CURRENT STATE OF ZOOPLANKTON IN THE LOWER COURSE OF THE UKSUNJOKI RIVER (NORTHERN LADOGA REGION)

Abstract

The zooplankton community of the lower course of a stream draining to the Lake Ladoga northern coast – the Uksujoki river was studied to estimate its current state as an essential structural and functional element of this lotic ecosystem, to assess the water quality based on zooplankton indicators and to determine what factors influence its development. Surveys in 2011–2016 showed the zooplankton community to include 53 taxa below the genus level, among them Rotifera – 16, Cladocera – 31, Copepoda – 6. During the surveys, 26 species and subspecies of zooplankton were recorded for the first time for this watercourse. Zooplankton quantities in the Uksunjoki were among the lowest among other 20 tributaries of Lake Ladoga during the spring, summer, and autumn seasons. The abundance varied from 10 to 2740 ind./m3, biomass from 0.0001 to 0.0330 g/m3. Quantitative indicators reached a maximum in September 2013. The highest season-averaged values of species richness, abundance, and biomass occurred in the summer period. In spring, Copepoda prevailed, represented by the nauplial and copepodite stages of Cyclopoida. In summer and autumn, cladoceran crustaceans from the family Chydoridae featured the largest numbers and biomass in the structure of the zooplankton community. Organisms that forage from the substrate surface (crawling-floating secondary filter feeders) predominated in the trophic structure of the zooplankton community. The development of species was stochastic. The most significant aquatic environment factors for the development of zooplankton in the Uksunjoki appear to be the water ionic composition and water discharge. As indicated by the zooplankton community, the water of the Uksunjoki River can be characterized as relatively clean and suitable for all types of water use.