Mixing processes in small arctic lakes during spring

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.

Mixing, stratification, and plankton under lake‐ice during winter in a large lake: Implications for spring dissolved oxygen levels

“…The river input of salt could enter the depths of the bay by flowing down the edges of the basin as dense gravity currents, driven by either heat fluxes from the shallow sediments in early winter (Fig. 1a), differential solar radiation fluxes (Ulloa et al 2019), or gravity currents driven by snowmelt plumes near the end of the winter (Cortés and MacIntyre 2020). These gravity currents are also likely present during Winter II (Ulloa et al 2019), but while they can still transport salt, they are likely not the major mechanism for heat transport in late winter.…”

Ice Cover and Extreme Events Determine Dissolved Oxygen in a Placid Mountain Lake

“…For RTR and Schmidt stability, we based thresholds for mixing on the patterns found (Supporting Information S4). While these thresholds are most likely lake specific because of basin morphology, there are few studies that also give non‐zero thresholds to indicate mixing (RTR: >20, Kortmann et al, 1994; <30, Siver et al, 2018; Wedderburn Number: <1, Read et al, 2011; <3, Cortés & MacIntyre, 2020; Schmidt stability: <50 indicates if not complete mixing, then very weak stratification, Read et al, 2011; Lewis et al, 2019; Sahoo et al, 2016).…”

Differential Heating Drives Downslope Flows that Accelerate Mixed‐Layer Warming in Ice‐Covered Waters

“…In lakes, these currents transport heat and biogeochemical tracers between nearshore and the interior basin (Brothers et al, 2017;Encinas Fernández et al, 2016;Farrow, 2004), supporting the biogeochemical connectivity between littoral and pelagic zones. Under-ice gravity currents have been observed, and their transport has been implicated in the basin-scale thermal structure and circulation of polar lakes (Cortés & MacIntyre, 2019;Kirillin et al, 2015).…”