Internal energy, the work of the wind, and the thermal stability in Lake Tyrifjord, southeastern Norway

Abstract

Lake Tyrifjord consists of Holsljord and Steinsfjord, two lakes of highly contrasting morphology, was used as the basis for a comparison of internal energy, the work of the wind, and thermal stability in two lakes under identical large-scale climatic conditions. Because of simultaneous temperature measurements available for the summer of 1973 from the two lakes, the present paper is based on these observations. The deep Holsfjord (z,,, = 295 m) accumulates internal energy of an order representative for its geographical situation. Its annual heat budget in 1973 was 129735 j cmP2. Steinsfjord (z,,, = 22 m) accumulated 60 per cent of the latter. At the stratifications representing maximum stability in 1973, the stability S in Holsfjord exceeded the corresponding value in Steinsfjord by a factor of 37, whereas the work of the wind B in Holsfjord was only 1.45 times the Steinsfjord B value. The much greater center of volume moment under homoiothermal conditions typical of Holsfjord was responsible for the great difference between energy conditions in the two lakes, as clearly demonstrated by the B + S values. The ratio (B + S)/B shows that the water masses of Steinsfjord would maintain homoiothermal conditions throughout the heating season with an increase in B (i.e. an increase in external energy supply) of ca 40 per cent. The additional energy supply needed to maintain homoiothermal conditions in Holsfjord was of the order of 1200 per cent of B for Holsfjord. Accordingly, the metalimnetic thermal discontinuity represents a far more effective mixing barrier in Holsfjord than in Steinsfjord. Calculations of the (B + S)/B ratios for other lakes show that this ratio is useful for a judgment of the degree of stagnancy of a lake's hypolimnetic water masses.

Morphometry and trophic state modify the thermal response of lakes to meteorological forcing

“…As described by several authors (Kjensmo, 1994;Ambrosetti & Barbanti, 2002b), for deep lakes, the annual maxima of Smith stability markedly exceed those of Birgean work, while for the shallow lakes B is generally higher than S. In that sense, Lake Monate behaved like a deep lake and Lake Varese as a shallow lake. Ambrosetti & Barbanti (2002b) found also a higher interannual variation of B compared to that of S in deep lakes while the opposite was true in shallow lakes.…”

“…As emphasised by Kjensmo (1994) for Birgean work it does not matter of how the energy enters the lake. In some lakes the water through-flow may be highly important for producing a certain density distribution, in other lakes the deeply penetrating solar irradiance may strongly contribute to the energy expressed by B.…”

“…In Lake Monate neither B nor S had significant interannual differences. As Birgean work can be defined as the amount of external energy needed to produce a certain observed density distribution beginning from a certain minimum heat content (Kjensmo, 1994), Ambrosetti & Barbanti (2002b) explained its larger variability in deep lakes with their earlier stratification due to which the surface layers are strongly affected by meteorological conditions until the moment of maximum stratification, and therefore this active component has a more important role than morphometry (the passive component). As their study was based on multiannual data from 31 Italian lakes of different morphometric characteristics, it has a high generalisation power and the opposite situation found in our study is rather an exception.…”

“…Still, as mentioned by Bowling (1990), these parameters are seldom calculated and employed as limnological working tools despite of their usefulness for comparing and contrasting the physical backgrounds of various lake environments. In most cases the annual values of these parameters are used and compared with morphometric characteristics of lakes (Gorham, 1964;Barbanti et al, 1996;Ambrosetti & Barbanti, 2002a, b) and more seldom the energetic variables are related with weather-driven seasonal changes (Bowling, 1990;Kjensmo, 1994).…”

Photography in limnology: documentation of lake color using a CCD camera

“…It has been the object of fundamental and applied limnological research (Ström 1945;Berge 1983;Kjensmo 1994). The station visited for our purpose was in a sheltered area, where the stands of vegetation may indicate local runoff from the village and farming activities.…”