Climate change and catastrophic lake drainage: what does this mean for Arctic ecosystems?
by Jana Tondu
Imagine you’re fishing on a remote northern lake. It’s a beautiful day, the sun is shining, the bugs aren’t too bad, and the fish are biting. You come back a few days later, hoping for the same success, only to find out the lake is gone. All you can see is exposed mud and, in the distance, puddles of water on what used to be a lake. You might panic. You might be sad. You might be confused.
These are some of the emotions that residents of Old Crow, Yukon, experienced when they learned that Zelma Lake drained catastrophically in June of 2007. Old Crow is home to the Vuntut Gwitchin First Nation, who hunt and fish on lakes within the Old Crow Flats — the largest wetland complex in the Yukon. Residents have stated that “lakes are rapidly turning to meadows,” and they are acutely aware of the toll climate change is taking on the landscape. They “can’t read the weather anymore” and thus, they “don’t trust [it] anymore.” The catastrophic drainage of Zelma Lake was an all too real experience of climate change hitting close to home.
Lake drainage events in permafrost environments like the Old Crow Flats are not uncommon. Indeed, a growing body of research suggests that the frequency of lake drainage events is increasing due to climate change. They can be triggered through a variety of mechanisms and can result in total or partial water loss. When a lake drains completely, a lake ecosystem can become a terrestrial one as vegetation grows within the empty lake bed. Lake drainage is often partial, with residual ponds remaining in the former lake bed.
Videos from the field show that Zelma Lake lost ~5.8 million m³ of water, equivalent to 2,300 Olympic sized swimming pools, and exposing ~5.2 km of lake bed. Zelma Lake is within the traditional hunting and trapping territory of a local family, who noted that two smaller lakes adjacent to Zelma’s southwest bank had drained the previous year and anticipated that Zelma would be next. In 2007, water levels rose after above-average precipitation from March to May. Precipitation (snow and rain) over these three months totalled 96.2 mm, three times higher than the long-term average (34.0 mm). As predicted, the high water levels breached the southwest bank. After losing approximately 80% of its water volume, pieces of Zelma Lake remain as residual waterbodies within a once larger lake basin.
Drainage events leave obvious physical scars on the landscape (landslides, slumped banks, exposed lake basins), but we wanted to know if water quality changes after a partial lake drainage event. Little is known about what happens to water quality after a partial drainage event, because no one has ever sampled residual waterbodies.
After Zelma Lake drained, we collected water samples in June and September from the largest residual waterbody from 2007–2012. Results showed a dramatic change in water quality, with a large increase in major ion (dissolved salts) and nutrient concentrations over the 5-year timeframe. The increase in ions indicates that the general health of Zelma Lake has declined. The high levels of nutrients fueled excessive algae growth, a phenomenon known as eutrophication.
But how comparable were our water quality data to the condition of Zelma Lake before it drained? Obviously, we couldn’t go back in time to collect samples, so we used paleolimnology, the study of lake sediments, to reconstruct the past condition of Zelma Lake. Lake sediments act as a natural archive, storing the chemical remains of the previous year’s water column and fossils of aquatic organisms in annual layers. This allows us to go back in time and look at historical conditions.
In August of 2010, in what looked like a James Bond attempt to collect data, we landed on Zelma Lake with an amphibious helicopter. The chopper was fitted with pontoons to land on the surface of the lake. Working from the base of the pontoon, we used a gravity corer to collect a sediment core from the lake. We cut the 34-cm long core into 0.5-cm thick hockey pucks and measured the geochemistry and concentration of algal pigments in each puck at the University of Waterloo’s WATERlab. Similar to radiocarbon dating used in archaeology, we used lead-210 dating to age each puck that we analyzed. Our sediment core dated back to approximately 1674 (~340 years ago).
The top two layers of the sediment core represented years following the drainage of Zelma Lake (2007–2010), while the remaining layers were from before the lake drained. The post-drainage sediments were drastically different than the pre-drainage sediments. In the post-drainage sediments we found species of cyanobacteria (blue-green algae), a group of diatoms, and purple-sulfur bacteria that were not present prior to the lake drainage event.
The presence of these unique algae groups and changes in water quality showed that the ecology of Zelma Lake changed substantially after the drainage event. As water rapidly evaporated from the former lake bed, dissolved ions were concentrated in the water, nutrients flowed into the remnant waterbodies from the nutrient-rich surrounding sediments, and the shallow remnant waterbodies developed low-oxygen conditions, which can cause sediments to release phosphorous.
As more lakes catastrophically drain across northern landscapes like Old Crow Flats, we can expect a shift in Arctic lake water quality. Lakes that are high in nutrients are more productive and have more algae and plant growth. This can reduce the number of fish in these lakes, because shallow water and excessive algae and plant growth can create low-oxygen environments. The cyanobacteria we observed in Zelma Lake occur in harmful algae blooms, which can be toxic to both wildlife and humans that use lake water for drinking.
For millennia, the Vuntut Gwitchin have relied on lakes like Zelma to support waterfowl, fish, and other wildlife (e.g., muskrats, moose). As scientists try to understand how these events will affect the physical landscape and internal lake ecology, communities like Old Crow try to understand how these changes will impact their traditional livelihoods and future.
Jana Tondu is a P. Biol. who received her M.Sc. from the Department of Biology at the University of Waterloo.
Read the full paper — Limnological evolution of Zelma Lake, a recently drained thermokarst lake in Old Crow Flats (Yukon, Canada) in Arctic Science.
