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.

When Rivers Meet the Sea: Carbon Cycling in the Gulf of Alaska

The Gulf of Alaska has numerous glacial inputs (Source: Jennifer Questel).

When rivers meet the sea, the sediment they carry becomes mixed into the ocean, where it makes quite a splash, biogeochemically speaking. In the subarctic North Pacific Ocean, for example, iron-rich sediment delivered from the continental margin spurs a wintertime phytoplankton bloom over 900 kilometers offshore. The presence of these terrigenous particles is felt up the food chain— the higher levels of iron in the water support larger diatom populations, which means more snacking for copepods, a type of zooplankton.

In the Gulf of Alaska, glacial meltwater is an important source of terrestrial particles. A recent study by Jessica Turner, Jessica Pretty, and Andrew McDonnell optically measured particles in the northern Gulf of Alaska, an area with extensive glacial inputs. This technique allowed the researchers to collect massive amounts of data with minimal lab work, maximizing the area they could survey, Jessica Pretty told GlacierHub. Their instrument measured a range of particle sizes, from some too small to be seen by the naked eye to others as large as paper clips.

Researcher Jessica Turner works with the optical profiling instrument (Source: Jennifer Questel).

Pretty and her coauthors found that in the Gulf of Alaska, particle concentrations are denser in two main places: where glaciers and rivers flow into the Gulf, and offshore, near the continental shelf break, where they are buoyed by waves, currents and tidal action. These small particles wield great influence, increasing biological productivity at the shelf break.

“The Gulf of Alaska is an interesting region,” said Pretty. “It has major freshwater input seasonally from melting glaciers and river runoff that eventually joins with Pacific waters and makes its way toward the Arctic.” The recent findings illuminate particle distribution in the northern Gulf of Alaska, yielding clues about how climate change may affect carbon cycling in the Gulf and parallel ocean systems.

Beyond local significance to the Gulf of Alaska ecosystem, the influence of these river-borne terrestrial particles scales up— globally, such sediment inputs impacts the carbon cycle, which regulates climate. The bits of rock Pretty tracked in the Gulf of Alaska are essentially tiny bundles of carbon, and when these bundles sink in the ocean, they drive what scientists have termed the “biological pump,” the process by which the ocean cycles organic and inorganic carbon, and sequesters carbon dioxide in the deep ocean.

Jessica Pretty observes an instrument deployment during a research cruise (Source: Jennifer Questel).

Because carbon dioxide is constantly exchanged between the upper layers of the ocean and lower levels of the atmosphere, concentrations become equal in the shallow ocean and low atmosphere over time. However, sinking particles remove carbon from this exchange. “The biological pump allows the ocean to store more carbon than it would be able to just from equilibration,” explained Pretty.

The ocean absorbs a quarter of the carbon dioxide released into the atmosphere each year, and so as carbon is pumped into the atmosphere, levels in the ocean increase in tandem. This leads to ocean acidification, which threatens many marine species. However, terrestrial carbon sequestration practices, like soil conservation and wildfire suppression, may be an important element of climate change mitigation.

Particle concentrations are high near glacial inputs, such as from the Fourpeaked Glacier (Source: Jennifer Questel).

As global climate warms and glaciers melt, higher glacial inputs will carry more sediment to the Gulf of Alaska and analogous ecosystems around the world. These minute particles will ramp up the global biological pump, increase carbon sequestration, and lead to a myriad of impacts yet unknown. In addition, seasonal changes, like an earlier springtime, may also spur earlier phytoplankton blooms, changing the dynamics of life in the sea. Through the movement of minuscule specks of rock, the Gulf of Alaska, and ultimately the whole ocean, will change.

Roundup: Midges, Rotifers, and Iron-Eating Bacteria

Each week, we highlight three stories from the forefront of glacier news.

 

Diversity of Midge Flies Near Italian Glaciers

From Insect Conservation and Diversity:

A winter-emerging midge. Courtesy of Flickr user thepiper351.
A winter-emerging midge. Courtesy of Flickr user thepiper351.

“A collection of approximately 100 000 chironomids (Diptera; Chironomidae) inhabiting glacial areas of the Southern Alps that were collected over a period of approximately four decades from 1977 to 2014 were analysed to evaluate the impact of environmental traits on the distribution of chironomid species. Although the list of species has not substantially changed over time, some rare species captured in the 1970s have not been collected in recent years, while other species have only been collected recently.”

Read more here.

Rotifers Colonize Maritime Glacier Ice

From Molecular Phylogenetics and Evolution:

Bdelloid rotifer, the species studied. Courtesy of Flickr user Ian Sutton.
Bdelloid rotifer. Courtesy of Flickr user Ian Sutton.

“Very few animal taxa are known to reside permanently in glacier ice/snow. Here we report the widespread colonization of Icelandic glaciers and ice fields by species of bdelloid Rotifera. Specimens were collected within the accumulation zones of Langjökull and Vatnajökull ice caps, among the largest European ice masses. Rotifers reached densities up to ∼100 individuals per liter-equivalent of glacier ice/snow, and were freeze-tolerant. ”

Read more here.

 

Bacteria Turn Iron into Food Under Glaciers

“Geochemical data indicate that protons released during pyrite (FeS2) oxidation are important drivers of mineral weathering in oxic and anoxic zones of many aquatic environments including those beneath glaciers.

Bacteria, courtesy of Flickr user AJ Cann.
Bacteria, courtesy of Flickr user AJ Cann.

Subglacial meltwaters sampled from Robertson Glacier (RG), Canada over a seasonal melt cycle reveal concentrations of S2O32- that are typically below detection despite the presence of available pyrite and several orders of magnitude higher concentrations of the FeS2 oxidation product sulfate (SO42-). Here we report the physiological and genomic characterization of the chemolithoautotrophic facultative anaerobe Thiobacillus sp. RG5 isolated from the subglacial environment at RG. The RG5 genome encodes pathways for the complete oxidation of S2O32-, CO2 fixation, and aerobic and anaerobic respiration with nitrite or nitrate.”

Read more here.