Highlights of the project results
Dynamics of water temperature (upper) and dissolved oxygen (lower) vertical structure
in winter seasons with "average" (left) and late (right) dates of ice cover formation. Depth axis is directed upward. |
Open-water weather conditions preceded ice cover formation are considered with regard to its effect
on further dynamics of water temperature and dissolved oxygen (DO) under ice.
Later ice formation leads to the partial decay of heat stored in bottom sediments invoking lesser warming of a water column.
Also, bacterial destruction of newly formed organic matter starting well before the ice cover forms, may "improve'
the DO regime in bottom layers due to lesser DO depletion.
Publications: |
Vertical patterns of photosynthetically active radiation (PAR) in lake Vendyurskoe and in the Petrozavodsk Bay (guba) of Lake Onego in April 2012 (upper panel), and ice conditions during the measurement period (lower panel).
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For the first time, observational data on dynamics of vertical PAR distribution in water of ice-covered lakes are obtained. High-quality data (registration one minute apart for a period 12-24.04.2012 in Lake Vendyurskoe) allow to reliably evaluate extinction coefficient values. Preliminary analysis of PAR data together with that on dynamics of vertical chlorophyll-a profiles in ice-covered lake Vendyurskoe revealed that algae developed during under-ice warming may decrease "winter" transparency in upper layers of a water column (shadowing effect).
Publications: |
Convective velocities in the vicinity of the lower boundary of convectively mixed layer CML (A, C) and in its middle (B, D) under different values of the extinction coefficient subject to the flux of solar radiation at the lower boundary of ice. Vertical axis - kinematic heat flux, horizontal axis - velocity. Calculations are based on results of observations in 1995-2012. |
Based on observational data from 1995-2012 on solar radiation at the lower ice boundary, vertical profiles of water temperature and chlorophyll-a for four groups of phytoplankton (greens,
blue-greens, diatoms and cryptophytes), and on previously received results of large-eddy simulations (Mironov eta al. 2002), a hypothesis considering the effect of convection on algae suspension within
CML has been verified. The estimates of vertical (updraught) convective velocities in the vicinity of the lower boundary of CML and in its middle are obtained from observational data on solar radiation and water temperature profiles.
Their values are higher than those of gravitational settling of algae cells for one-two orders of magnitude.
Publications: |
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A.
 B. (A)Ice/snow thickness and water temperature in Lake Mueggelsee, Germany in winter 2005/2006.
Red rectangle in the near-bottom area marks the period of strong internal waves at the beginning stage of the ice-covered period.
(B) Under-ice water temperatures in Lake Paajarvi in March 2009 showing a signature of the 3-rd vertical ("4-layered") internal wave mode.
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The first quantitative description is achieved for the seasonal variability of the internal basin-scale internal waves in ice-covered lakes.
The waves under ice have much larger periods and amplitudes than in the open-surface conditions (conditioned by weak density stratification) and,
therefore, are strongly affected by the Earth rotation. At the beginning of the ice-covered period, the waves appear
at the density interface near bottom (Fig. 1). In deeper lakes, internal waves exist longer. Their multilayer vertical structure
is maintained by combined effects of the heat flux from sediment and solar heating (Fig. 2).
Publications:G. Kirillin, C. Engelhardt, S. Golosov et al. (20100 Seiches in ice-covered lakes, in: M. Leppäranta (Ed.), Proc. 20th IAHR Int Symp on Ice, p. 140
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 Bottom: fine-scale data on the dissipation rates in the lower 1m of the water column measured with a hi-res acoustic doppler profiler during the same period. |
The insight into the anatomy of turbulent mixing under ice is achieved at the unpreceded level of resolution.
First direct measurements of the shear microstructure across the water column under ice provided the information on the energy budget of convective mixing,
mixing variability within the diurnal radiation cycle and maximum intensity of convection at the moment of ice breakup. High-resolution acoustic velocity
profiling in the bottom boundary layer and in the ice boundary layer provided high-resolution data on the fine structure of the boundary layer turbulence.
The results have disclosed an important role of the lateral density variations within a lake in production of the boundary currents and turbulence.
The information achieved on the mixing energy budget allows further estimation of the role played by turbulence in the water-sediment mass transport and ice melting at the final stage of the ice-covered period.
Publications:Kirillin G., M. Leppäranta, A. Terzhevik, et al. 2012 Physics of seasonally ice-covered lakes: A review // Aquat. Sci.74: 659-682
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Vertical patterns of chlorophyll-a (Chl-a) (for four groups and its total content) in Lake Vendyurskoe in April 2011. Horizontal axis - time; vertical axis: left panel - depth, m; right panel - non-dimensional depth (each reading divided by the CML lower depth). Chl-a in the right panel is presented in a non-dimensional form (each profile is normalised by its maximum value). Red lines on graphs: Left panel - the lower CML depth; right panel - the lower euphotic depth.
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Observational data on dynamics of vertical Chl-a distribution in water reveal that the algae development under ice takes place mostly within CML, thus confirming the hypothesis on algae suspension by the under-ice convection. Presence of algae up to the lower boundary of the euphotic layer can likely be explained by settling in night hours when convection is suppressed.
Publications: |
 Upper panel: Effect of RTI mechanism on heat transfer and water temperature dynamics in the benthic boundary layer BBL during the period 11-31.12.2009. Upper curves correspond to the distance of 8 cm from the sediment surface. Lower panel: Effect of RTI mechanism on mass transfer and dissolved oxygen (DO) concentration dynamics in the benthic boundary layer BBL during the period 11-31.12.2009. Upper curves correspond to the distance of 66 cm from the sediment surface. Horizontal axis - simulation time, min, starting from 11.12.2009, 0 h, 0 min.
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It is found that an important role in formation of the bottom mixed layer with corresponding fluctuations of water temperature and DO concentration is played by Rayleigh–Taylor instability (RTI), which develops due to the effect of chemical and biochemical processes in the vicinity of the water-sediment interface. This mechanism, if invoked, provides effective DO transfer from upper layers to the interface. At the expense of high gradient maintenance in DO concentration between near-bottom water and that transported, the DO accelerated consumption by bacterioplankton and bottom sediment is supported. The RTI model developed can be used to evaluate heat and DO budgets in ice-covered lakes.
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