Quantitative Characteristics and Age Structure of Apatania crymophila Larvae on Aquatic Moss of the Yenisei River Downstream of а High-Pressure Hydroelectric Plant

The seasonal dynamics of the abundance, size–age composition, and production of the larvae of the caddisfly Apatania crymophyla McLachlan, 1880, which form high-density aggregations on water moss in the thermally and hydrologically altered section of the Yenisei River downstream of the Krasnoyarsk Hydroelectric Plant (HEP) dam, are addressed. The abundance of the larvae inhabiting water moss was highest in October 2022 and 2023 and in August 2021. The maximal abundance of the larvae on moss varied between 1242 and 4037 individuals kg–1 of fresh moss, and the fresh biomass of the larvae varied from 1.78 to 3.15 g kg–1 of fresh moss. The level of water in the river affected the quantitative parameters and the number of generations of A. crymophila. In 2022, when the water level was abnormally low, the number of new generations in the population was smaller and the production of the larval biomass over the season was lower (by a factor of 1.8) than in 2023, when the water level was close to the long-term annual average. Hence, the conditions in the thermally altered section of the Yenisei River downstream of the Krasnoyarsk HEP are favorable for the development of A. crymophila; one of these conditions is the abundance of aquatic moss in this section, which provides the invertebrates with resources favoring the maintenance of the high-density population. The results show that the invertebrates inhabiting aquatic moss should be taken into account when estimating organic matter production in the Yenisei River.