Glacial Rivers Release Mercury into High Arctic Watersheds

Lake Hazen Canada on GlacierHub
On the shore of Lake Hazen, Nunavut, Canada (Source: Judith Slein/Flickr).

Mercury is a contaminant which poses environmental health risks to terrestrial and aquatic ecosystems around the world, especially in the Arctic. A recent study in Environmental Science & Technology traces the source of mercury concentrations in Lake Hazen to increased flow in glacial rivers. Lake Hazen, located in Nunavut, Canada, is the High Arctic’s largest lake by volume, and reaches depths up to 267m.

There are both natural and anthropogenic sources of mercury. Global mercury emissions have been declining, specifically after ratification of the Minamata Convention. However, as anthropogenic sources decrease, climate change could be increasing natural sources of mercury—if in a less direct fashion than emissions.

Mercury is stored in permafrost and glacial ice, so as permafrost thaws and ice melts, downstream ecosystems could be impacted. Microbes can also transform mercury into a poisonous neurotoxin called methylmercury, which impacts the nervous system. Both can bioaccumulate in organisms, especially at higher levels of the food chain.

“The primary focus of the research program at Lake Hazen is on understanding the biogeochemistry of freshwater ecosystems downstream of the glaciers of the Northern Ellesmere Icefield,” said Kyra St. Pierre, the study’s lead author, in an interview with GlacierHub. St. Pierre, who conducted this research as a part of the Department of Biological Sciences at the University of Alberta, Canada, went on to say that the study aimed to explain how recent warming patterns might impact biogeochemical cycles in the future.

Henrietta Nesmith glacier river on GlacierHub
One of the glacial rivers that feeds into Lake Hazen, flowing from the Henrietta Nesmith glacier (Source: Judith Slein/Flickr).

Lake Hazen receives meltwater—and up to 94 percent of total mercury inputs—primarily from three glacial rivers. The study showed that most mercury from these rivers flowed into the lake in particulate form. This means that the particles carrying mercury are not dissolved, making the water flowing into Lake Hazen more turbid, or cloudy, than the lake’s existing water. Due to the weight of the particles it carries, turbid water is also very dense. The increased weight creates what is called a turbidity current, which efficiently deposits most of the mercury particles in the bottom of the lake.

St. Pierre named these turbidity currents the study’s most surprising result, because it revealed important aspects of how Lake Hazen’s watershed functions. “Not only do [turbidity currents] transport mercury from the surface but also oxygen and other nutrients directly to the depths of the lake,” she said.

This study is distinctive in that it approached mercury cycling at a watershed-scale instead of looking at individual system components. St. Pierre called this one of the study’s most important attributes, explaining that if, for example, they had decided to focus simply on Lake Hazen’s outflows, they would have concluded that mercury concentrations were extremely low.

western part of Lake Hazen on GlacierHub
The western part of Lake Hazen in the summer of 1997 (Source: Ansgar Walk/Wikimedia Commons_).

Lake Hazen’s turbidity currents make it a huge mercury sink. Despite huge mercury inputs from glacial rivers, the lake’s main outflow, the Ruggles River, discharges relatively small amounts of mercury and methylmercury. The researchers found that the lake sequestered over 95 percent of total mercury inputs to the system annually. Downstream in the Ruggles River, mercury concentrations rose exponentially, a result of erosion and thawing permafrost.

The High Arctic is extremely sensitive to increasing temperatures and precipitation in the context of anthropogenic climate change. Craig Emmerton and Jennifer Graydon, researchers at the University of Alberta, spoke to GlacierHub about some of the larger implications of this study. “The High Arctic is among the most rapidly changing regions on Earth and its climate is expected to become warmer and wetter,” they said, pointing out the potential role of glaciers and permafrost as developing sources of mercury with the power to contaminate freshwater and marine ecosystems.

“I think we can safely infer that as warming continues in High Arctic latitudes, we can expect a greater delivery of mercury from the cryosphere to downstream ecosystems,” said St. Pierre. Though Lake Hazen retains most mercury inputs from glacial rivers, the researchers found a 3.4-times greater water volume and 2-times higher delivery of total mercury in the notably warm summer of 2015, than in the much cooler summer of 2016. So, as glaciers continue to melt, more mercury will inevitably make its way downstream.

Henrietta Nesmith River delta on GlacierHub
The Henrietta Nesmith River delta on the northwest coast of Lake Hazen (Source: Ansgar Walk/Wikimedia Commons).

Lake Hazen’s depth and size draw close similarities to High Arctic fjord systems. The researchers showed that these turbidity currents also occur in fjords indirectly fed by land-terminating glaciers. Almost 70 percent of arctic glaciers are land-terminating glaciers, and so could be important sources of mercury for marine ecosystems. More, fjords fed by marine-terminating glaciers can flow directly into high productivity zones, increasing potential for bioaccumulation in organisms and into coastal food webs.

Ultimately, this study highlights an important discovery—even with reduction of direct anthropogenic sources of mercury, there is a lingering, growing anthropogenic driver—climate change.

Climate Change in the High Arctic: Lake Hazen’s Response

High above the Arctic Circle, far from the footprint of human civilization, a significant indication of human-induced climate change has manifested in Lake Hazen, the largest lake by volume north of the Arctic Circle. The lake and surrounding glacial environment are experiencing rapid change as the climate warms, ice cover declines, and glaciers retreat. A recent study in Nature Communications examines these physical drivers and their impacts on the lake’s ecological composition and the physiological condition of its only fish species, the Arctic Char. These changes, unprecedented in 300 years, have serious ramifications for local indigenous populations who rely on the lake’s ecosystem services.

Map of Lake Hazen watershed
Map outlining the Lake Hazen watershed and changes in surrounding glacier surface temperatures from 2000 to 2012 (Source: Lehnherr et al.).

In northern Ellesmere Island, the farthest north of the islands that compose the Canadian Arctic Archipelago, summer air temperatures increased by 1 degree Celsius during the 2001 to 2012 period in comparison to the period 1986 to 2000. Climate model simulations suggest temperatures are expected to increase 3.2 degrees Celsius by 2100. These changes have the potential to dramatically alter local ecosystems.

The study’s research team, which included experienced Arctic scientists from a diverse set of backgrounds, grew over time, according to Igor Lehnherr, who spoke with GlacierHub. From a scientific standpoint, the team knew that glacial masses were shrinking in other parts of the Arctic, along with summer lake ice cover. From this basis, according to Lehnherr, it was ”a matter of bringing everyone on board with all the different expertise required to quantify each of these various aspects.”

The study’s authors note that few previous studies have evaluated ecosystem-scale changes to climate change in inland watersheds. Lehnherr cited the need for a multidisciplinary team and baseline data to “quantify how much the system has changed and what drivers are responsible for ecological change” as challenges to study.

Photo of Eureka Sound on Ellesmere Island
Eureka Sound on Ellesmere Island (Source: Stuart Rankin/Creative Commons).

The researchers benefitted from over 50 years of scientific research on Lake Hazen, helping this recent study fill part of this knowledge gap by analyzing how the lake’s ecosystem has responded to climate change. The study does this through four distinct, yet interconnected focuses: watershed warming and declining lake ice cover, hydrological changes within the watershed, recent changes in the paleo-lake record, and ecological shifts in the lake itself.

Watershed Warming and Declining Lake Ice Cover

From 2000 to 2012, summer air temperatures in the Lake Hazen watershed rose by 2.6 degrees Celsius, with most of the rise occurring after 2007. These higher air temperatures, in turn, warmed the soil. Spring-time soil temperatures were 4 degrees Celsius higher from 2007 to 2012 than they were from 1994 to 2006. The lake warms particularly in late spring, when it is still covered by ice, and in early summer, when ice cover finally breaks up. Overall, the lake’s warming trend is causing ice to melt earlier in the summer and freeze later in the fall. This is in addition to an increase in ice-free area by 3 km2 per year since 2000, which was found to be related to August lake surface temperatures.

Hydrological Changes within the Watershed

Glaciers within the Lake Hazen watershed are the main hydrological driver. Because of warming temperatures, these glaciers are experiencing mass-balance losses. Positive feedback loops play a role in this loss, as high surface temperatures melt ice, subsequently decreasing reflectivity, which allows the surrounding surface to absorb more solar radiation, speeding up melting.

Figure detailing trends in lake surface temperatures, onset dates for ice melt and freeze up, and ice cover.
Figure detailing trends in lake surface temperatures, onset dates for ice melt and freeze up, and ice cover (Source: Lehnherr et al.).

Mean rates of annual glacial runoff have increased significantly in recent years. This increase has raised water levels in Lake Hazen by almost a meter since 2007. Finally, the large increase in glacial runoff into Lake Hazen has lowered the time that water stays in the lake (before leaving by the lake’s one outflow stream) from a historical average of 89 years to 25 years today.

Recent Changes in the Paleo-Lake Record

The increase in glacial runoff entering Lake Hazen has driven sediment accumulation rates to levels eight times higher than a 1948 baseline period. Most runoff is deposited by glacier-fed rivers that empty into the lake, leading to the increased mixing and oxygenation of the lake’s once stable and anoxic bottom waters.

More sediment deposition has also given rise to increased levels of anthropogenic contaminants, such as mercury and pesticides, in lake sediments. In addition, organic carbon accumulation rates in the lake have increased by an astonishing 1000 percent, much higher than the 50 percent increase in most North American boreal lakes.

Ecological Shifts

To assess the impact of the lake’s changes outlined above on its ecology, the authors used micro-fossil counts of algae. Before widespread warming (prior to 1890), when the lake was covered with ice almost year-round, algal fossils were rare. However, after warming (post 1890), when more areas of the lake became ice-free, nearshore algal species boomed.

After remaining relatively stable for much of the 20th century, the lake’s ecological composition changed in the late 1980s when planktonic species succeeded benthic species. This change was driven by a longer ice-free period where the deep waters of the lake were exposed to light for more months each year.

Photo of Lake Hazen
Lake Hazen (Source: Igor Lehnherr).

Lake Hazen’s one fish species, the Arctic Char, has also been negatively impacted by climate warming. Lehnherr notes that the team might have expected ice-free summers to increase the lake’s primary productivity, subsequently increasing biomass and leading to healthier and thriving Char populations. However, this has yet to occur; instead, amplified lake turbidity due to the raised levels of glacial river discharge has hindered the ability of the visually reliant Char to feed on midges and other Char, harming their physiological condition.

Implications

These changes have negative effects on the lake’s ecology and also on indigenous communities that inhabit the area. These communities rely on the lake as a source of food in an otherwise desolate region. While the future of High Arctic ecosystems is far from certain, Lehnherr points to the need for more multidisciplinary studies that encompass entire watersheds as a key to the better assessment of climate change impacts.