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Seasons of change: Investigating the influence of time, seasonality, and climate change on mining-related arsenic toxicity to plankton communities in lakes from the Yellowknife, Northwest Territories, Canada

Seasons of change: Investigating the influence of time, seasonality, and climate change on mining-related arsenic toxicity to plankton communities in lakes from the Yellowknife, Northwest Territories, Canada

Amanda Little

The impacts of climate and land-use change have begun to degrade the natural resiliency of ecosystems, affecting even the most remote locations at both the local and global scale. Lakes, which serve as catchment sites for entire landscapes, have been particularly impacted and many have become highly concentrated with harmful contaminants due to a combination of inputs from atmospheric deposition, downstream flow from polluted aquatic sites, and runoff from terrestrial surfaces. The combination of increased pollution with the accelerating impacts of climate change has resulted in the large-scale degradation of otherwise historically stable and resilient aquatic ecosystems.

As a result, many aquatic ecosystems are currently undergoing community reorganization in response to rapidly changing disturbance event regimes which can deviate significantly from those historically experienced by these systems. This has led to the uncoupling of the evolved adaptive capacity responses of stressed aquatic ecosystems, which in turn has led to significant time lags between contaminant exposure and biological recovery, regime shifts, and the establishment of new, alternative stable states in these systems. All of these processes fundamentally change the structure of biological communities, resulting in a loss of critical ecosystem function and services. Lakes experiencing multiple stressors are considered to be at the highest risk of ecosystem disruption, as prolonged exposure to multiple, novel disturbances can lead to the collapse of a system’s resistance to and ability to revert to pre-exposure conditions.

One of the most notorious sites of environmental degradation in Canada is the greater Yellowknife region located in the Northwest Territories. With a remediation bill expected to exceed $900 million, the abandoned gold mine Giant Mine is currently ranked as Canada’s second largest environmental liability. Situated on top of 237,000 tonnes of buried toxic arsenic trioxide, the degrading mine site is a significant threat to both human and environmental health. Operational from 1948-2004, Giant Mine is reported to have released approximately 20,000 tonnes of arsenic trioxide (As2O3) into the environment, which has created a 15 km radius of extreme mining-related contamination surrounding the abandoned mine site.

Despite the known presence of this large zone of contamination in the greater Yellowknife region, very little research has explored the current state of the environment outside of Giant Mine’s property lease boundaries. In particular, very little is known about the long-term effects of mining-related arsenic contamination in exposed aquatic systems, and how it has, and likely continues to impact organisms and ecosystems in the region. In addition to widescale legacy mining contamination, the Yellowknife region is undergoing increasing rates of urbanization and the accelerated effects of climate change. Increasing temperature and changing precipitation regimes are expected to lead to significant changes in the physical land and waterscapes, with important examples including increasing flooding frequency, the formation of new hydrological connections, thawing permafrost, and decreasing lake ice cover. All of these changes have been shown to impact critical biogeochemical cycles, including arsenic and other mining-related contaminants. For example, warmer temperatures elevate microbial activity in the surface sediments of lakes and rivers. Since the release of arsenic from sediments is largely a microbially-mediated process, an increase in temperature will result in more arsenic being released back into the water column30. It is therefore expected that water arsenic concentrations will increase in the region, especially during warm, summer months. This has important toxicological implications for humans since summer is when lakes are most frequented for recreational activities like swimming and fishing.

Amanda collecting water samples in Yellowknife, Northwest Territories

Unfortunately, many of the environmental variables which control oxygen availability in aquatic systems are also impacted by climate change (e.g., increasing water temperature, earlier onset of thermal stratification, earlier ice break up) and will therefore likely influence arsenic toxicity and mobility as well. The increased input of nutrients from land-use change and urbanization has also been shown to influence arsenic. Therefore, the combined effects of land-use and climate change are likely influencing arsenic toxicity and mobility in the Yellowknife region. It is possible that different times of the year may result in lower toxicological thresholds than currently advertised by regulatory agencies. In addition, aquatic communities experiencing multiple and/or worsening disturbances, may be more vulnerable to future novel stressors introduced by climate and land-use change in the region. This could lead to significant ecosystem restructuring, potentially impacting higher trophic organisms with important connections to humans (e.g., snails, fish). It is therefore critical that more research be done on the biogeochemical cycling of arsenic and its ecotoxicological impacts in aquatic ecosystems.

In order to address these research gaps, my PhD dissertation investigates how arsenic has influenced aquatic ecosystems in the Yellowknife region, what the implications are under climate and land-use change, and how this may impact both human and environmental health. In order to appropriately explore these relationships, my research is divided into three main themes: 1) exploring how arsenic has shaped lake community structures through time; 2) assessing modern plankton community structure across a spatial gradient of arsenic, and; 3) observing the seasonal aquatic cycling of arsenic and its impacts on plankton communities. This research uses a combination of space-for-time substitutions, paleolimnological, and modern-day sampling techniques in order to investigate how arsenic has impacted lake ecosystems in the Yellowknife region. This combination will serve to position my research within a novel “paleo-ecotoxicological” framework, whereby arsenic concentrations (“dose”) and the community assemblage changes (“response”) preserved in collected sediment cores can be used to develop and test hypotheses about how lakes in the Yellowknife region have been altered by long-term exposure to mining-related arsenic contamination.

The combination of these approaches allows for the investigation of arsenic and its subsequent ecological impacts along both spatial and temporal scales, while also contributing relevant ecotoxicological data from real world settings, allowing for the indirect investigation of human impacts on the environment within the appropriate geographical and historical contexts. This research will directly inform ongoing restoration and freshwater conservation efforts in and around Yellowknife, and specifically assist the Yellowknives Dene First Nation in understanding both the current and future risks of legacy arsenic exposure to their water and fish. In addition, this research will also provide new information regarding arsenic toxicity thresholds in aquatic systems, how arsenic cycling and toxicity are likely to be influenced by climate change, and highlight specific times of the year when arsenic may be more harmful to both human and environmental health.