The Effect of Salinity and Nutrient Load on the Ecosystems of Selected Lakes in the South of Siberia

Abstract

Salinity largely determines the species diversity and structure of the food web in lake ecosystems. Many studies have been focused on the salinity tolerance of various plankton species. There are fewer studies investigating the modifications in the food web interactions under the effect of salinity. An appropriate system for such studies is closely located lakes of different salinity. We took samples during the summer stratification and analyzed the physicochemical and biological characteristics of 20 lakes (salinity 0.1-35 g/l) in southern Siberia in order to provide general description of this system and to test a number of hypotheses about the effect of salinity on the structure of the food web. The analysis revealed two key factors structuring ecosystems of the studied lakes. The species composition of zooplankton was determined by salinity, with a decrease in the number of species and the transformation of the community from a diverse community of zooplankton (salinity up to 3 g/l) to the dominance of large daphnia (3-8 g/l), copepods (> 8 g/l), small cladocerans and rotifers (20-30 g/l), and Artemia (> 30 g/l). The top predator (fish) was eliminated at salinity above 10 g/l. The biomass of zooplankton did not depend on salinity, but significantly increased in lakes with an increased nutrient load. The nutrient load also had a significant effect on the concentration of total phosphorus, turbidity and transparency of water, and the concentration of chlorophyll “a”. Thus, we recorded the structuring effect of the nutrient load (bottom-up control) on the biomass of the trophic levels and the parameters of water transparency, while salinity transformed the trophic chain from the top, leading to the disappearance of fish and a change in the dominant species of zooplankton. At the same time, salinity did not affect the biomass of the trophic levels. In order to detect top-down effects in the ecosystems of saline lakes, it is necessary to perform a comparative analysis of the seasonal dynamics of ecosystems of lakes with different salinity