Fluctuations in water temperature of Lake Zamkowe

Transcription

1 Limnological Review 4 (2004) Fluctuations in water temperature of Lake Zamkowe Adam Choiński*, Jacek Kanikowski** * Adam Mickiewicz University, Department of Hydrology and Water Management, Institute of Physical Geography and Environmental Planning, Fredry 10, Poznań ** Ekoenergia Poznań, Grochowska 49A/1, Poznań Abstract: On the basis of long-term (401 days) measurements of air and water temperature in Lake Zamkowe every 15 minutes at four depths, a set of nearly 200,000 data was obtained. It allowed short-term pulsations in water temperature to be determined. It was found that the greatest number of fluctuations exceeding 0.1 o C at hourly intervals (5.8% of the observation period) occurred at a depth of 30 m. Most of the observed fluctuations seem to generated by wind-induced water lifting, which means they are the effect of internal waves. Key words: hydrology, physical limnology, thermal conditions. Lake Zamkowe, a typical glacial finger lake, is situated in the Wałcz Lakeland. Its most important characteristics include a substantial volume of water (17 million m 3 ) given the relatively small area (132.8 ha), an exceptionally great mean depth (12.9 m), and a considerable mean gradient of the bottom (7 o 35'). The lake lies partly in the town of Wałcz and is one of the largest closed lakes in Poland. The aim of the article is to analyse changes in the lake-water temperature over a long measurement period at various depths. The result should be an answer to the question of whether, and to what extent, the water is thermally stable. Because of the scarcity of high-frequency measurements of the deeper parts of the lake waters, we know little about their stability. It is generally believed that the near-surface zone is decidedly more mobile as a result of direct wind action. The near-bottom zone, in turn, is more stable, which is reflected in minor variations in water temperature over the course of the year. Is it so indeed at short time intervals? To answer this question, an analysis was made of water temperatures near the bottom and near the surface. Measurements were taken with the help of platinum resistance sensors with a measuring range of o C to o C and accurate to 0.01 o C at four depths: 0.9, 1.5, 25.0 and 30.0 metres. Additionally, air temperature was measured on the shore 2 m above ground. The choice of depths was intended to allow an assessment of the possibility of using the lake's thermal energy. The measurements were performed every 15 minutes from 1 November 1994 to 17 April Discounting the occasional breakdowns (e.g., owing to atmospheric discharges), this yielded data for 401 days, or 9,624 hours. With four measurements per hour at the four depths and one of the air, a total of 192,480 data were obtained. On the basis of measurements taken every 15 minutes, mean diurnal temperatures (Fig. 1) and diurnal temperature fluctuations (Fig. 2) were determined. Hence, each diurnal figure is an arithmetic mean of 96 measurements. The distributions of the mean diurnal water temperatures generally follow variations in the mean diurnal temperatures of the air, but the resemblance is decidedly closer for the nearsurface data. Diagrams depicting the near-bottom zone are markedly flattened. In July/August

2 34 Adam Choiński, Jacek Kanikowski Fig. 1. Mean diurnal temperatures ( o C) of air and water

3 Fluctuations in water temperature of Lake Zamkowe 35 20,0 18,0 16,0 14,0 12,0 10,0 8,0 6,0 4,0 2,0 0, ,0 6,0 5,0 4,0 3,0 2,0 1, ,0 6,0 5,0 4,0 3,0 2,0 1, ,5 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0, ,5 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0, Fig. 2. Daily variations in air and water temperatures ( o C)

4 36 Adam Choiński, Jacek Kanikowski 1995 the temperatures at 0.9 m exceeded 23 o C for a fortnight or so, and at a depth of 1.5 m they were even higher than 25 o C. The maximum mean diurnal temperatures at 0.9 m and 1.5 m were recorded on the same day, 1 August 1995, and reached o C and o C, respectively. In the near-bottom zone, high temperatures were recorded from May to November 1995 and ranged between 5.5 o C and 6.0 o C. A maximum cooling of nearsurface waters (at depths of 0.9 m and 1.5 m), i.e. to below 1 o C, occurred from mid-january to the end of February When there was a winter with ice cover, the temperature of water under the ice did not drop below 1 o C, oscillating around 2 o C. In the near-bottom zone, minimum temperatures, i.e. below 1 o C, were recorded in February A diametrically opposite distribution occurred during the severe winter of 1996 with a permanent ice cover. The near-bottom temperatures oscillated around 3.5 o C at a depth of 30 m and 2.5 o C at 25 m. Thus, the lack of ice cover contributed to the cooling of the nearbottom layers by 2 o C to 3 o C. It is to be expected that each time a permanent ice cover develops (and isolates the lake water from external factors), the temperature of the near-bottom layers will rise by this order of magnitude. The use of a continuous method of measurement of such high frequency made it possible, for the first time in Poland, to determine daily variations in temperature for such a long period of time. The variations at depths of 0.9 m and 1.5 m (Fig. 2) are closely correlated with one another and those of the air temperature. From the first decade of April to the third one of October, the greatest diurnal fluctuations attain up to 7 o C in June. Over the remaining period they are much smaller and rarely exceed 1 o C. Interestingly enough, daily variations in temperature in the winter are of the same order for a mild (without ice) and a severe season (with ice cover). At depths of 25 m and 30 m, the variations are not closely correlated and are stochastic in nature, determined by hydrodynamic factors. It should be noted, however, that during an ice-free winter the amplitudes are much greater, about 1.7 o C, than during a severe winter, when they drop below 1 o C. The highest observed diurnal amplitudes in the near-bottom zone were considerable and attained 4.2 o C at 25 m in March 1995 and 4.5 o C at 30 m in January From the third decade of March to the first one of October, daily oscillations of temperature at a depth of 30 m were even twice as big than those at 25 m. The long measurement series allowed not only the scale of temperature variations at various depths to be determined; it also made it possible to find the frequency of their occurrence. It was assumed that those temperature fluctuations were significant for the analysis that exceeded 0.1 o C over an hourly interval. Table 1 lists the frequencies of temperature fluctuations at 0.1 o C intervals, starting with 0.11 o C up to 2.0 o C. Use was made of data for 401 days, or 9,624 hours. As can be seen, the greatest number of cases (563 hours, or 5.8% of the observation time) are fluctuations at a depth of 30 m. What may seem surprising is the fact that there are more fluctuations (of more than 0.1 o C) near the bottom than the surface. The greatest proportion of variations are slight ones, i.e. between 0.11 o C and 0.20 o C. However, even larger ones (i.e., above 0.1 o C over an hour) are much more frequent in the near-bottom than the near-surface zone. What factors are responsible for this pattern? In the case of near-surface waters, they can include: the warming of water associated with the daily insolation cycle, varying cloud amount, wind action, i.e., mixing, wind-generated water lifting, etc., precipitation that can cool or warm surface waters, and variations in the inflow of surface waters. In the case of near-bottom waters, their thermal fluctuations can be determined by: internal waves and any disturbances connected with seiching, biochemical processes occurring in bottom deposits, and inflows of the pulsating type of groundwater, e.g. crenogenic mixing. Figures 3 7 present selected examples of water temperature pulsations over short periods, i.e. a few days. Because of the abundance of data, it is impossible to illustrate variations for the entire period under analysis. It should be realised that the observed pulsations may be resultants of several overlapping factors mentioned earlier. Figures 3 and 4 depict changes in temperature under ice cover. In both cases the scale of nearbottom pulsations is much larger than of those at 25 metres. In the case registered between 29 January and 5 February 1996 (Fig. 4), there is no correlation

5 Fluctuations in water temperature of Lake Zamkowe 37 between variations at 25 m and 30 m, in contrast to a clear correlation observed after 7 January 1996 Fig. 3. Patterns of temperature fluctuations (description in the text)

6 38 Adam Choiński, Jacek Kanikowski Fig. 4. Patterns of temperature fluctuations (description in the text)

7 Fig. 5. Patterns of temperature fluctuations (description in the text) Fluctuations in water temperature of Lake Zamkowe 39

8 40 Adam Choiński, Jacek Kanikowski Fig. 6. Patterns of temperature fluctuations (description in the text)

9 Fig. 7. Patterns of temperature fluctuations (description in the text) Fluctuations in water temperature of Lake Zamkowe 41

10 42 Adam Choiński, Jacek Kanikowski (Fig. 3). The cyclicity and scale of pulsations in the latter case can be associated with seiching. The seiche-induced shifts in the water near the bottom can be more pronounced than above, which results from the fact, determined by the geometry of the lake basin, that the same amount of water moves in a space more constricted than higher up. The differences in the scale of vertical temperature fluctuations may also be a consequence of differences in water density in the profundity and above it. In the case of the near-surface zone, the scale of fluctuations at 0.9 m is clearly about twice as big as at 1.5 m. Interestingly enough, at both depths the diagrams of January/February 1996 (Fig. 4) are synchronous, as opposed to the situation in early January when the concordance is not so great and in a few cases distinct inversion can even be observed. Table 1. Number of pulsations (at hourly intervals) in temperature intervals at selected depths o C 0,9 m 1,5 m 25 m 30 m > Total Fig. 5 illustrates continuous (i.e., of several days' duration) temperature fluctuations in near-surface waters that are of a similar scale. They are not closely correlated, however, and neither can a dependence be seen between them and changes in air temperature. In the near-bottom zone, in turn, there are distinct several-hours' pulsations at depths of 30 m and 25 m. These are closely correlated, and the scale of variations near the bottom is twice as large as at 25 m. In Fig. 6, the peaks of pulsations coincide at all the analysed depths, both in the near-surface and near-bottom zones. However, there is no correlation in this respect between the two zones. The fluctuations at 0.9 m and 1.5 m are of the same order, while those at 30 m are about twice as big as at 25 m. No association can be detected between variations in the temperature of the air and the subsurface waters. Fig. 7 presents a pattern in which temperature variations at depths of 0.9 m and 1.5 m are coincidental and similar as to scale while not being correlated with those of the air temperature. In the near-bottom zone, in turn, the situation is different from the previous ones: fluctuations at 25 m do not coincide with those at 30 m, are three times as big, and occur even when there are none at 30 m (1 July 2 Aug. 1995). Most of the pulsations presented (especially those near the bottom) seem to result from windgenerated water lifting, i.e. they are the effect of internal waves. The facts presented above show how complicated short-term variations in water temperature can be. Those irregular ones near the bottom, of the order of a few hundredths of a degree centigrade, can be associated with the supply of thermal energy from biological processes occurring in the bottom/ water contact zone. Those on a larger scale forming various pulsation patterns are usually the result of seiching. However, temperature measurements at 15-minute intervals are not frequent enough to determine their periods; a continuous record would be the best in this case. Another useful thing would be a record of wind and pressure conditions, because they would allow a precise determination of the onset time of seiches. Although such measurements were not performed in this research, they are planned to be carried out in the future, and thus this problem will finally be carefully analysed.

11 Adam Choiński, Jacek Kanikowski 43 Streszczenie W okresie od 1.XI.1994 do 17.IV.1996 prowadzono pomiary temperatury wód Jeziora Zamkowego co 15 minut na czterech głębokościach (0,9; 1,5; 25 i 30 metrów) oraz powietrza. Łącznie uzyskano blisko 200 tys. danych. Określono średnie temperatury dobowe oraz dobowe wahania temperatur. Stwierdzono, Ŝe podczas surowej zimy pokrywa lodowa pełniąca rolę izolatora, powoduje podwyŝszenie temperatur wód w strefie przydennej o 2 do 3 o C. Największe dobowe wahania temperatury wody dochodziły w strefie podpowierzchniowej do 7 o C w czerwcu, zaś w strefie przydennej osiągnęły 4,5 o C w styczniu. Przy załoŝeniu, iŝ istotne do analizy są te fluktuacje, które w interwale godzinnym są większe od 1 o C określono częstotliwość ich występowania na czterech głębokościach. Najwięcej fluktuacji tego typu zarejestrowano na głębokości 30 m, tj. 563 przypadki (godziny), co stanowi 5,8% okresu obserwacyjnego. Zaskakujący jest fakt, Ŝe więcej przypadków fluktuacji temperatury wody (zarówno tych powyŝej 0,1 o C, jak i tych powyŝej 1,0 o C) ma miejsce przy dnie niŝ przy powierzchni. Spośród wielkiej liczby obserwacji wybrano kilka przykładów ukazujących róŝne układy pulsacyjne. Są więc przypadki krótkookresowych fluktuacji temperatury wody pod lodem, bez pokrywy lodowej, większe na głębokości 30 metrów od tych na głębokości 25 metrów, sytuacje odwrotne od powyŝszej, pulsacje kilkugodzinne i kilkudniowe itp. Większość z zaobserwowanych pulsacji (szczególnie tych przydennych) wydaje się być pochodnymi piętrzeń wiatrowych, a więc jest to efekt fal wewnętrznych. Jak wynika z uzyskanych danych jest wiele pytań pozostających bez pełnych odpowiedzi. Aby je uzyskać naleŝy w przyszłości prowadzić pomiary ciągłe, co pozwoli ustalić okresy sejsz oraz równolegle rejestrować warunki anemobaryczne, co z kolei pozwoli ustalić momenty wzbudzeń fal wewnętrznych.