Structure of the Food Web of a Northern Humified Lake: Seasonal Features

The relevance of studying the structure of food chains in high-latitude lakes characterized by oligotrophy and a significant influx of allochthonous organic matter from adjacent marshes is obvious. An attempt is made to identify carbon sources in various trophic groups (zooplankton, zoobenthos, and fish) and analyze the trophic relationships between them, taking into account seasonal dynamics in a small subarctic humified lake located in the north of Karelia (the coast of the White Sea). To achieve this goal, an isotopic analysis of stable carbon (δ13C) and nitrogen (δ15N) isotopes was carried out in the tissues of consumers and potential producers in different seasons (summer, autumn, winter, and spring) of 2019–2020. The results of the study reveal significant differences in the structure of the lake food web between the open water and ice-covered periods, which is likely due to changes in the level of phytoplankton primary production, sources and rates of allochthonous organic matter input, and a significant energy contribution from heterotrophic organisms associated with methanotrophs. The important role of methane carbon in the formation of organic matter in this water body is indicated by low δ13C values (<–40‰) in consumers such as zooplankton (dominated by Eudiaptomus graciloides) and chaoborids (Chaoborus sp.), apparently actively involved in the transfer of methane carbon within food chains. In summer, a significant contribution of diazotrophic nitrogen fixed by planktonic cyanobacteria acts as an important source of nutrients. Seasonal variations in δ15N values and trophic position are determined for the common perch (Perca fluviatilis). The data contribute to the understanding of the formation mechanisms of energy and trophic links in a humified lake food web with the influence of allochthonous organic matter.