Extreme warming and regime shift toward amplified variability in a far northern lake

Abstract

Mean annual air temperatures in the High Arctic are rising rapidly, with extreme warming events becoming increasingly common. Little is known, however, about the consequences of such events on the ice‐capped lakes that occur abundantly across this region. Here, we compared 2 years of high‐frequency monitoring data in Ward Hunt Lake in the Canadian High Arctic. One of the years included a period of anomalously warm conditions that allowed us to address the question of how loss of multi‐year ice cover affects the limnological properties of polar lakes. A mooring installed at the deepest point of the lake (9.7 m) recorded temperature, oxygen, chlorophyll a (Chl a) fluorescence, and underwater irradiance from July 2016 to July 2018, and an automated camera documented changes in ice cover. The complete loss of ice cover in summer 2016 resulted in full wind exposure and complete mixing of the water column. This mixing caused ventilation of lake water heat to the atmosphere and 4°C lower water temperatures than under ice‐covered conditions. There were also high values of Chl a fluorescence, elevated turbidity levels and large oxygen fluctuations throughout fall and winter. During the subsequent summer, the lake retained its ice cover and the water column remained stratified, with lower Chl a fluorescence and anoxic bottom waters. Extreme warming events are likely to shift polar lakes that were formerly capped by continuous thick ice to a regime of irregular ice loss and unstable limnological conditions that vary greatly from year to year.

The littoral zone of polar lakes: inshore–offshore contrasts in an ice-covered High Arctic lake

“…The ice cover allowed inverse thermal stratification along with chemical stratification by preventing exposure to the atmosphere and wind-induced mixing. Ice has a low thermal conductivity, limits heat loss in the atmosphere and allows solar heating of the water column to temperatures well above those of the overlying air: Ward Hunt Lake had under-ice water temperatures up to 7°C, yet air temperatures averaged around 1.7°C in July (Bégin et al 2020). This greenhouse effect has been modelled in other ice-covered lakes of the Arctic (Vincent et al 2008) and Antarctica (Obryk et al 2019).…”

“…Complete loss of ice cover during warm summers allows mixing of the water column and ventillation of heat to the atmosphere, and water temperatures can then drop to near freezing (Schindler et al 1974;Doran et al 1996;Bégin et al 2020). The vertical structure that was lost in Ward Hunt Lake in 2016 was reinstated in summer 2017 beneath the first-year ice cover, but with lower temperatures (Bégin et al 2020).…”

“…Complete loss of ice cover during warm summers allows mixing of the water column and ventillation of heat to the atmosphere, and water temperatures can then drop to near freezing (Schindler et al 1974;Doran et al 1996;Bégin et al 2020). The vertical structure that was lost in Ward Hunt Lake in 2016 was reinstated in summer 2017 beneath the first-year ice cover, but with lower temperatures (Bégin et al 2020). The presence of ice cover hence allows a stable and relatively warm environment to develop, which may stimulate primary production (Markager et al 1999) and microbial food web activity (Wrona et al 2006).…”

“…3), suggesting a balance between gain and loss processes. The bottom waters of Ward Hunt Lake are well oxygenated in July, but reach anoxia at the beginning of winter that persists until spring (Bégin et al 2020). In contrast, frequent mixing in the ice-free moat zone likely keeps the water and the microbial mats well oxygenated until freeze-up, which translates into a lower proportion of anaerobic bacteria (Mohit et al 2017).…”

“…In all years, even in the past when the lake was covered by 4-m thick perennial ice, a moat of open water has extended around more than half the shoreline, persisting for several weeks each summer (Paquette et al 2015;Bégin et al 2020). We sampled along transects from the lake-edge moat to sub-ice waters in the middle of the lake to address the hypothesis that there would be an inshoreoffshore discontinuity in limnological properties that is directly linked to the ice cover.…”