by Altrisha Rodrigues

In the early 1950s, the City of Yellowknife in the Northwest Territories was the site of a booming gold mining industry. Giant Mine, located 5 km north of the city, produced thousands of ounces of gold by roasting vast amounts of arsenopyrite ore, resulting in the release of highly toxic arsenic trioxide dust into the atmosphere. There were a couple of strategies implemented to remediate this situation. One of them included collecting harmful arsenic waste in baghouses and storing it in the ground, where experts were sure the permafrost would keep it contained long after the closure of Giant Mine. Today, the rapid thawing of permafrost over the past decade has made Giant Mine the most expensive contaminated site cleanup project in Canada’s history, with remediation costs estimated at $4.38 billion. However, this cost does not cover clean up of arsenic contaminated lakes within a 30-km radius of Giant Mine that have reported alarmingly high concentrations of arsenic. Even though mining activities ceased in 2004, only minimal chemical or biological recovery of lakes has been documented.

Bioindicators are organisms that provide insights into environmental health. Bioindicators are widely used in the field of paleolimnology, which is a science that uses biological, chemical, and physical remains preserved in the sediments of inland waters to reconstruct environmental change. Chaoboridae (Order: Diptera), commonly called “phantom midges”, leave identifiable fossil remains preserved in lake sediments that can be useful bioindicators of freshwater food web changes, including the presence or absence of fish. Chaoborus are flies that have an aquatic larval stage, and their mandibles can preserve well in the sediments due to the presence of chitin, a polysaccharide commonly found in the exoskeleton of many animals. Four Chaoborus species are reported in Canadian freshwaters. Some species have nocturnal feeding patterns coupled with an ability to migrate efficiently in the water column, which allows them to coexist with fish. Species that are incapable of vertical migration are heavily preyed upon by fish and cannot sustain a viable population in lakes where fish are present.

For my EUCURA project, I analyzed Chaoborus mandibles found in surface sediments of 16 lakes that ranged in depth from 4-6 m, all located within a 30 km radius of Giant Mine. The goal was to determine fish presence/absence in these lakes of intermediate depth, to support ongoing research using zooplankton as bioindicators of impact and recovery from arsenic emissions.  Fish are a strong determinant of zooplankton community structure that must be accounted for in the use of zooplankton as bioindicators for arsenic ecotoxicity. Lakes shallower than 4 m tend to be fishless, while lakes deeper than 6 m typically have fish. It is in the intermediate depth range where fish presence/absence is difficult to infer. Sampling fish is extremely difficult in remote Yellowknife lakes that are only accessible by helicopter, and Chaoborus mandibles in the sediments offer an efficient alternative for inferring fish presence/absence.

I isolated Chaoborus mandibles from the sediments using standardized paleolimnological techniques and identified them to the species level under the microscope. Out of the 16 lakes analysed, 9 lakes contained 10 or more Chaoborus mandibles. The most abundant species were Chaoborus americanus and Chaoborus trivittatus. C americanus is a species that does not migrate within the water column, and instead remains in shallower depths, making them unable to coexist with fish. C. trivittatus is capable of migration but is more common in lakes with very low fish biomass. Of the 9 lakes with Chaoborus, 3 lakes also contained migratory Chaoborus species such as Chaoborus punctipennis and Chaoborus flavicans, which can co-exist with complex fish populations, while the remaining 6 lakes contained only C trivittatus and C americanus. Since C trivittatus is co-existing with C americanus, this indicates that lakes are fishless.

The results produced from this research will be incorporated into a multi-indicator ecotoxicological study of the extent of arsenic contamination from Giant Mine, which is a collaboration between several EUC faculty, graduate, and undergraduate students (including me), the Government of Northwest Territories, the Aurora Research Institute, and researchers from Laurier University.  

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Altrisha Rodrigues is a recipient of the EUC Undergraduate Research Award (EUCURA) and worked with Jennifer Korosi, Amanda Little and Randelle Adano in this Yellowknife, Northwest Territories project.