Georgiy Kirillin

Physics of seasonally ice-covered lakes: a review

Recently, the attention to the ice season in lakes has been growing remarkably amongst limnological communities, in particular, due to interest in the response of mid- and high-latitude lakes to global warming. We review the present advances in understanding the governing physical processes in seasonally ice-covered lakes. Emphasis is placed on the general description of the main physical mechanisms that distinguish the ice-covered season from open water conditions. Physical properties of both ice cover and ice-covered water column are considered. For the former, growth and decay of the seasonal ice, its structure, mechanical and optical properties are discussed. The latter subject deals with circulation and mixing under ice. The relative contribution of the two major circulation drivers, namely heat release from sediment and solar heating, is used for classifying the typical circulation and mixing patterns under ice. In order to provide a physical basis for lake ice phenology, the heat transfer processes related to formation and melting of the seasonal ice cover are discussed in a separate section. Since the ice-covered period in lakes remains poorly investigated to date, this review aims at elaborating an effective strategy for future research based on modern field and modeling methods.

Physical background of the development of oxygen depletion in ice-covered lakes

The effect of the heat interaction between a water column and sediments on the formation, development, and duration of existence of anaerobic zones in ice-covered lakes is estimated based on observational data from five frozen lakes located in northwestern Russia and North America. A simple one-dimensional model that describes the formation and development of the dissolved oxygen deficit in shallow ice-covered lakes is suggested. The model reproduces the main features of dissolved oxygen dynamics during the ice-covered period; that is, the vertical structure, the thickness, and the rate of increase of the anaerobic zone in bottom layers. The model was verified against observational data. The results from the verification show that the model adequately describes the dissolved oxygen dynamics in winter. The consumption rates of DO by bacterial plankton and by bottom sediments, which depend on the heat transfer through the water–sediment interface, are calculated. The results obtained allow the appearance of potentially dangerous anaerobic zones in shallow lakes and in separate lake areas, which result from thermal regime changes, to be predicted.

Software, Data and Modelling News: FLake-Global: Online lake model with worldwide coverage

An online lake-modeling tool FLake-Global allows virtually instantaneous estimation of the seasonal cycle of temperature and mixing conditions in any shallow freshwater lake around the world. The tool is built on the basis of the lake model FLake (www.lakemodel.net) and a year- long near-surface meteorological data from the NCEP/NCAR Reanalysis Project (http://www.esrl.noaa.gov/psd/data/reanalysis). FLake-Global gives estimates of the surface and bottom water temperatures, mean temperature of the water column, surface mixed layer depth, and ice cover duration and thickness, using geographical coordinates, mean depth to the bottom and water transparency of the lake in question, and the NCEP atmospheric forcing as input. The tool is suitable for a wide spectrum of applications, including general limnology, lake management and restoration, fisheries, and recreation.

Some features of the thermal and dissolved oxygen structure in boreal, shallow ice-covered Lake Vendyurskoe, Russia

The 5-year-long (2001–2005) studies of the winter thermal structure and the dissolved oxygen (DO) dynamics in Lake Vendyurskoe, Russia, a typical boreal shallow mesotrophic lake of glacial origin, revealed still poorly studied features of lake-wide dynamics, such as net lateral heat flux towards deeper parts of a lake and development of the anaerobic zone over the deepest points of the lake basin. We estimated magnitude of the heat transport along the bottom slope based on scaling analysis. The seasonal changes in DO concentration appear to be controlled mostly by biochemical consumption. We identify four factors controlling the extent of anoxic zones in shallow ice-covered lakes: (1) the amount of organic matter stored in the bottom layers, including the sediments surface during the autumnal bloom; (2) the length of the ice-covered period; (3) heat content of bottom sediments; and (4) the initial water temperatures at the time of the ice cover formation.

A parameterized model of heat storage by lake sediments

A model of seasonal heat storage by lake sediments is proposed oriented at applications in climate modeling and at lake parameterization in numerical weather prediction. The computational efficiency is achieved by reformulating of the heat transfer problem as a set of ordinary differential equations for evolution of the temperature wave inside the upper sediment layer. Arising temperature and depth scales completely replace the conductivity of the sediment in the heat transfer equation and can be easily achieved from the lake water temperature observations without any data on the sediment thermal properties. The method is proposed for the scales estimation from the inverse solution of the model equations in special case of the constant water-sediment heat flux in ice-covered lakes. The method is tested on data from sediments of Lake Krasnoye, North-Western Russia. The long-term (1961-2002) modeling of temperature in German lakes Muggelsee and Heiligensee with a coupled one-dimensional model of lake water column and sediments has demonstrated an appreciable effect of the sediment heat storage on near-bottom temperatures in both lakes. Thus, incorporation of the sediment layer into lake temperature models can essentially improve, at low computational costs, the model performance, especially for shallow lakes. In addition, a better forecast of near-bottom temperature evolution on climatic scales can provide a better understanding of the response of lake benthic communities to global warming.

Lake ice phenology in Berlin-Brandenburg from 1947–2007: observations and model hindcasts

Rising northern hemispheric mean air temperatures reduce the amount of winter lake ice. These changes in lake ice cover must be understood in terms of resulting effects on lake ecosystems. Accurate predictions of lake ice phenology are essential to assess resulting impact. We applied the one-dimensional physical lake model FLake to analyse past variability in ice cover timing, intensity and duration of Berlin-Brandenburg lakes. The observed ice phenology in two lakes in the period 1961–2007 was reconstructed by FLake reasonably well and with higher accuracy than by state-of-the-art linear regression models. Additional modelling results of FLake for 38 Berlin-Brandenburg lakes, observed in the winter of 2008/09, were quite satisfactory and adequately reproduced the effects of varying lake morphology and trophic state. Observations and model results showed that deeper and clearer lakes had more ice-free winters, later ice cover freezing and earlier ice cover thawing dates, resulting in shorter ice-covered periods and fewer ice-covered days than shallow and less clear lakes. The 1947–2007 model hindcasts were implemented using FLake for eight Berlin-Brandenburg lakes without ice phenology observations. Results demonstrated past trends of later ice start and earlier ice end, shorter ice cover duration and an increase in ice-free winters.

Basin-scale internal waves in the bottom boundary layer of ice-covered Lake Müggelsee, Germany

We performed high-resolution temperature measurements under ice cover in Lake Müggelsee, Germany, during the winter of 2005–2006. Intense seiche-like temperature oscillations developing after the ice-on have been encountered in a thin water layer above the sediments. The oscillations were initiated immediately after lake freezing by the release of the potential energy of the thermocline slope and existed for several weeks without appreciable external forcing. The oscillations were associated with a basin-scale internal waves existing in the lower stratified part of the water column. The weakness of the density stratification under ice ensured the long wave periods, exceeding the period of geostrophic inertial oscillations at the lake’s latitude. As a result, two frequency peaks were present in the oscillations corresponding to two rotational waves, one of Kelvin-wave type and another of Poincaré type wave. The rotational character provided long dissipation times of the waves and allowed the oscillations to persist in lake several weeks. Temperature measurements in the upper several centimeters of the sediment demonstrated that oscillations of the near-bottom temperature produced vertical density instability and pore-water convection in the upper sediments.

Seeking a compromise between pharmaceutical pollution and phosphorus load: Management strategies for Lake Tegel, Berlin.

Lake Tegel (Berlin, Germany) is controlled by two main inflows: inflow #1 (River Havel) is heavily phosphorus-laden, whereas inflow #2 is an artificial confluence that includes discharge from a municipal wastewater treatment plant distinguished by high levels of phosphorus and pharmaceuticals. To reduce the phosphorus load on the lake, a phosphorus elimination plant (PEP) is situated at inflow #2. Moreover, the two inflows are short-circuited by a pipeline that transfers part of the inflow #1 water to the PEP and finally releases it into inflow #2. The pipeline and the PEP have contributed to a continuous reduction in the total phosphorus concentration of Lake Tegel in the past 25 years. We investigate the question of whether the existing lake pipeline can also be used to reduce the amount of pharmaceuticals in Lake Tegel originating from inflow #2 by dilution with water from River Havel, by diverting part of inflow #2 around the lake, or by a combination of both strategies. The circulation pattern of Lake Tegel is complicated by complex bathymetry and numerous islands and is therefore highly sensitive to winds. We tested seven different management scenarios by hydrodynamic modeling for a period of 16 years with the two-dimensional version of the Princeton Ocean Model (POM). None of the scenarios provided a strategy optimal for both pharmaceuticals and phosphorus. Nonetheless, compound regimes, such as alternating the pipe flow direction or adding another pipeline, allowed the most abundant pharmaceutical (carbamazepine) to be reduced while maintaining the current phosphorus level. This study demonstrates the ability of immediate lake regulation measures to maintain water quality. In the case of Lake Tegel, the pipeline can be fully effective with regard to pharmaceuticals only in combination with additional efforts such as advanced pharmaceutical treatment of wastewater and/or phosphorus reduction in the River Havel catchment.

Axisymmetric circulation driven by marginal heating in ice‐covered lakes

Below the temperature of maximum density (TMD) in freshwater lakes, heating at the lateral margins produces gravity currents along the bottom slope, akin to katabatic winds in the atmosphere and currents on continental shelves. We describe axisymmetric basin-scale circulation driven by heat flux at the shorelines in polar Lake Kilpisjarvi. A dense underflow originating near the shore converges toward the lake center, where it produces warm upwelling and return flow across the bulk of lake water column. The return flow, being subject to Coriolis force, creates a lake-wide anticyclonic gyre with velocities of 2–4 cm s-1. While warm underflows are common on ice-covered lakes, the key finding is the basin-scale anticyclonic gyre with warm upwelling in the core. This circulation mechanism provides a key to understanding transport processes in (semi) enclosed basins subject to negative buoyancy flux due to heating (or cooling at temperatures above TMD) at their lateral boundaries.

Present state of the Aral Sea: diverging physical and biological characteristics of the residual basins.

Latest data on the hydrophysical and biological state of the residual basins of the Aral Sea are presented and compared. Direct, quasi-simultaneous observations were carried out in the central part of the Western Large Aral Sea, the northern extremity of the Large Aral known as Chernyshev Bay, Lake Tshchebas, and the Small Aral Sea in October 2014. The Large Aral Sea and Lake Tshchebas transformed into hyperhaline water bodies with highly special taxocene structure. The Small Aral Sea was a relatively diverse brackish ecosystem, which was rather similar to the pre-desiccation environment. The Small Aral Sea and Lake Tshchebas exhibited a fully-mixed vertical structure, whereas the Western Large Aral Sea was strongly stratified. Our data show that during desiccation, different parts of the Aral Sea experienced different environmental conditions, resulting in qualitative and quantitative differences in the physical and biological regimes among the different residual basins.