Evolution and dynamics of the vertical temperature profile in an oligotrophic lake

[摘要] In this study, the fine-scale responses of a stratified oligotrophic karstic lake (Kozjak Lake, Plitvice Lakes, Croatia; the lake fetch is 2.3  km , and the maximum depth is 46  m ) to atmospheric forcing on the lake surface are investigated. Lake temperatures measured at a resolution of 2  min at 15 depths ranging from 0.2 to 43  m , which were observed during the 6 July–5 November 2018 period, were analyzed. The results show thermocline deepening from 10  m at the beginning of the observation period to 16  m at the end of the observation period, where the latter depth corresponds to approximately one-third of the lake depth. The pycnocline followed the same pattern, except that the deepening occurred throughout the entire period approximately 1  m above the thermocline. On average, thermocline deepening was 3–4  cm d −1 , while the maximum deepening (12.5  cm d −1 ) coincided with the occurrence of internal seiches. Furthermore, the results indicate three different types of forcings on the lake surface; two of these forcings have diurnal periodicity – (1) continuous heat fluxes and (2) occasional periodic stronger winds – whereas forcing (3) corresponds to occasional nonperiodic stronger winds with steady along-basin directions. Continuous heat fluxes (1) produced forced diurnal oscillations in the lake temperature within the first 5  m of the lake throughout the entire observation period. Noncontinuous periodic stronger winds (2) resulted in occasional forced diurnal oscillations in the lake temperatures at depths from approximately 7 to 20  m . Occasional strong and steady along-basin winds (3) triggered both baroclinic internal seiches with a principal period of 8.0  h and barotropic surface seiches with a principal period of 9  min . Lake currents produced by the surface seiches under realistic-topography conditions generated baroclinic oscillations of the thermocline region (at depths from 9 to 17  m ) with periods corresponding to the period of surface seiches ( ≈ 9  min ), which, to the best of our knowledge, has not been reported in previous lake studies.

[学科分类] 妇产科学