village) up to the site above the Novosibirsk reservoir (Taradanovo village). To assess the longitudinal distribution of zooplankton, we collected samples from 17 river sections in different phases of a hydrological cycle, i.e. during the periods of flood (May–June) and summer low water (July–August) 2016. A total of 103 species and forms of zooplankton were detected. Rotifera dominated in species number. In various river parts and hydrological cycle phases, species number, abundance and biomass of zooplankton differed significantly. Their maximum typically fell on the period of summer low water. Abundance and biomass of zooplankton tended to increase downstream. In most parts of the river, Rotifers was dominant in number, whereas copepods – in biomass (juvenile stages). Zooplankton distribution along the transversal profile of the river was characterized by the increased species number, abundance and biomass near the right bank (unlike the left one) that is, probably, due to the floodplain appearance on the right bank. An increase in zooplankton abundance was also observed in sites with a low flow velocity (river bays) and below the large tributaries. The assessment of water quality in the study site of the Ob river was performed based on the zooplankton composition and structure. A saprobic index for zooplankton is evidence of oligo- betamesosaprobic conditions.