Institute of Water Problems of the North Karelian Research Centre

Abstract. We present a systematic compilation of previously published Holocene proxy climate records from the Arctic. We identified 170 sites from north of 58° N latitude where proxy time series extend back at least to 6 cal ka (all ages in this article are in calendar years before present – BP), are resolved at submillennial scale (at least one value every 400 ± 200 years) and have age models constrained by at least one age every 3000 years. In addition to conventional metadata for each proxy record (location, proxy type, reference), we include two novel parameters that add functionality to the database. First, "climate interpretation" is a series of fields that logically describe the specific climate variable(s) represented by the proxy record. It encodes the proxy–climate relation reported by authors of the original studies into a structured format to facilitate comparison with climate model outputs. Second, "geochronology accuracy score" (chron score) is a numerical rating that reflects the overall accuracy of 14C-based age models from lake and marine sediments. Chron scores were calculated using the original author-reported 14C ages, which are included in this database. The database contains 320 records (some sites include multiple records) from six regions covering the circumpolar Arctic: Fennoscandia is the most densely sampled region (31% of the records), whereas only five records from the Russian Arctic met the criteria for inclusion. The database contains proxy records from lake sediment (60%), marine sediment (32%), glacier ice (5%), and other sources. Most (61%) reflect temperature (mainly summer warmth) and are primarily based on pollen, chironomid, or diatom assemblages. Many (15%) reflect some aspect of hydroclimate as inferred from changes in stable isotopes, pollen and diatom assemblages, humification index in peat, and changes in equilibrium-line altitude of glaciers. This comprehensive database can be used in future studies to investigate the spatio-temporal pattern of Arctic Holocene climate changes and their causes. The Arctic Holocene data set is available from NOAA Paleoclimatology.

Penetrative convection is discussed where the instability is driven by radiative heating of water below the temperature of maximum density. Convection of this type occurs in ice‐covered freshwater lakes in late spring, when the snow cover vanishes and solar radiation is absorbed beneath the ice cover. The vertical temperature structure, bulk mixed layer scaling, and mixed layer deepening are examined for a number of temperate and polar lakes. A bulk mixed layer scaling for this type of convection is based on energy arguments underlying the classical Deardorff convective scaling. The depth of the convective layer serves as an appropriate length scale. However, a modified scale that takes account of the energetics of a distributed radiation source term replaces the surface buoyancy flux velocity scale used by Deardorff. The scaling compares favorably with large‐eddy simulations of turbulence kinetic energy (TKE) and with both observations and large‐eddy simulations of the TKE dissipation rate. Mixed layer deepening is simulated with a model of convection beneath lake ice. The model describes the structure of the stably stratified layer just beneath the ice with a stationary solution to the heat transfer equation; the structure of the entrainment layer is parameterized with a zero‐order jump approach. The entrainment equation is derived from the mixed layer TKE budget and bulk mixed layer scaling. Entrainment regimes characteristic of convection beneath ice are analyzed. It is shown that if the Deardorff convective velocity scale is replaced with a scale incorporating the distributed buoyancy flux, the entrainment equation describing atmospheric and oceanic convective boundary layers also applies beneath the ice. Model predictions compare well with data from observations in a number of lakes. We propose and compare with observations an extension of the mixed layer model that allows for the inclusion of salinity. Although the salt concentration is low in most temperate and polar lakes, its dynamical effect can be significant close to the temperature of maximum density.

Abstract. Empirical evidence demonstrates that lakes and reservoirs are warming across the globe. Consequently, there is an increased need to project future changes in lake thermal structure and resulting changes in lake biogeochemistry in order to plan for the likely impacts. Previous studies of the impacts of climate change on lakes have often relied on a single model forced with limited scenario-driven projections of future climate for a relatively small number of lakes. As a result, our understanding of the effects of climate change on lakes is fragmentary, based on scattered studies using different data sources and modelling protocols, and mainly focused on individual lakes or lake regions. This has precluded identification of the main impacts of climate change on lakes at global and regional scales and has likely contributed to the lack of lake water quality considerations in policy-relevant documents, such as the Assessment Reports of the Intergovernmental Panel on Climate Change (IPCC). Here, we describe a simulation protocol developed by the Lake Sector of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) for simulating climate change impacts on lakes using an ensemble of lake models and climate change scenarios for ISIMIP phases 2 and 3. The protocol prescribes lake simulations driven by climate forcing from gridded observations and different Earth system models under various representative greenhouse gas concentration pathways (RCPs), all consistently bias-corrected on a 0.5∘ × 0.5∘ global grid. In ISIMIP phase 2, 11 lake models were forced with these data to project the thermal structure of 62 well-studied lakes where data were available for calibration under historical conditions, and using uncalibrated models for 17 500 lakes defined for all global grid cells containing lakes. In ISIMIP phase 3, this approach was expanded to consider more lakes, more models, and more processes. The ISIMIP Lake Sector is the largest international effort to project future water temperature, thermal structure, and ice phenology of lakes at local and global scales and paves the way for future simulations of the impacts of climate change on water quality and biogeochemistry in lakes.

We investigated radiatively driven under-ice convection in Lake Onego (Russia) during 3 consecutive late winters. In ice-covered lakes, where the temperature of water is below the temperature of maximum density, radiatively driven heating in the upper water column induces unstable density distributions leading to gravitational convection. In this work, we quantified the key parameters to characterise the radiatively driven under-ice convection: (1) the effective buoyancy flux, B∗ (driver), and its vertical distribution; (2) the convective mixed-layer thickness, hCML (depth scale); and (3) the convective velocity,w∗(kinematic scale). We compared analytical w∗ scaling estimates to in situ observations from high-resolution acoustic Doppler current profilers. The results show a robust correlation between w∗and the direct observations, except during the onset and decay of the solar radiation. Our results highlight the importance of accurately defining the upper limit of hCML in highly turbid water and the need for spectrally resolving solar radiation measurements and their attenuation for accurate B∗ estimates. Uncertainties in the different parameters were also investigated. We finally examined the implications of under-ice convection for the growth rate of nonmotile phytoplankton and provide a simple heuristic model as a function of easily measurable parameters.

Convection in an ice‐covered lake, driven by the absorption of solar radiation, is investigated by means of temperature microstructure technique. This type of convection typically occurs in spring, when melting snow on the ice cover enables solar radiation to penetrate into the water body. The diurnal dynamics of the stratification system of five distinct layers is analyzed by means of consecutive CTD profiles and with the aid of a one‐dimensional model. The model solves the transfer equation of heat and salinity and includes convective procedures to react on density instabilities. This study is focused on the turbulent kinetic energy (TKE) balance. The stratification analysis reveals the importance of several processes for the TKE balance, namely: (1) the entrainment into the top layer from the convective layer below, (2) the inflow of water from melted ice, and (3) the volumetric solar heating. Enabled by the analysis of the temperature microstructure profiles, two TKE budgets are presented. The temporally averaged budget reveals a vertical distribution of generation and dissipation rate similar to the case of cooling‐induced convection in a surface boundary layer. But contrary to this reference regime, a transition layer was found in the upper convective layer, where both rates drop back to zero toward the layer above. The second TKE budget is spatially averaged over the convective layer but resolves the diurnal dynamics. The generation rate and dissipation rate feature similar diurnal dynamics, where the dissipation lags on average by 1.5 hours. The temporal change rate of TKE was found to be on the same order of magnitude as the generation rate and the dissipation rate, while the export rate of TKE out of the convective layer was found to be less significant.

Abstract. The northern Eurasian regions and Arctic Ocean will very likely undergo substantial changes during the next decades. The Arctic–boreal natural environments play a crucial role in the global climate via albedo change, carbon sources and sinks as well as atmospheric aerosol production from biogenic volatile organic compounds. Furthermore, it is expected that global trade activities, demographic movement, and use of natural resources will be increasing in the Arctic regions. There is a need for a novel research approach, which not only identifies and tackles the relevant multi-disciplinary research questions, but also is able to make a holistic system analysis of the expected feedbacks. In this paper, we introduce the research agenda of the Pan-Eurasian Experiment (PEEX), a multi-scale, multi-disciplinary and international program started in 2012 (https://www.atm.helsinki.fi/peex/). PEEX sets a research approach by which large-scale research topics are investigated from a system perspective and which aims to fill the key gaps in our understanding of the feedbacks and interactions between the land–atmosphere–aquatic–society continuum in the northern Eurasian region. We introduce here the state of the art for the key topics in the PEEX research agenda and present the future prospects of the research, which we see relevant in this context.

This chapter summarises the climatic and environmental information that can be inferred from proxy archives over the past 12,000 years. The proxy archives from continental and lake sedimentsSediments include pollen, insect remnants and isotopic data. Over the Holocene, the Baltic Sea area underwent major changes due to two interrelated factors—melting of the Fennoscandian ice sheet (causing interplay between global sea-level rise due to the meltwater and regional isostatic rebound of the earth’s crust causing a drop in relative sea levelSea level ) and changes in the orbital configuration of the Earth (triggering the glacial to interglacial transition and affecting incoming solar radiationRadiation and so controlling the regional energy balance). The Holocene climate history showed three stages of natural climate oscillations in the Baltic Sea region: short-term cold episodes related to deglaciationDeglaciation during a stable positive temperature trend (11,000–8000 cal year BP); a warm and stable climate with air temperature 1.0–3.5 °C above modern levels (8000–4500 cal year BP), a decreasing temperatureTemperature trend; and increased climatic instability (last 5000–4500 years). The climatic variation during the Lateglacial and Holocene is reflected in the changing lake levels and vegetationVegetation , and in the formation of a complex hydrographical network that set the stage for the Medieval Warm Period and the Little Ice Age of the past millennium.

This article presents a review of the current data on the level of paleolimnological knowledge about lakes in the Russian part of the northern Eurasia. The results of investigation of the northwestern European part of Russia as the best paleolimnologically studied sector of the Russian north is presented in detail. The conditions of lacustrine sedimentation at the boundary between the Late Pleistocene and Holocene and the role of different external factors in formation of their chemical composition, including active volcanic activity and possible large meteorite impacts, are also discussed. The results of major paleoclimatic and paleoecological reconstructions in northern Siberia are presented. Particular attention is given to the databases of abiotic and biotic parameters of lake ecosystems as an important basis for quantitative reconstructions of climatic and ecological changes in the Late Pleistocene and Holocene. Keywords: paleolimnology, lakes, bottom sediments, northern

A field study on current structure and circulation characteristics in Lake Vendyurskoe, a small, shallow, icecovered lake in Karelia, Russia, is presented. The current velocity magnitudes were generally found to be small. The most pronounced currents had an oscillating character, with velocity amplitudes on the order of millimeters per second. The oscillation period, obtained from spectral density calculations, corresponded to that of a barotropic uninodal seiche. The seichelike nature of the current oscillations was supported by the results from analysis of icelevel fluctuations, giving identical periods and a phase shift of one‐fourth the period between the two types of oscillations. Mean currents measured during the winter were on the order of millimeters per second. Because Lake Vendyurskoe does not have any significant river inflow or outflow during winter, the most probable cause of these currents is horizontal temperature (pressure) gradients. Scaling analysis indicated that these currents are geostrophic. This was supported by theoretical estimates, based on observed horizontal temperature gradients, being of the same order as the observed currents. The mean current velocities increased considerably after spring convection from <1 to several millimeters per second.

Field experiments were conducted during two melting periods, April 2006 and April 2007, in Lake Vendyurskoe. The observation programme included weather, ice and snow thickness and structure, water temperature and solar radiation transfer through the ice. Albedo showed a systematic decrease from 0.5–0.8 for wintertime dry ice and snow to 0.1 for wet bare ice in spring, with spatial standard deviation of about 10%. The e-folding depth of light level was 60–80 cm for congelation ice and 15 cm for snow-ice. Light transmissivity of the ice cover increased from melting but decreased from ice deterioration; it varied between 0.25–0.35 in 2006, while in 2007 there was a systematic trend from 0.1 to 0.5 in six days. The heat budget was governed by net solar radiation with daily peaks up to 400–500 W m−2 on clear days. The average daily melt was 1.2 cm at the surface, 0.5 cm at the bottom and 1–2 cm (thickness equivalent) in the interior.

The quality of lake ice is of uppermost importance for ice safety and under-ice ecology, but its temporal and spatial variability is largely unknown. Here we conducted a coordinated lake ice quality sampling campaign across the Northern Hemisphere during one of the warmest winters since 1880 and show that lake ice during 2020/2021 commonly consisted of unstable white ice, at times contributing up to 100% to the total ice thickness. We observed that white ice increased over the winter season, becoming thickest and constituting the largest proportion of the ice layer towards the end of the ice cover season when fatal winter drownings occur most often and light limits the growth and reproduction of primary producers. We attribute the dominance of white ice before ice-off to air temperatures varying around the freezing point, a condition which occurs more frequently during warmer winters. Thus, under continued global warming, the prevalence of white ice is likely to substantially increase during the critical period before ice-off, for which we adjusted commonly used equations for human ice safety and light transmittance through ice.

During late winter (18 March–7 April 1994), temperature and current measurements were made in Lake Vendyurskoe, Russia, including three surveys at six cross sections of the lake. Also, the temperature profile evolution was registered with two thermistor chains at two stations (bottom depths of 7.6 and 11.5 m) until the time of ice breakup. Temperature gradients were measured just below the ice cover and in the upper 10‐cm layer of the bottom sediments. The isotherms were found to be almost horizontal and evenly spaced vertically, so no conditions for large‐scale, density‐induced currents existed. The heat flux from sediments to water ranged from 0.6 to 2.0 W m −2 . These values were inversely related to the depth. The heat flux from water to ice ranged from 0.7 to 1.2 W m −2 . When water heating from solar radiation penetration became apparent, this flux increased by a factor of two. When solar radiation increased, convection occurred in the upper layers of the water column. When solar radiation heating became significant at the beginning of spring, the average net heat flux at the ice‐water interface during daytime was 7.7 W m −2 . Weak currents (few mm s −1 ) with a seiche‐like character were observed, which most likely resulted from ice‐cover oscillations.

Lake Ladoga in northwestern Russia is Europe's largest lake. The postglacial history of the Ladoga basin is for the first time documented continuously with high temporal resolution in the upper 13.3 m of a sediment core (Co1309) from the northwestern part of the lake. We applied a multiproxy approach including radiographic imaging, (bio‐)geochemical and granulometric analyses. Age control was established combining radiocarbon dating with varve chronology, the latter anchored to a correlated radiocarbon age from a lake close by. The age‐depth model reveals the onset of glacial varve sedimentation at 13 910±140 cal. a BP , when Lake Ladoga was part of the Baltic Ice Lake. Linear extrapolation of published retreat rates of the Scandinavian Ice Sheet provides a formation age of the Luga moraine close to Lake Ladoga's southern shore of 14.5–15.9 cal. ka BP , older than previously assumed. Varve sedimentation covers the Bølling/Allerød interstadial, the Younger Dryas stadial and the Early Holocene. Varve‐thickness variations, conjoined with grain‐size and geochemical variations, inform about the relative position of the Scandinavian Ice Sheet and the climate during the deglaciation phase. The upper limit of the varved succession marks the change from glaciolacustrine to normal lacustrine sedimentation and post‐dates the drainage of the Baltic Ice Lake as well as the formation of the Salpausselkä II moraine north of Lake Ladoga, by c . 250 years. The Holocene sediment record is divided into three periods in the following order: (i) a lower transition zone between the Holocene boundary and c . 9.5 cal. ka BP , characterized by mostly massive sediments with low organic content, (ii) a phase with increased organic content from c . 9.5 to 4.5 cal. ka BP corresponding to the Holocene Thermal Maximum, and (iii) a phase with relatively stable sedimentation in a lacustrine environment from c . 4.5 cal. ka BP until present.

Among the numerous processes that govern the functioning of a lake ecosystem, the regime of dissolved oxygen (DO) is of primary importance. The DO content is strongly affected by the temperature regime, mixing conditions and by the duration of the ice-covered period. These are formed due to atmospheric forcing and are, therefore, subject to variations in regional climate. Despite the large amount of data revealing the physical effect on the biological and chemical regimes in lakes, there is still insufficient understanding, both qualitative and quantitative, of how a lake ecosystem would be affected by changes in the lake temperature and mixing conditions due to changes in the atmospheric forcing. Below, the study of shallow lakes’ response to climatic changes using the coupled FLake–FLakeEco modelling system is presented. The results obtained reveal the extreme vulnerability of the lakes’ ecosystems to changes in atmospheric forcing. In ‘future’ climate the permanent existence of potentially dangerous anaerobic zones in shallow lakes is expected. The projected decreased oxygen concentrations are caused by: (1) the reduced oxygen flux from the atmosphere to the lakes due to increased temperature; and (2) strengthened density stratification of the water columns which would prevent aeration of the near-bottom layers.

The statistical relationships between lake ice phenology (freeze and break-up dates, ice duration), air temperature and North Atlantic Oscillation (NAO) index are analysed for eight lakes in Karelia from 1950 to 2009. Linear trends over this time period are estimated. It is shown that in the last 20 years trends in the timing of ice phenomena are more evident than in the entire 60-year period. The statistical relationship between lake ice phenology and variability of regional air temperature is used in the empirical model to assess ice-related events on previously unstudied lakes in NW Russia.

A relationship between indirect chemical indices of organic matter content of surface waters and light absorption in the visible and ultraviolet ranges is discussed. The allochthonous and autochthonous types of organic matter are found to essentially differ in the rate of light absorption. Techniques are proposed for the calculation of the concentrations of allochthonous and autochthonous organic matter by the integral rate of light consumption in the visible region and the bichromate oxidability of water, as well as by three indirect indices: chemical oxygen demand, permanganate oxidability, and water color index. The mean concentrations of allochthonous and autochthonous organic matter in large lakes (Ladoga, Onega, and Baikal) and in rivers and smaller water bodies in Karelia (>300 water objects) are analyzed. Allochthonous organic matter was found to predominate in most surface waters of Karelia (>80%), while allochthonous organic matter predominates only in lakes with a small specific catchment area (supposedly, <5) and in highly eutrophic lakes.

This study investigated climate change impacts on watershed parameters as well as on water balance, water temperatures, and ice-cover regime for 2 large lakes in western Russia, Lakes Ladoga and Onego. Parts of the study occurred during a period of pronounced warming and limited ice cover on Lake Ladoga, which precluded winter surveys of the lake. This limitation notwithstanding, the research provides background climatic and hydrological conditions for the joint Russian–Swiss project “Lake Ladoga – Life under the ice.” For both Lakes Onego and Ladoga from 1955 to 2017, air temperature, precipitation, and evaporation data all showed increasing trends, and inflow, outflow, and water levels showed no discernible changes from baseline conditions. Thermal and ice conditions of Lake Onego showed greater sensitivity to climatic changes than corresponding conditions for Lake Ladoga because of the latter’s significantly smaller volume and lower heat content. The total ice-cover duration for Lake Onego decreased ∼20 days during the observation period. Detailed data analysis also revealed key characteristics of the Shuya River discharge into Petrozavodsk Bay, where a major part of the fieldwork occurred. River discharge into Lakes Ladoga and Onego increased in the winter. These processes collectively can change the hydrochemical conditions, water quality, and habitat.

The results of studies of water chemistry carried out in the water body in 1965–2009 are presented. This period features a considerable industrial and agricultural rise in the lake drainage basin, followed by a decline since the 1990s, manifesting themselves in the dynamics of some hydrochemical characteristics of water bodies. The multipurpose use of Onega Lake is shown to cause its pollution and eutrophication. The results of studies are used to evaluate the major components of the external load onto the lake and trends in variations of chemical characteristics under the effect of natural and anthropogenic factors.

Oxygen conditions in ice-covered lakes depend on many factors, which, in turn, are influenced by a changing climate, so detection of the oxygen trend becomes difficult. Our research was based on data of long-term measurements of dissolved oxygen (2007–2020), water temperature, under-ice solar radiation, and snow-ice thickness (1995–2020) in Lake Vendyurskoe (Northwestern Russia). Changes of air temperature and precipitation in the study region during 1994–2020 and ice phenology of Lake Vendyurskoe for the same period based on field data and FLake model calculations were analyzed. The interannual variability of ice-on and ice-off dates covered wide time intervals (5 and 3 weeks, respectively), but no significant trends were revealed. In years with early ice-on, oxygen content decreased by more than 50% by the end of winter. In years with late ice-on and intermediate ice-off, the oxygen decrease was less than 40%. A significant negative trend was revealed for snow-ice cover thickness in spring. A climatic decrease of snow-ice cover thickness contributes to the rise of under-ice irradiance and earlier onset of under-ice convection. In years with early and long convection, an increase in oxygen content by 10–15% was observed at the end of the ice-covered period, presumably due to photosynthesis of phytoplankton.

Abstract Ground ice and sedimentary records of a pingo exposure reveal insights into Holocene permafrost, landscape and climate dynamics. Early to mid‐Holocene thermokarst lake deposits contain rich floral and faunal paleoassemblages, which indicate lake shrinkage and decreasing summer temperatures (chironomid‐based T July ) from 10.5 to 3.5 cal kyr BP with the warmest period between 10.5 and 8 cal kyr BP. Talik refreezing and pingo growth started about 3.5 cal kyr BP after disappearance of the lake. The isotopic composition of the pingo ice (δ 18 O − 17.1 ± 0.6‰, δD −144.5 ± 3.4‰, slope 5.85, deuterium excess −7.7± 1.5‰) point to the initial stage of closed‐system freezing captured in the record. A differing isotopic composition within the massive ice body was found (δ 18 O − 21.3 ± 1.4‰, δD −165 ± 11.5‰, slope 8.13, deuterium excess 4.9± 3.2‰), probably related to the infill of dilation cracks by surface water with quasi‐meteoric signature. Currently inactive syngenetic ice wedges formed in the thermokarst basin after lake drainage. The pingo preserves traces of permafrost response to climate variations in terms of ground‐ice degradation (thermokarst) during the early and mid‐Holocene, and aggradation (wedge‐ice and pingo‐ice growth) during the late Holocene.