Current state of zooplankton and zoobenthos in the lower stream of the Chulym river (Obi river basin)

Abstract

A comparison was made of the numerical indicators of zooplankton and zoobenthos of the Chulym River in comparison with those obtained in 1972–1973 in order to assess the transformation of the ecosystem that has occurred over 50 years. It is shown that the zooplankton of the Chulym River has undergone dramatic changes, both qualitatively and quantitatively. The number of zooplankton increased 4–5 times due to small rotifers. At the same time, the zooplankton biomass, and therefore the feed value, decreased by more than half due to a reduction in the number of large crustaceans. The species composition of structure-forming zooplankton species has changed. The number of rotifer species in the riverbed decreased from 55 rotifers to 34. The species of Trichocerca family dropped out of the dominant composition and representatives of the river began to dominate Brachionus family. There is a significant change in the structure of zoobenthos: the number of organisms in the Chulym River bed is 5 times lower than in the 1970s. The species composition in benthic communities has remained almost unchanged over 50 years, while the proportion of some heterotopic insects (caddis flies, mayflies, stoneflies) has increased, and the proportion of oligochaetes, mollusks and chironomids in the biomass has decreased by 4–5 times. In the accessory reservoirs of Chulym, the abundance and biomass of zooplankton and zoobenthos over the past 50 years have remained more or less stable, which may indicate the presence of chronic pollution that affects the ecosystem of the main river channel and does not have time to have a negative impact on the accessory reservoirs during floods. The water quality indicators of the Chulym River and its subsidiary reservoirs, according to biotic indices, are low and correspond to a low or poor water quality class. Such conditions are unfavorable for the further development of zoobenthos and, accordingly, ichthyofauna, for which bottom invertebrates are the food supply.