The role of internal feedbacks in shifting deep lake mixing regimes under a warming climate

Abstract

Climate warming is causing changes in the physics of deep lakes, such as longer summer stratification, increased water column stability, reduced ice cover, and a shallower depth of winter overturns. An ultimate consequence of warming would be a transition to a different mixing regime. Here we investigate the role of physical, chemical, and biological feedback mechanisms that unfold during a shift in mixing regime, and whether these feedbacks could prompt and stabilise the new regime. Although climate, interannual temperature variation, and lake morphometry are the main determinants of a mixing regime, when climate change causes shifts in mixing regime, internal feedback mechanisms may gain in importance and modify lake ecosystem functioning. We review the role of these feedbacks in three mixing regime shifts: from polymictic to seasonally stratified, from dimictic to monomictic, and from holomictic to oligomictic or meromictic. Polymictic lakes of intermediate depth (c. 3–10 m mean depth) could experience seasonal stratification if a stratification event triggers phytoplankton blooms or dissolved organic matter release, reducing transparency and therefore further heating the surface layer. However, this feedback is only likely to have influence in small and clear lakes, it would be easily disturbed by weather conditions, and the resulting stratified state does not remain stable in the long term, as stratification is lost in winter. The ice‐albedo feedback might cause an accelerated shift from ice‐covered (dimictic) to ice‐free (monomictic) winters in sufficiently deep (mean depth 50 m or more) lakes, where temperature memory is carried over from one winter to the next. Nevertheless, there is an ongoing debate into whether this process can persist during natural weather variations and overcome self‐stabilising mechanisms such as thermal insulation by snow. The majority of studies suggest that a gradual transition from dimictic to monomictic is more likely than an abrupt transition. A shift from a holomictic to a meromictic regime can occur if anoxia is triggered by incomplete mixing and an increase in deep‐water density—through the accumulation of solutes—exceeds a density decrease by hypolimnetic warming. A shift to meromixis would strongly alter the biology of a lake and might be difficult to reverse. If solutes accumulate only minimally in the hypolimnion, an oligomictic regime is formed, in which years with complete and incomplete mixing alternate. Understanding the importance of feedback mechanisms and the role of biogeochemistry when lakes shift in mixing regime could lead to a better understanding of how climate change affects lake ecosystems.

Stratification strength and light climate explain variation in chlorophyll a at the continental scale in a European multilake survey in a heatwave summer

“…This is of particular concern for resolving the climate warming effect on lakes and the positive feedbacks on eutrophication of lakes (Sinha et al 2017; Deng et al 2018). However, thermal stratification, which will likely increase with climate warming (Woolway and Merchant 2019), is an important feature governing lake ecosystems as it affects both nutrient availability and light climate (Schwefel et al 2016), generating complex feedbacks for the biota (Mesman et al 2021). The importance of these factors may dominate when lakes are not nutrient limited.…”

Classifying Mixing Regimes in Ponds and Shallow Lakes

“…Lakes have historically been classified based on mixing regimes (Hutchinson & Loffler, 1956; Lewis Jr., 1983) as mixing frequency drives key ecosystem processes, including benthic oxygen availability, sediment resuspension, nutrient and carbon recycling, and greenhouse gas production and emissions (Andersen et al., 2019; Mesman et al., 2021; Vachon et al., 2019; Wilhelm & Adrian, 2007). While mixing patterns in deepwater lakes are well established (Hutchinson & Loffler, 1956), shallow waterbodies have more complex mixing regimes that still need resolution.…”

Cyanobacterial blooms in oligotrophic lakes: Shifting the high‐nutrient paradigm

“…In the stratified summer months, the epilimnion of deep oligotrophic lakes will probably become more nutrient deficient due to an earlier onset of stratification, with nutrients isolated in the hypolimnion for extended periods (Shimoda et al., 2011). With a shift from holomixis (full mixing once per year) to oligomixis (full mixing rarely), nutrients may be isolated from the epilimnion for multiple years (Mesman et al., 2021). Hence, although climate warming may lead to anoxia and enhanced release of nutrients from the sediment, nutrient availability in the productive upper zones of the lake may actually decrease and become available only in years with full overturn (see O’Reilly et al., 2003; Yankova et al., 2017).…”