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

Abstract

<jats:p>Abstract. 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. </jats:p>

Internal seiches in a karstic mesotrophic lake (Prošće, Plitvice Lakes, Croatia)

“…Observed Prošće Lake temperatures (panel a) and vertical temperature gradients over depths from 3 to 27 m for the two lakes, Prošće and Kozjak (1 st and 12 th in the lake chain) during the period from 6 July at 00:00 LST to 4 November 2019 at 00:00 LST (panels b and c, respectively). a thermal structure differs from that which was found for the deepest part of the 12 th lake in the chain (Kozjak) in which during the period of lake stratification, the observed temperatures of the bottom layers were approximately 4 °C (Klaić et al, 2020).…”

“…9 (bottom) shows vertical squeezing and stretching of the deep metalimnion which generated the second vertical mode (e.g., Münnich et al, 1992). We note that higher vertical modes were not observed for nearby Kozjak Lake (Klaić et al, 2020), which is a somewhat larger and deeper lake of comparable size but with more complex bathymetry and an inlet that is approximately 1.2 km distant from the Prošće outlet (although the path of water flow between the two lakes is somewhat longer).…”

“…In addition to the standard statistical procedures, spectral analysis as described in more detail in Klaić et al (2020) was conducted. Specifically, the power spectral density (PSD) was determined using Welch's method (Welch, 1967) (also known as the weighted overlapped segment averaging (WOSA) method or periodogram averaging method).…”

“…A recent review of studies addressing the abiotic properties and processes in Plitvice Lakes (Klaić et al, 2018) pointed to a lack of investigations in the field of physical limnology (this is also valid for other Croatian lakes). Thus far, when considering physical topics, only barotropic, surface seiches (Gavazzi, 1919;Pasarić and Slaviček, 2016) for the two largest lakes, Prošće and Kozjak (1 st and 12 th in the lake chain, respectively) and baroclinic, internal seiches in Kozjak (Klaić et al, 2020) have been studied. In comparison with surface seiches, internal seiches (e.g., Mortimer, 1953;Heaps, 1961;Münnich et al, 1992;Fricker and Nepf, 2000) are far more important for lakes as the restoring forces (which strive to restore equilibrium at the surface between the two distinct layers) are generally proportional to the density difference between the two layers.…”

SIMO v1.0: Simplified model of the vertical temperature profile in a small warm monomictic lake

“…Although the model satisfactorily reproduced the temporal variations in the lake temperatures at the hourly scale, it was not able to reproduce the internal seiches that were previously documented for both lakes (Klaić et al, 2020a(Klaić et al, , 2020b). This finding is not surprising since the present model is based solely on the energy-budget approach; thus, except for vertical mixing of the two adjacent layers under unstable stratification, it does not account for any hydrodynamic behavior.…”