Millions of lakes worldwide are distributed at latitudes or elevations resulting in the formation of lake ice during winter. Lake ice affects the transfer of energy, heat, light, and material between lakes and their surroundings creating an environment dramatically different from open-water conditions. While this fundamental restructuring leads to distinct gradients in ions, dissolved gases, and nutrients throughout the water column, surprisingly little is known about the resulting effects on ecosystem processes and food webs, highlighting the lack of a general limnological framework that characterizes the structure and function of lakes under a gradient of ice cover. Drawing from the literature and three novel case studies, we present the Lake Ice Continuum Concept (LICC) as a model for understanding how key aspects of the physical, chemical, and ecological structure and function of lakes vary along a continuum of winter climate conditions mediated by ice and snow cover. We examine key differences in energy, redox, and ecological community structure and describe how they vary in response to shifts in physical mixing dynamics and light availability for lakes with ice and snow cover, lakes with clear ice alone, and lakes lacking winter ice altogether. Global change is driving ice covered lakes toward not only warmer annual average temperatures but also reduced, intermittent or no ice cover. The LICC highlights the wide range of responses of lakes to ongoing climate-driven changes in ice cover and serves as a reminder of the need to understand the role of winter in the annual aquatic cycle.Plain Language Summary Millions of lakes worldwide freeze during winter. The formation of lake ice dramatically alters lakes by isolating them from their surrounding landscape and atmosphere. The thickness and optical qualities of ice and snow regulate the amount of solar radiation entering lakes, while shielding them from wind energy. Consequently, lake ice is an important factor regulating physical mixing dynamics within lakes, and structuring vertical thermal and chemical gradients. Although organisms from bacteria to fish have adapted to the winter environment, we lack a comprehensive understanding of how variation in lake ice cover conditions affects fundamental ecosystem processes or food web structure. Here, we combine a synthesis of the current literature with three novel case studies to develop the Lake Ice Continuum Concept as a framework for understanding how key aspects of the physical, chemical, and ecological structure and function of lakes vary along a continuum of energy inputs mediated by winter climate. This framework is useful for understanding changes associated with seasonal ice dynamics and for predicting how lakes may respond to climate change.

The Arctic is covered with lakes that freeze over for a significant part of the year. Ice formation leads to the establishment of a distinct environment within the lake relative to the open-water period by limiting gas exchange at the air-water interface, wind-driven mixing (Denfeld et al., 2018), allochthonous input of terrestrial carbon and nutrients (Bertilsson et al., 2013), and also reducing the amount of light penetrating through the water column (Kirillin et al., 2012). All of these factors-light penetration, nutrient concentrations, and thermal structure of the water column-affect biomass and community structure of phytoplankton. Despite the fact that ice covers Arctic lakes for the majority of the annual cycle, most of what we know about phytoplankton communities and biogeochemical dynamics in these lakes is based on the open-water season. Considering the rapid changes in climate affecting phenology, ice-out dates, and abiotic conditions

While the sentinel nature of freshwater systems is now well-recognized, widespread integration of freshwater processes and patterns into our understanding of broader climate-driven Arctic terrestrial ecosystem change has been slow. We review the current understanding across Arctic freshwater systems of key sentinel responses to climate, which are attributes of these systems with demonstrated and sensitive responses to climate forcing. These include ice regimes, temperature and thermal structure, river baseflow, lake area and water level, permafrost-derived dissolved ions and nutrients, carbon mobilization (dissolved organic carbon, greenhouse gases, and radiocarbon), dissolved oxygen concentrations, lake trophic state, various aquatic organisms and their traits, and invasive species. For each sentinel, our objectives are to clarify linkages to climate, describe key insights already gained, and provide suggestions for future research based on current knowledge gaps. We suggest that tracking key responses in Arctic freshwater systems will expand understanding of the breadth and depth of climate-driven Arctic ecosystem changes, provide early indicators of looming, broader changes across the landscape, and improve protection of freshwater biodiversity and resources.

South Ural is aterritory with aunique geographical position and heterogeneous natural conditions. Unexplored biodiversity of the terrestrial cyanobacteria of this territory is very high. We undertook a fl oristic study covering all botanical-geographical zones of the Bashkiria and Bredinskiy district of the Chelyabinsk region. In atotal of 85 soil samples collected, 56 species of cyanobacteria were identifi ed. The number of cyanobacteria was highest in the boreal-forest zone (39 species) and notably lower in the other zones (18, 29, and 24 species for broad-leaved forest, forest steppe and steppe regions, respectively). Leptolyngbya voronichiniana, Leptolyngbya foveolarum,cf. Trichocoleus hospitus, Pseudophormidium hollerbachianum, Nostoc cf. punctiforme, Microcoleus vaginatus, Phormidium breve, Phormidium dimorphum, Phormidium corium,and Leptolyngbya cf. tenuis were detected in all studied zones. Trichormus variabilis and Cylindrospermum majus were detected in the forest zone, Phormidium ambiguum was typical for forest-steppe and steppe zones, Pseudophormidium hollerbachianum and Nostoc cf. commune were most abundant in the steppe. Humidity and heterogeneity of the substrate were likely the most important factors infl uencing terrestrial cyanobacteria diversity. For full understanding of the biodiversity of cyanobacteria in the South Urals, future molecular-genetic research is necessary. Bashkiria / Leptolyngbya / Oculatella / Phormidium /soil / Trichocoleus 85 samples of soil and microbiotic crusts from 12 localities were studied: 17 collected from boreal forests, 21 from broad-leaved forests, 29 from forest-steppes, and 18 from steppes (Table 1). Fig. 1. Terrestrial cyanobacteria of the South Ural region: map of the sampling sites. Boreal forest zone (A): 1-B eloretsk region, 2-n ear Pavlovka village; Broad-leaved forest zone (B): 3-B akaly, 4-I glino, 5-K rasnousolskiy; Forest-steppe zone (C): 6-n ear Dyurtyuli town, 7-n ear Tolbazy village, 8-e dge of Bolsheustikinskoye village, 9-n ear Georgievka village; Steppe zone (D) of Bashkiria and Bredinskiy district of Chelyabinsk region: 10-n ear Sibay town, 11-n ear Yangelskiy village, 12-near Arkaim monument. Synechococcales Pseudanabaenaceae Pseudanabaena papillaterminata (Kiselev) Kukk 1 Leptolyngbyaceae Leptolyngbya foveolarum (Rabenhorst ex Gomont) Anagnostidis &K omárek Leptolyngbya cf. fragilis (Gomont) Anagnostidis &K omárek Leptolyngbya cf. hansgirgiana Komárek in Anagnostidis Leptolyngbya cf. subtilissima (Kützing ex Hansgirg) Komárek in Anagnostidis 2 Leptolyngbya cf.t enuis (Gomont) Anagnostidis &K omárek Leptolyngbya cf. nostocorum (Bornet ex Gomont) Anagnostidis &K omárek Leptolyngbya сf. notata (Schmidle) Anagnostidis &K omárek 2 Leptolyngbya voronichiniana Anagnostidis &K omárek 6 55 Oculatella sp. 1 2 Oculatella sp. 2 2 Oculatella sp. 32 cf. Trichocoleus hospitus (Hansgirg ex Forti) Anagnostidis 6 44 Pleurocapsales Hyellaceae Myxosarcina cf. tatrica (Starmach) Komárek &Anagnostidis 1 Chroococcales Aphanothecaceae Aphanothece stagnina (Spreng.) A....

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