THE OXYGEN REGIME OF A SHALLOW LAKE

The year-round measurement data of water temperature and dissolved oxygen content in a small boreal Lake Vendyurskoe in 2007–2013 were used to explore the hydrophysical prerequisits of anoxia and accumulation and emission of greenhouse gases. Typically, anoxia appears in the bottom layers of lakes in mid-winter and during the summer stagnation. The thickness of the benthic anaerobic zone (dissolved oxygen concentration <2 mg·l–1) reached one meter in the end of the winter and at the peak of the summer stratification, except for the extremely hot summer of 2010, when it reached five meters. Synoptic conditions had a crucial influence on the formation and destruction of the benthic anaerobic zones in summer. The most favorable oxygen dynamics was observed during the cold summers of 2008, 2009, and 2012, when the repeated full mixings of the water column occurred under conditions of the cyclonic weather. In the winter periods, the early dates of ice season resulted in the most pronounced deficiency of oxygen.


INTRODUCTION
Dissolved oxygen (DO) is one of the most important parameters in the lake ecosystem.Oxygen depletion can cause large changes in the population of fish, leading, in extreme cases, to the massive fish kill [Greenbank, 1945;Barica & Mathias, 1979].The lack of DO leads to the activation of anaerobic processes and to accumulation of harmful greenhouse gases such as carbon dioxide (CO 2 ) and methane (CH 4 ) in the bottom layers of the water column and upper bottom sediments [Golosov et al., 2007;McGinnis et al., 2015].The significant emission of such gases can take place after the full mixing of the water column, for example, as a result of the ice breakup or after erosion of the seasonal thermocline at the end of the summer [Lohila et al., 2015].Anaerobic conditions occur in the bottom layers of mesotrophic and eutrophic boreal lakes during summer thermal stratification due to insufficient aeration of the water column [Efremova et al., 2015].In the winter periods, the main cause of the oxygen reduction in shallow lakes is the absence of the gas exchange with the atmosphere and the suppression of photosynthesis.The rate of DO reduction is determined by the intensity of its diffusion in sediments and bacterial consumption in the water column [Hargrave, 1972;Boylen & Brock, 1973;Mathias & Barica, 1980;Brekhovskih, 1988].The amount of organic matter, initial water temperature at the start of the ice season, and heat flux from the sediments are the factors that have the major influence on the rate of oxygen reduction in a lake during winter [Terzhevik et al., 2009].In addition, hydrodynamic processes, such as geostrophic circulation, non-linear internal waves, seiches, and seiche-induced convection in upper sediments, can have a significant impact on the rate of oxygen reduction [Mackenthun & Stefan, 1998 To date, the seasonal oxygen regime of the shallow boreal lakes is described in general [Golosov et al., 2007[Golosov et al., , 2012;;Terzhevik et al., 2009Terzhevik et al., , 2010;;Zdorovennov et al., 2011]; however, many aspects of its dynamics at a finer scale remain poorly understood.
The aim of the current study is to investigate the thermal and oxygen seasonal dynamics of a shallow boreal lake from the standpoint of the development of favourable conditions for the accumulation and release of greenhouse gases, including anoxia in near-bottom waters as a prerequisite.

MATERIALS AND METHODS
A small mesotrophic Lake Vendyurskoe (southern part of Karelia, Russia, 62°10'N, 33°10'E) was chosen as the object of our study.It is a shallow polymictic lake (surface area 10.4 km 2 , volume 54.8•10 6 m 3 , maximal and mean depths 13.4 and 5.3 m, respectively) (Fig. 1), a typical representative of the lakes of water-glacial origin widespread in North-Western Russia and Fennoscandia [Terzhevik et al., 2010].Lake Vendyurskoe is a popular site for fishing used for fish farming and recreational purposes.
The climate of the study area can be characterized as temperate continental with some marine features.Winters are long and relatively mild, and summers are short and cool.The weather during the year is highly variable due to the frequent cyclones approaching the area from the west.Cloudy weather occurs on more than half of the days each year.In winter, thawing often occurs, and in spring and sometimes in early summer, short frosty periods are normal.
Previous studies of the oxygen regime of the lake were carried out in 2000-2006 based on the field measurements of DO content made in the vertical sounding regime at 22 stations in April and October.Estimates of winter DO consumption rate were obtained [Terzhevik et al., 2009[Terzhevik et al., , 2010]].However, such sporadic surveys could not describe the annual course of DO dynamics.
The year-round measurements with 1-min intervals during 2007-2013 allowed filling this gap.We measured the water temperature and DO content within the water column using chains equipped with sensors TR-1060 (range -5…+35 °C, accuracy ± 0.002 °C, resolution < 0.00005 °C) and DO-1050 (range 0-150 %, accuracy ±1 %FS) (RBR Ltd., Canada), which were spaced on a rope at the intervals of 0.5-1 m, the top sensor being located at the depth of 2.5 m.The rope was stretched between the anchor and the buoy.

RESULTS AND DISCUSSION
The seasonal dynamics of the thermal regime of Lake Vendyurskoe included spring and summer heating (2.5-3.5 months), autumn cooling (3-4.5 months), winter heating (5-6 months) and spring under-ice convection (one month).The water column was inversely stratified during winter with temperature and density increasing with depth, it was homothermal after the ice breakup and during autumn cooling, and it was unstable or had a mild thermal stratification during summer heating.
Ice season.
where С is DO concentration, γ is the total rate of oxygen consumption; [γ] = s -1 , t time.This equation has an analytical solution: where С 0 is the initial DO concentration at the very beginning of the ice season.
Evolution of relation C t /C 0 shows that DO concentration in the lake decreased by more than one-third during the winter, compared with the initial value (Fig. 2A).The reduction of oxygen concentration lasted from 140 to 170 days every winter.The maximal values of the total rate of oxygen consumption γ reached 0.5-1.5•10 - s -1 in the first week of ice period and then rapidly dropped, not exceeding 0.5•10 -8 s -1 from the third month until the end of winter (Fig. 2B).The greatest variation in the range of γ was observed in the first month of ice period, while later the rate of oxygen consumption became comparable across different years.As shown previously [Golosov et al., 2007[Golosov et al., , 2012 Thus, anaerobic conditions may exist in the bottom layers of Lake Vendyurskoe for fourfive months of each winter, and the probability of a significant accumulation of greenhouse gases in the bottom layers of the lake at the end of winter is high.A decrease of the icecovered period leads to the less pronounced deficiency of oxygen in the bottom layers of lakes and a reduced risk of large greenhouse gases emissions after the ice breakup.
Ice breakup lasts for several days accompanied with a complete mixing of the water column and vertical uniform distribution of the water temperature and dissolved oxygen content [Gavrilenko et al., 2014].The near-bottom anaerobic zone usually disappears after the ice breakup; at this moment, a substantial release of greenhouse gases into the atmosphere may occur.
Measurements carried out in small and medium-size boreal lakes show that concentration of greenhouse gases changes significantly during the ice-covered period.Greenhouse gas concentrations change little in the under-ice layer, while in the bottom layers there is rapid growth at the beginning of winter, taking place against the backdrop of a sharp reduction of oxygen [  (Fig. 3).The thickness of the anaerobic zone did not exceed one meter, and its lifespan was not more than two-three weeks.A danger of the accumulation of greenhouse gases was minimal.
In contrast, a pronounced thermal stratification occurred during a period of hot windless weather in July and early August of 2010, 2011 and 2013 (Fig. 3).
The most dramatic situation was observed in the summer of 2010 as a result of the stationary anticyclone which blocked the western transfer of air masses [Trenberth & Fasullo, 2012 At the stage of the autumn cooling the lake is nearly homothermal; the water temperature gradually decreases, and the oxygen content increases with the growth of its solubility.At this stage, the oxygen saturation occurs at the bottom layers, and there is no risk of the accumulation of greenhouse gases.

CONCLUSIONS
Throughout the year, the ice season and long periods of warm windless weather during summer are potentially dangerous in terms of the accumulation of greenhouse gases.The greatest risk of greenhouse gas emissions exists for several days after the ice breakup and at the time of the erosion of the seasonal thermocline at the end of the summer.The observational data suggest that oxygen consumption is much faster in summer than in winter.This is evidenced by a more rapid increase in the thickness of the anaerobic zone in the bottom layers: just one month of summer thermal stratification leads to anaerobic zone thickness greater than three meters, while the five months of winter stagnation translate into the onemeter anaerobic zone.Consequently, the risk of accumulation of the large amounts of greenhouse gases in summer is also pronounced, even when the periods of hot windless weather are short.At the same time, cold summers are favorable for the oxygen saturation of the water column and do not present a risk in terms of the accumulation of greenhouse gases in the lake.

Fig. 1 .
Fig. 1.Scheme of Lake Vendyurskoe with the location of the measurement sites (circles).

;
Terzhevik et al., 2009Terzhevik et al., , 2010]], the rate of oxygen consumption is determined by several factors, i.e. (1) the initial freezing temperature of the lake, (2) the magnitude of the heat flux from the sediments and (3) the amount of the organic matter accumulated in waters of the lake.Our estimates of γ for winters of 2007-2013 are in a good agreement with the estimates done for the winter seasons of2001-2002and 2003 -2006[Terzhevik et al., 2010]].
[Zdorovennov et al., 2013]November to early December and ended in the first half of May, thus lasting five to six months[Zdorovennov et al., 2013].The bottom water temperature in the central part of the lake rose to 4.5-5.5 °C by the end of winter[Zdorovennov et al.
[Terzhevik et al., 2010rate of DO consumption γ for the winter seasons of 2007-2013 were calculated in accordance with the approach outlined in[Terzhevik et al., 2010]: