Browsing by Subject "Thermal stratification"

Sort by: Order: Results:

Now showing items 1-2 of 2
  • Silvonen, Soila; Niemistö, Juha; Csibrán, Adrián; Jilbert, Tom; Torma, Péter; Krámer, Tamás; Nurminen, Leena; Horppila, Jukka (2021)
    Hypolimnetic withdrawal (HW) is a lake restoration method that is based on the removal of phosphorus (P) along with near-bottom water. While it has often proven to be effective, the method also sets challenges: it is about balancing between effective P removal and maintenance of the thermal stratification of the lake. The success of different HW projects has been reviewed in some studies retrospectively, but scientific literature still lacks studies that use detailed data on the lake biogeochemistry to scale and optimize the method in advance, and to predict the outcomes of the restoration measure. In the current study, we investigated the seasonal biogeochemistry, P stocks and thermal stratification of a eutrophic lake (Lake Kymijarvi/Myllypohja basin, southern Finland) to determine an optimal withdrawal rate, to assess its effects on stratification, and to evaluate the expected success of HW. We found that by adjusting HW with P diffusive fluxes from the sediment (diffusion-adjusted HW), it is possible to remove a notable part of the cycling P without causing major disturbances to the thermal stratification even in a relatively shallow lake. Our results show that HW can have great potential in lake restoration: diffusion-adjusted HW in our study lake could increase the annual P output by 35-46%, shifting the P budget of the lake to negative. We thus propose a novel approach to optimize HW on the basis of the diffusive flux of P from the sediment, with the goal of extracting P continuously at an equivalent rate to the diffusive flux. We finally discuss how this can be achieved more effectively with HW based on a closed-circuit system. (c) 2020 The Authors. Published by Elsevier B.V.
  • Huang, Wenfeng; Zhang, Jinrong; Leppäranta, Matti; Li, Zhijun; Cheng, Bin; Lin, Zhanju (2019)
    A large number of lakes and ponds are unevenly distributed over the Qinghai-Tibet Plateau (QTP). Little is known about their ice processes and thermal regimes. Seasonal ice mass balance, thermal regime and stratification of under-ice water were investigated in a shallow thermokarst lake in central QTP using in situ observations and global reanalysis data. Congelation ice grew to 60–70 cm depth while continuous surface sublimation caused a total ice loss of over 30 cm. The bulk lake temperature below ice remained above 3.0 °C through the ice freezing period and rose gradually up to 7–9 °C during the melting period. The vertical thermal structure of under-ice water consisted of a stable strongly stratified interfacial layer (IL) and an underlying convective layer reaching the lake bottom. A warm layer existed just beneath the IL since the middle equilibrium period and increased in thickness and temperature. There was seasonal variation of IL depth and temperature gradient in response to the ice thermodynamics and atmospheric conditions. The calculated daily water-ice heat flux (Fw) was of 10 W m–2 magnitude. Seasonal variation of Fw manifested both diffusive and convective heat transport to the ice-water interface. This study suggests that strong penetration of solar radiative flux is the dominant contributor to high Fw, which results in a relatively thin ice compared with other equivalent high-latitude climate.