Thermohaline structure and stability of the water column The thermal characteristics of the water column in Lake Limnopolar differed notably

depending on the ice condition (Fig. 4.15). For all years, the water column during ice-free periods was almost isothermal. Over summer 2001-02 the maximum water temperature at 2 m depth reached 6.4 ºC. On the other hand, for summers 2002-03 and 2003-04 the maximum temperatures were 7.2 ºC and 4.2 ºC, respectively. It is also remarkable that the water temperature remained between 2 and 4ºC colder over the entire summer 2003-04 than in the other two studied years.

Similar to temperature, conductivity profiles for summer 2001-02 were characterized by a uniform distribution (Fig. 4.15). Variations over time were also narrow. Mean values over the water column increased gradually from 52 to 77 µS cm^-1. During summer 2002-03, no important variations were observed in the

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conductivity profiles, though the mean was somewhat higher in mid January (76 µS cm^-1) compared to mid February (60 µS cm^-1). In late December of summer 2003-04, the values for almost the entire water column were also quite homogeneous, although higher conductivities (137.9 µS cm^-1) were observed near the bottom. By mid January, the conductivity gradient was more pronounced, from approximately 2 m to the bottom, when circulation in the lake increased and the inlet water flowed into the lake. At this time, conductivities ranged from 12.3-204.4 µS cm^-1. This vertical gradient remained nearly constant until thawing at the end of January, when the largest differences in conductivity were observed between the surface of the lake and the bottom. When ice melted completely in February, conductivity became constant with depth.

Figure 4.14. Changes of extinction coefficient (k) of PAR irradiance of the ice cap and in the water column during summer 2003/2004.

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Figure 4.15. Vertical profiles of temperature (solid line) and conductivity (dashed line) in Lake Limnopolar used to make stability calculations at different dates of summers 2001-02, 2002-03 and 2003-04.

The vertical profiles of stability, expressed as Brünt-Väissäla frequencies (N²), obtained from the three summers are depicted in figure 4.16. Both temperature and conductivity profiles determined the shapes observed. The N² values for summer 2001-02 were consistently below 1x10^-4 s^-1 throughout the entire water column, indicating a complete mixing, even when the lake was still partially covered by ice (22^nd December). Only on 29^th January was a slight increase in surface water observed as a consequence of somewhat higher temperatures. In contrast, a short, inverse stratification period was established over January 2004. Just before this period, in late December, only near-bottom and surface stratification peaks were observed, due to the slight increase in conductivity and the onset of inverse thermal stratification. Nevertheless, as observed in figure 4.16, the pycnocline was only obvious at 2 m during mid January. This pycnocline deepened slightly, settling down to 2.5 m depth at the end of January. Afterwards, by mid February, the ice cover thawed completely and holomixis occurred. As a result, during the pycnocline period, two layers of the water column were isolated, namely, an upper mixing layer (varying between 2 and 2.5 m depth) and a lower, fairly stagnant layer, which can be considered as a epilimnion and a hypolimnion, respectively.

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Figure 4.16. Vertical stability in the water column in Lake Limnopolar at different dates expressed as the vertical profiles of Brünt-Väissäla frequency

In addition to the above profiles, a CTD probe was positioned at 2 meters depth to continuously register temperature with a frequency of 30 min. This procedure was followed in the same way for the three summers, from 28-Dec-2001 to 11-Jan-2002, from 16 to 31-Jan-2003, and from 30-Dec-2003 to 23-Jan-2004. As observed in figures 4.17 and 4.18 for summers 2001-02 and 2002-03, respectively, the occurrence of temperature oscillations in the water column was verified, suggesting either thermocline tilting or internal seiches. To determine what forced this tilting of temperature, the temperature coupling to both wind and solar radiation was analyzed via cross-correlation analysis. The cross-correlation data that we present here are those obtained for interval time of a day. Additional analyses were made to determine whether time periods over a day also reported a periodic auto-correlation, but no significant signal was observed between either pair of variables.

Further, given that the cross-covariance might be not necessarily symmetric about zero, both positive and negative lags were computed for this timeframe.

During summers 2001-02 and 2002-03, over the ice-free periods, water temperature at two meters depth changed in a quasi-regular oscillation that matched well with the dial cycle of solar radiation and air temperature, suggesting a pattern of a diurnal slight stratification and mixing. As demonstrated by cross-correlation analysis (Fig. 4.19), the correlation was somewhat higher for 2001-02 compared with the following summer. This analysis furthermore demonstrated the way by

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which the water temperature of lake lagged behind the causal variables. Thus, the cross-correlation coefficients between solar radiation and water temperature were generally moderate and peaked positively and negatively at lags 6 and -6 hours, respectively. This closely coincided with that observed for the air temperature. There was not autocorrelation, however, at -6 hour lags for summer 2002-03. Lags also varied compared to summer 2001-02 for the rates of water temperature change.

Thus, coefficients in this case were even higher for periods of 0 and 12 hours in a positive and negative way, respectively.

In contrast, the temperature fluctuation exhibited less covariance with the wind velocity, as observed in the autocorrelograms (Fig. 4.19). However, it was detected as a negative effect that lagged 19 and 12 hours for summers 2001-02 and 2002-03, respectively. For summer 2001-02, this negative effect was moderate and affected temperature rather than its rate of variation. In contrast, the effect was weaker for summer 2002-03, although in this case both temperature and its variation rate were similarly influenced. Additionally, for summer 2001-02 in particular, a moderate, positive effect of wind velocity on water temperature was observed, which lagged -18.5 hours.

Notably, wind velocities for both years exhibited some structure other than periodic that dramatically affected the temperature evolution, indicating the existence of a larger scale coherent wind force. For instance, as shown in figure 4.17, three clear discontinuities occur in temperature data over summer 2001-02 that match with stronger wind events on days 2, 5 and 12. As observed in the same figure for the rate of temperature change, the increases were largest just before these events. In the case of summer 2002-03, the changes in temperature induced by wind were weaker, as mentioned before, such that a constant increase of temperature took place during the first 8 days (Fig. 4.18). However, an increase of wind velocity of around 3 fold during days 9 and 10 triggered water column mixing and, as a result, a notable drop in the water temperature.

The thermal regime for summer 2003-04 differed notably from the two preceding years because an ice cap was present in the lake. As a consequence of this isolation from the atmospheric forcing, no significant correlations were observed between water temperature and environmental variables. However, there was a weak, perceptible positive co-variation between water temperature variation and both solar irradiance and wind velocity (Fig. 4.20). However, the coefficients observed for wind velocity were the residuals of its covariance with solar radiation, clearly indicating that no direct causality existed between wind velocity and water temperature. A noteworthy outcome observed from this summer was that the

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correlation between solar radiation and water temperature differed in phase when compared to previous summers. Higher coefficients, both positive and negative, occurred at lags -2 and 10 hours, which implies that the target variable’s response was delayed 2 hours when lake was ice covered compared with that observed for the ice-free periods.

Figure 4.17. Time-series of environmental parameters and thermal characteristics of Lake Limonopolar at summer 2001-02. Plots are wind velocity (a), solar radiation (b), air temperature (c), water temperature in lake at two meters depth (d), and variation of temperature in lake at two meters depth (d). Data were collected from 28^th December to 11^th January.

The time courses for both radiative heat (gained by the lake) and sensible heat (lost by the lake) over summers 2001-02 and 2002-03 are shown in figures 4.21 and 4.22, respectively. The residual energy stored in the lake, indicated with a red line in these figures, was partially resolved by subtracting sensible heat from radiative flux. As observed for two summers, the level of energy stored into the lake

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followed the diurnal cycle upon radiation balance, that is, maximum values occurred in the middle of the day and minimum values during the night. In contrast, wind cools down the lake surface during the afternoon hours as observed by the increase of sensible heat flux going to the lake (i.e., negative values). This was a common trend for the two summers; though heat fluxes out of the lake were, on average, higher for summer 2001-02. The highest episodic sensible heat losses occurred over summer 2002-03, coinciding with lower solar irradiation. As observed in figure 4.22, this implied a negative balance of heat, close to -100 W m^-2

Figure 4.18. Time-series of environmental parameters and thermal characteristics of Lake Limonopolar at summer 2002-03. Plots are wind velocity (a), solar radiation (b), air temperature (c), water temperature in lake at two meters depth (d), and variation of temperature in lake at two meters depth (d). Data were collected from 16^th to 31^st January.

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2001-02

2002-03

Figure 4.19. Cross-correlations coefficients (CCF) between environmental parameters and temperature (a) and temperature variation (b) at two meters depth in Lake Limnopolar at summers 2001-02 (up) and 2002-03 (below). Environmental parameters are solar radiation (black), wind velocity (clear grey), and air temperature (dark grey).

Figure 4.20. Cross-correlations coefficients (CCF) between environmental parameters and temperature (a) and temperature variation (b) at two meters depth in Lake Limnopolar at summer 2003-04. Environmental parameters are solar radiation (black), wind velocity (clear grey), and air temperature (dark grey).

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Figure 4.21. Heat and radiation fluxes at the water–air interface of Lake Limnopolar as calculated for the period from 12-28-01 to 01-11-02 based on 30 min interval measurements.

The series of heat fluxes are radiation (black), sensible heat (grey) and the balance among them (red).

Figure 4.22. Heat and radiation fluxes at the water–air interface of Lake Limnopolar as calculated for the period 01-16-03 to 01-31-03 based on 30 min interval measurements. The series of heat fluxes are radiation (black), sensible heat (grey) and the balance between them (red).

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4.3.5. Heat content variation and thermal diffusivity in the lake