Lake Kuetsjarvi (in the lower reaches of the Pasvik River, Murmansk Region, Russia) in the border area between Russia and Norway, is one of the most polluted water reservoirs in the European Arctic. The operation of the Pechenganikel Smelter located on its shores has led to the extremely high concentrations of heavy metals observed in the waters and sediments of the lake. Long-term comprehensive studies of the ecosystem of Lake Kuetsjarvi have made it possible to identify the response of its components to the global and regional change in the environment and climate as a whole, resulting in increased water toxicity and eutrophication, reduction in the number of stenobiont species of aquatic organisms against the background of an increase in the number of eurybiontic and invasive species. Modern communities of Lake Kuetsjarvi are the result of a combination of long-term changes in the abiotic environment and biotic interactions. Heavy-metal pollution of Lake Kuetsjarvi, observed since the 1930s, has led to the formation of a community that is resistant to this type of impact and supports large populations of adapted species. Adaptations of communities to the dynamics of the environmental conditions that their members are exposed to include changes in the species composition, quantitative indicators, ratios between individual taxonomic groups, and the population structure. The development of sympatric forms that differ in the ecological niches they occupy, morphology, and life cycle strategies, including the transition to a short-cycle survival strategy, allows whitefish to remain the dominant species and maintain high population numbers. Unlike the organismal level, responses to medium-term environmental changes on the population and community level are less specific and characterized by stronger inertia.

Ключевые слова: Coregonus lavaretus внутривидовые группировки антропогенное загрязнение оз. Куэтсъярви Аннотация: Впервые на основе подробного исследования биологии сига Coregonus lavaretus субарктического оз. Куэтсъярви (нижнее течение р. Пасвик, Мурманская область), являющегося одним из наиболее техногенно загрязненных природных водоемов Евро-Арктического региона, выделены четыре внутривидовые группировки рыб: медленнорастущий малотычинковый сиг, быстрорастущий малотычинковый сиг, медленнорастущий среднетычинковый сиг, быстрорастущий среднетычинковый сиг. Специализация этих сигов на определенном виде ресурсов обусловливает их морфологические и поведенческие различия, пространственную дифференциацию и, в конечном итоге, различные экологические ниши.

The article gives an overview of the limnological research historyat INEP KSC RAS since the late 1980s. until now. The most important results of complex work are presented, including the study of the hydrochemical composition of water and bottom sediments, as well as biota (plankton, benthos and fish) of the Murmansk region lakes. The prospects for the scientific research development of are shown.

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1. High latitude ecosystems are experiencing the most rapid warming on earth, expected to trigger a diverse array of ecological responses. Climate warming affects the ecophysiology of fish, and fish close to the cold end of their thermal distribution are expected to increase somatic growth from increased temperatures and a prolonged growth season, which in turn affects maturation schedules, reproduction and survival, boosting population growth. Accordingly, fish species living in ecosystems close to their northern range edge should increase in numerical importance and possibly displace cold-water adapted species. 2. We aim to document if and how population level effects of warming mediated by individual level responses to increased temperatures, shift community structure and composition in high latitude ecosystems. 3. We studied 11 cool-water adapted freshwater fish populations in communities dominated by cold-water adapted species to investigate changes in the relative importance of cool-water fish during the last 30 years of rapid warming in high latitude lakes. In addition, we studied the individual level responses to warming to clarify the potential mechanisms underlying the population effects. 4. Our long-term series‘ (1991-2020) reveal a marked increase in numerical importance of the cool-water fish species, perch, in ten out of eleven populations, and in most fish communities the cool-water species is now dominant. Moreover, we show that climate warming affects population level processes via direct and indirect temperature effects on the individuals. Specifically, the increase in abundance arises from increased survival of 0+ individuals, faster juvenile growth and ensuing earlier maturation, all boosted by climate warming. 5. The speed and magnitude of the response to warming in these high latitude fish communities strongly suggest that cold-water fish will be displaced by fish adapted to warmer water. Consequently, management should focus on climate adaptation limiting future introductions and invasions of cool-water fish and mitigating harvesting pressure on cold-water fish.