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Fish play a key role in the trophic dynamics of lakes. With climate warming, complex changes in fish assemblage structure may be expected owing to direct effects of temperature and indirect effects operating through eutrophication, water level changes, stratification and salinisation. We reviewed published and new long-term (10-100 years) fish data series from 24 European lakes (area: 0.04-5,648 km; mean depth: 1-177 m; a north-south gradient from Sweden to Spain). Along with an annual temperature increase of about 0.15-0.3°C per decade profound changes have occurred in either fish assemblage composition, body size and/or age structure during recent decades and a shift towards higher dominance of eurythermal species. These shifts have occurred despite a reduction in nutrient loading in many of the lakes that should have benefited the larger-sized individuals and the fish species typically inhabiting cold-water, low-nutrient lakes. The cold-stenothermic Arctic charr has been particularly affected and its abundance has decreased in the majority of the lakes where its presence was recorded. The harvest of cool-stenothermal brown trout has decreased substantially in two southern lakes. Vendace, whitefish and smelt show a different response depending on lake depth and latitude. Perch has apparently been stimulated in the north, with stronger year classes in warm years, but its abundance has declined in the southern Lake Maggiore, Italy. Where introduced, roach seems to take advantage of the higher temperature after years of low population densities. Eurythermal species such as common bream, pike-perch and/or shad are apparently on the increase in several of the lakes. The response of fish to the warming has been surprisingly strong and fast in recent decades, making them ideal sentinels for detecting and documenting climate-induced modifications of freshwater ecosystems. [ABSTRACT FROM AUTHOR]

Lakes in Europe have a bipolar distribution by latitude with higher lake densities in the north (58–65° N) and south (38–48° N). By area, 95% of the large lakes (>100 km2) are located at altitudes lower than 100 m above sea level (ASL) and only 1% lie higher than 1,000 m ASL. Physically large lakes exhibit several similarities to seas and oceans in their thermal structure and circulation dynamics. From the chemical point of view, lakes are important accumulation sites for substances transported from the watershed or built up in the lake itself but they may contribute positively to global greenhouse gas emission. Fauna and flora of ancient large lakes such as the Caspian Sea and Lake Ohrid include large numbers of endemic species, which become endangered if conditions change because of direct human impact, alien species invasions or climate change. Large lakes offer a wide range of ecosystem services to society, the multiple use of which creates multiple pressures on these water bodies such as nutrient load and toxic pollution, modification of hydrology and shore line structure, and shifting of the food web balance by stocking or harvesting various species. Although large lakes are among the best-studied ecosystems in the world, the application to them of environmental regulations such as the European Water Framework Directive is a challenging task and requires that several natural and management aspects specific to these water bodies are adequately considered. [ABSTRACT FROM AUTHOR]

The population dynamics of fish in northern lakes is strongly influenced by climatic factors. In this study, we investigated whether there is a link between the late 1980s climate regime shift in Europe and the collapse of vendace (Coregonus albula) population at the same time in Lake Peipsi. Until the end of the 1980s, vendace was very abundant in the lake, but then its catches sharply declined. This decline inspired investigations into the extreme weather events preceding the vendace collapse using data on daily water temperatures and ice phenology together with commercial fishery statistics since 1931 and test catch data since 1986. We identified using advanced statistical methods that the hot summer of 1988, which was accompanied by a severe cyanobacterial bloom and extensive fish kill, and the subsequent non-permanent ice cover and early ice-offs in 1989 and 1990 in Lake Peipsi were the main reasons for the disappearance of vendace from catches in 1991. Moreover, a negative correlation appeared between catches of the predatory pikeperch (Sander lucioperca) and vendace. Predation pressure as well as fish habitat degradation caused by lake eutrophication may contribute to the instability of the vendace population too. Our study showed that extreme weather events such as heat waves in summer and non-permanent ice-cover in winter in consecutive years may have long-lasting harmful effects on the population abundance of cool-water fish species such as vendace whose eggs usually develop under an ice cover in north-temperate lakes. [ABSTRACT FROM AUTHOR]

Lake eutrophication is a pervasive problem globally, particularly serious in agricultural and densely populated areas. Whenever nutrients nitrogen and phosphorus do not limit phytoplankton growth directly, high growth rates will rapidly lead to biomass increases causing self-shading and light-limitation, and eventually CO 2 depletion. The paradigm of phytoplankton limitation by nutrients and light is so pervasively established, that the lack of nutrient limitation is ordinarily interpreted as sufficient evidence for the condition of light limitation, without considering the possibility of limitation by inorganic carbon. Here, we firstly evaluated how frequently CO 2 undersaturation occurs in a set of eutrophic lakes in the Pampa plains. Our results confirm that conditions of CO 2 undersaturation develop much more frequently (yearly 34%, summer 44%) in these agriculturally impacted lakes than in deep, temperate lakes in forested watersheds. Secondly, we used Generalized Additive Models to fit trends in CO 2 concentration considering three drivers: total incident irradiance, chlorophyll a concentration, and lake depth; in eight multi-year datasets from eutrophic lakes from Europe, North and South America, Asia and New Zealand. CO 2 depletion was more often observed at high irradiance levels, and shallow water. CO 2 depletion also occurred at high chlorophyll concentration. Finally, we identified occurrences of light- and carbon-limitation at the whole-lake scale. The different responses of chlorophyll a and CO 2 allowed us to develop criteria for detecting conditions of CO 2 limitation. For the first time, we provided whole-lake evidence of carbon limitation of phytoplankton biomass. CO 2 increases and eutrophication represent two major and converging environmental problems that have additive and contrasting effects, promoting phytoplankton, and also leading to carbon depletion. Their interactions deserve further exploration and imaginative approaches to deal with their effects., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)