Master of Science
Environment and Sustainability
Henry, Hugh A. L.
One of the first steps of methylmercury (MeHg) bioaccumulation, uptake by phytoplankton, is regulated by dissolved organic matter (DOM). Methylmercury binds strongly with DOM, especially high-molecular weight (HMW) compounds containing reduced sulphur; this binding decreases MeHg bioavailability. Climate-induced changes were measured at the landscape level using ArcGIS; land-cover changes between 2005-2010 were assessed around lakes in Mackenzie Valley, NWT. The observed changes, a 10% increase in forest-classified area and up to 15% decrease in wetland area, will likely increase delivery of HMW DOM and MeHg to lakes. Partitioning experiments were conducted to examine effects of DOM ligand quality on MeHg uptake by Chlamydomonas reinhardtii algae. In cysteine solution (reduced sulphur DOM), algae took up 89% less MeHg than in EDTA solution and 172% less MeHg than in ‘no DOM’ solution. Together, geospatial and experimental analyses indicate that changing DOM composition will likely cause MeHg bioavailability to decrease in subarctic lakes.
Summary for Lay Audience
Methylmercury (MeHg) is a toxic chemical that exists in nature in very small amounts. Tiny plant-like cells (algae) take up MeHg from lake water, and the MeHg transfers to animals that eat algae, such as fish. Even small doses of MeHg are harmful and humans that often eat these fish can experience impairment from mercury poisoning. Algae take up MeHg at different rates, depending on what other chemicals are in the water: this especially includes dissolved organic matter (DOM), which is produced by decaying plants and animals. MeHg binds strongly to large DOM and DOM that contains thiols (a part of DOM structure that contains sulphur). In water with these types of DOM, algae take up less MeHg. Using satellite images from 2005-2010, I measured changes in forest and wetland area in Mackenzie Valley, Northwest Territories, because this area of Canada is changing rapidly due to climate warming. Both the overall area of forest and the type of trees present can affect the type and amount of DOM in runoff. Wetlands are also important because they produce DOM and MeHg and increase the amount of both that enters lakes. Generally, I found that forest area, especially evergreen trees, increased in the Mackenzie Valley, and some study regions had decreased wetland area. As a result, DOM entering lakes likely became heavier and contained more sulphur, and MeHg production around lakes likely decreased, possibly making MeHg less available to algae. In a lab, I studied the effect of different types of DOM in water on the amount of MeHg taken up by algae. I confirmed that algae took up much less MeHg in water with DOM that contained sulphur. Together, these studies show that landscape change can greatly affect the ability of algae to take up MeHg. In the case of increased forest area and decreased wetland area, MeHg might be less of a hazard to lake wildlife, but in areas with intense deforestation and large wetlands, high levels of MeHg might build up in fish.
Simone, Kyra Lyn, "Climate Change and Dissolved Organic Matter in Subarctic Lakes: Relevance to Methylmercury Bioavailability" (2019). Electronic Thesis and Dissertation Repository. 6601.
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Available for download on Sunday, October 18, 2020