Salt‐Fingering in Seasonally Ice‐Covered Lakes

Abstract

The past decade has seen a significant increase in studies on the physical processes occurring under lake ice. These processes had been previously understudied (Kirillin et al., 2012), despite playing a key role in the yearly variability of important physical lake properties, such as temperature and oxygen (Cortés & MacIntyre, 2020;B. Yang et al., 2021). While progress has been made, there remain many open questions concerning the physical, biogeochemical, and biological processes that are occurring under lake ice (Jansen et al., 2021). One process in particular, cryoconcentration (or the rejection of solutes such as salts from freezing ice), may be an important process for both under-ice transport and has the potential to suppress turnover in lakes (Pieters & Lawrence, 2009).The diffusion rates of salt and heat in water are different. This difference can result in what are known as double diffusive instabilities. Depending on how the salt and temperature are stratified, these instabilities will lead to two different types of flows. If the concentration of salt is unstably stratified (as is the case in cryoconcentration), and the temperature is stably stratified, double diffusive instabilities lead to the formation of long finger-like plumes called salt fingers (Turner, 2001). These salt fingers have been observed in the ocean (Kunze et al., 1987) and play a key role in the vertical transport of heat in the Arctic (Rudels et al., 2009). The other type of double diffusive instability, diffusive convection, occurs when the temperature is unstably stratified and the salt is stably stratified, such as in Lake Kivu. For more information on double diffusive convection in lakes, see Bouffard and Wüest (2019).The rejection of salt from ice in inland water (with salinities <24 ppt) is complicated by a nonlinear equation of state. Unlike seawater, freshwater systems have a temperature of maximum density above their freezing temperature (Chen & Millero, 1986). This nonlinear temperature dependence enables an inverse stratification under ice, with colder water above and warmer water below. Despite this stable temperature stratification, sufficient cryoconcentration may lead to salt fingering, which is complicated by the nonlinear equation of state (Özgökmen & Esenkov, 1998). Motivated by the work of Bluteau et al. (2017), Olsthoorn et al. (2020 used numerical simulations to model these salt fingers and the resultant transport of salty water from the ice-water interface to the lake bottom. The present paper builds upon this previous work by performing a set of physical experiments to visualize the salt exclusion-induced plumes.However, salt fingers are not the only physical process responsible for transporting salts under ice. Mortimer and Mackereth (1958) discussed how bottom heat flux and sediment respiration can generate gravity currents that transport heat and solutes to the deepest portion of the lake. Furthermore, MacIntyre et al. ( 2018) showed

Variability in lake bacterial growth and primary production under lake ice: Evidence from early winter to spring melt

“…Recent advances in winter limnology have greatly improved the understanding of physical winter processes (Cavaliere et al 2021; Olsthoorn et al 2022) and under‐ice communities (Tran et al 2018; McMeans et al 2020; Shchapov et al 2021). Yet, much remains unknown, especially considering links between the physical limnology and communities, resource availability, and food web functions, as well as their variability on spatial and temporal scales.…”