Structure of the ice cover

In the years 2007–13, the ice phenomena usually occurred between 17 November and 15 May on Morskie Oko lake, while the ice cover occurred between 26 November and 3 May. Ice-cover thickness varied from 0.40 to 1.14 m, with a maximum yearly mean of 0.67 m. The ice structure was often multilayered, consisting of two layers of ice between which was a layer of water and/or slush (water with snow crystals). The bottom layer of ice was usually thicker than the upper layer.

The formation of such a cover is connected with several-day-long warmer periods (usually in the second phase of winter) when the ice surface or the snow covering it may partly melt. Rainfall may also occur. Water gathering on the surface begins to refreeze, with the temperatures below zero, but depending on the intensity of snowfall it may be to a different extent ‘saturated’ (pervaded), which is why it takes the form of slush (snow mud). In its final phase, winter is usually too mild to freeze the whole layer of water, so multilayered ice is formed, i.e. ‘inter-water’ ice in various stages of water crystallization (Reference Choiński, Kolendowicz, Pociask-Karteczka and SobkowiakChoiński and others, 2010; Reference Pociask-Karteczka and ChoińskiPociask-Karteczka and Choiński, 2012). Other studies indicate weather conditions as the main cause of the multilayered structure of the ice cover (e.g. Reference Catalan, Ballesteros, Camarero, Felip and GaciaCatalan and others, 1992; Reference Duguay, Flato, Jeffries, Ménard, Morris and RouseDuguay and others, 2003).

Physical and chemical properties of ice, snow and water in lake Morskie Oko

The salt content in the structure of ice measured as electrical conductivity of water from the melted ice cover (upper, middle and bottom parts) was quite diversified in the years 2007–13, ranging from 7 to 38 μS cm⁻¹, and mostly did not exceed 25 μS cm⁻¹, whereas the electrical conductivity of lake water ranged from 18 to 29 μS cm⁻¹. The highest values of electrical conductivity in relation to the bottom, middle and upper parts of the ice cover were observed in 2012 and were 36, 27 and 24 μS cm⁻¹ respectively. The lowest values for all three layers were observed in 2010, falling below 16 μS cm⁻¹. The analysis of results in relation to the profiles showed higher pollution in the bottom layer of ice than in the upper part. Conductivity values measured in the Morskie Oko lake ice cover are similar to other high-mountain lakes (Reference Lepparanta, Tikkanen and VirkanenLepparanta and others, 2003), and indicate a contribution of impurities. This can be attributed to processes associated with ice-cover formation (Reference Taguchi, Guo and YamaguchiTaguchi and others, 2001; Reference Luo, Chen and HanLuo and others, 2010), as well as during the cover persistence (Reference Bordonsky and KrylovBordonsky and Krylov, 2000; Reference KangKang, 2005).

Analysis of the pH of water from the melted ice cover showed values in the range 6.2–6.9, and no relationship was observed between the pH and the location of the layer from which the ice sample was collected. Considering the fact that pH is the resultant of the presence of acidifying and alkalizing ions, as a well as the content of dissolved CO₂, one may assume that the influence of external factors does not contribute significantly to changes in the pH of the ice cover.

In water from the melted ice cover, chloride (max 69%) and sulphate (max 51%) ions dominate among the anions, and ammonium (max 66%) and calcium (max 78%) ions among the cations. The lowest contributors to the ionic balance are fluoride and magnesium ions (Table 1). In the upper layer of ice, the highest concentrations were mainly those of chlorides (in six cases out of nine) and sulphates (three cases out of nine), while nitrate ions reached lower concentrations (Fig. 3). In the middle and bottom layers of the ice cover, the highest concentrations were observed mainly for sulphates, in six out of nine and six out of ten cases respectively (Figs 4 and 5). In samples from these layers, increased concentrations were also measured for nitrate anions (maximum 5.99 and 6.98 mg L⁻¹ respectively), which was not observed in the upper part of the ice cover (maximum 2.18 mg L⁻¹). The highest concentrations of cations occurred in the case of calcium ions in the bottom layer of the ice (median 1.02 mg L⁻¹), and lowest concentrations in the upper part (median 0.47 mg L⁻¹). At the same time, measured values occurred in the broad range between 0.06 mg L⁻¹ (2007) and 6.93 mg L⁻¹ (2012). The maximum concentration of ammonium ions (1.81 mg L⁻¹) was observed in the upper layer. Nevertheless, in this layer, average concentration values were the lowest in the whole profile (median 0.41 mg L⁻¹). The contribution of ammonium ions was the highest in the bottom layer (median 0.72 mg L⁻¹). Sodium ions occurred mostly on a similar level of concentration regardless of the layer from which the ice sample was collected, which is confirmed by similar median values ranging between 0.20 and 0.30 mg L⁻¹. A similar distribution of pollutants in a profile was also observed in the case of potassium and magnesium ions, but their concentrations were lower. The medians ranged between 0.12 and 0.19 mg L⁻¹ and between 0.03 and 0.05 mg L⁻¹ respectively (Table 1; Figs 3–5). It should be noted that the penetration of ions in the structure of ice strictly depends on the borderline conditions (Reference Bryk and HaymetBryk and Haymet, 2004). During 7 years of research, different atmospheric conditions have been observed, marked by varying ice structure, so conclusions should be formulated with particular care. Nevertheless, a few studies worldwide indicate similar results (Reference CatalanCatalan, 1989; Reference Bordonsky and KrylovBordonsky and Krylov, 2000; Reference Lepparanta, Tikkanen and VirkanenLepparanta and others, 2003).

Table 1.

Results of investigation of samples (snow and ice covers, water) from Morskie Oko lake in years 2007–13 (mg L⁻¹) (relative standard deviation <5%)

Fig. 3.

Chemical characteristics of Morskie Oko lake ice cover (top ice) in the period 2007–13 (a and b represent parallel drillings, spaced ∼100 m apart).

Fig. 4.

Same as Figure 3 but for middle ice.

Fig. 5.

Same as Figure 3 but for bottom ice.

It is noteworthy that significant differences between samples from the same layers were observed within the ice cover, despite the analogous depth of sample collection (Figs 3–5). In the years 2007–09, when two drillings (Figs 3–5; a and b) were made, differences between the sampling points were observed in all layers (top, middle, bottom), with the most significant differences occurring in the surface part (2009), where the difference in mineralization represented by electrical conductivity was >100% (7 and 16 μS cm⁻¹). Moreover, depending on the ion type, differences within particular layers were distributed irregularly. In 2007, at one point (central part of the lake), higher concentrations of chloride ions but lower concentrations of sulphate were observed, while the other ions occurred at similar concentration levels. The variable thickness of the ice cover over the lake, as well as different physicochemical properties of ice samples collected at the same depth, confirm the heterogeneous chemical composition caused by external factors and the kryochemical effect (Reference Bartels-RauschBartels-Rausch and others, 2012a).

In the years 2007–13, the ice on lake Morskie Oko was usually covered by a snow layer several cm to several tens of cm thick. The exception was 2010, when there was no snow cover in the sample collection period (March). Earlier that year, precipitation did not exceed 100 mm month⁻¹ (Reference PajakPajak, 2011). Consequently, wind, snow sublimation and melting caused the lack of snow cover. Analysis of the chemical composition of snow lying on the surface of the ice cover over lake Morskie Oko showed similar or lower concentrations of particular ions in comparison with the results obtained for the upper part of the ice cover (Table 1). Chlorides dominated among the anions, while ammonium and sodium ions prevailed among the cations. However, it should be emphasized that the analyzed samples were the old (seasonal) snow, which underwent numerous changes with time, such as metamorphism and elution of admixtures from the crystal structure (Reference CatalanCatalan, 1989; Reference Catalan, Ballesteros, Camarero, Felip and GaciaCatalan and others, 1992; Reference Lilbæk and PomeroyLilbæk and Pomeroy, 2008; Reference Bartels-RauschBartels-Rausch and others, 2012b). The monitoring of atmospheric pollutant loads over the Tatra Mountains allows for the observation that wintertime precipitation brings 7.4 kg SO₄ ²⁻ ha⁻¹, 3.7 kg Cl⁻ ha⁻¹, 3.0 kg NO₃ ⁻ ha⁻¹, 2.1 kg Ca²⁺ ha⁻¹, 1.4 kg Na⁺ ha⁻¹ and 2.3 kg K⁺ ha⁻¹ (e.g. November 2010–March 2011; Reference PajakPajak, 2011, Reference Pajak2012). Atmospheric deposition (including pollutants), as well as other factors, i.e. ionic permeability of ice lattice in relation to the reduction of temperature, ice crystal size and incorporation of ions in the ice lattice, contributes to the chemical composition of ice cover. It can be seen that Na⁺ : Cl⁻ ratios by weight in snow (0.28) and ice (0.24–0.40) are closer to the sea water (0.55), indicating that they are primarily atmospheric in origin, derived from sea salt via precipitation. Molar ratios of NH₄ ⁺ : SO₄ ²⁻ in snow and ice (4 : 1 to 2 : 1 respectively) are closer to the ideal ratio 2:1, indicating the formation of (NH₄)₂SO₄ by post-deposition photochemical reactions in the atmosphere. Also the Ca²⁺ : Cl⁻ ratio by weight is much higher (4.50) than that of snow (0.15) and ice (0.38–1.18). The ion Ca²⁺ could be mainly derived from the weathering of rocks in the drainage basins.

Electrical conductivity of water in the lake ranged from 18 to 29 μS cm⁻¹ (20–25 μS cm⁻¹ predominantly), with the highest contributions from sulphate and nitrogen ions and calcium cations. Atmospheric deposition is considered to be the source of these ions in the Tatra reservoirs, while the alkaline ions, including the calcium ion, mainly originate from the reaction of weathering and processes of ion exchange in catchment soil (Reference RzychonRzychon, 2009).

Comparison of the oligotrophic water of lake Morskie Oko (predominantly 20–25 μS cm⁻¹) with the ice cover reveals lower mineralization of ice (most often in the range 7–25 μS cm⁻¹). The average value of total ions, 118 μEq L⁻¹, is lower than that of water (163 μEq L⁻¹). Also, the order of abundance of ions in lake water under the ice (Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ > NH₄ ⁺; NO₃ ⁻ > SO₄ ²⁻ > Cl⁻ > F⁻) is different from that of ice cover (top: NH₄ ⁺ > Na⁺ > Ca²⁺ > K⁺ > Mg²⁺; Cl⁻ > NO₃ ⁻ = SO₄ ²⁻ > F⁻, middle: NH₄ ⁺ > Ca²⁺ > K⁺ > Mg²⁺ > Na⁺; NO₃ ⁻ > Cl⁻ > SO₄ ²⁻ > F⁻, bottom: NH₄ ⁺ > Ca²⁺ > Na⁺ > K⁺ > Mg²⁺; Cl⁻ > NO₃ ⁻ > SO₄ ²⁻ > F⁻). Lower medians of concentrations for nitrate, sulphate, magnesium and calcium ions are observed in the ice cover than in the lake water. However, it is interesting that, in the case of chloride, potassium and especially ammonium ions, higher average concentration values are found in the ice cover (Table 1). The catchment characteristic in terms of geochemical processes, as well as the annual, biological cycle of the reservoir, may play a significant role. Similar results were obtained by Reference Lepparanta, Tikkanen and VirkanenLepparanta and others (2003) for an oligotrophic ice-covered lake in Finland, while this phenomenon was not observed in lakes with higher mineralization of water.

On the other hand, the ice cover on the sea has been studied in numerous works (e.g. by Weeks, Bartels-Rausch). It has been stated that the binding impurities from fresh and salty water into the ice may vary due to different process conditions and ice-cover structure (Reference Vrbka and JungwirthVrbka and Jungwirth, 2005; Reference Luo, Chen and HanLuo and others, 2010; Reference Timco and WeeksTimco and Weeks, 2010; Reference LepparantaLepparanta, 2015) and in ice cover on the sea some salinity variation in the ice cover profile is usually observed. The differentiation in salinity in ice is connected with the migration of salt downward through the ice (Reference Timco and WeeksTimco and Weeks, 2010). A similar effect was observed in Morskie Oko lake ice, although ionic concentration (salt content) in oligotrophic water and sea water varies considerably. It is very likely that the salinity of lake water affects the cycle of freezing and defreezing observed in the high-altitude lake (Reference LepparantaLepparanta, 2015).

Based on our study, it can be stated that the analysis of the chemical composition of the ice cover on lake Morskie Oko is very significant, especially since the environmental effects, in the form of increased ion concentrations (Reference RzychonRzychon, 2009), decreased water pH (Reference Kopacek, Stuchlík and HardekopfKopacek and others, 2006) and release of toxic compounds to the water depth (Reference Borowiak, Polkowska and PrzyjaznyBorowiak and others, 2006), have already been observed in some mountain lakes (Reference Kurzyca, Choiński, Kaniecki and SiepakKurzyca and others, 2009).