Mixing processes in small arctic lakes during spring

• Published in: Limnology and Oceanography
• Main Authors: Cortés, Alicia, MacIntyre, Sally
• Other Authors: NSF ARC, NSF's Arctic Research Support and Logistics Program, NSF PLR, U.S. NSF, Arctic Natural Sciences (ARC) LTER
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2019
• Subjects: Arctic
• Online Access: http://dx.doi.org/10.1002/lno.11296; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Flno.11296; https://onlinelibrary.wiley.com/doi/pdf/10.1002/lno.11296; https://onlinelibrary.wiley.com/doi/full-xml/10.1002/lno.11296; https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.1002/lno.11296

Abstract Small ice‐covered lakes are stratified by temperature and solutes. Using time series measurements and profiles of temperature, specific conductance (SC), and dissolved oxygen obtained during spring 2014 and 2015, we identified the physical processes occurring under the ice and at ice‐off in two ~ 2 ha, 10‐m‐deep arctic lakes. The lakes are distinguished from other freshwater, ice‐covered lakes by solutes initially stabilizing the density stratification when temperature decreased in the lower water column and, with one exception, stabilizing it during warming. With an ice cover 1 m thick, wind‐forced internal waves occurred, with 2nd vertical mode waves prevalent where stratification was weak. Snowmelt induced near‐surface chemical stratification such that diurnal thermoclines formed with stable temperature stratification in a ~ 4‐m‐thick layer. Horizontal exchange was mediated by internal waves and gravity currents induced by greater heating near shore and as incoming snowmelt displaced water in shallow regions. Toward ice‐off, the gravity currents reduced temperature stratification between the snowmelt‐induced near‐surface pycnocline and the bottom pycnocline but slight increases in SC precluded radiatively driven convection. Snowmelt retention was greater with rapid spring heating. The lakes did not mix by ice‐off. With moderate winds, Wedderburn numbers decreased below 3 at ice‐off, and the near‐surface pycnocline upwelled and then deepened due to internal wave‐induced mixing. The concomitant downward mixing of heat caused a rapid onset of thermal stratification, and that, combined with incomplete mixing under the ice, led to persistence of near‐bottom depletion in oxygen and increased density and dissolved solutes.