Methylmercury photodemethylation in Kajimkujik lakes

Klapstein, Sara Jane (2017) Methylmercury photodemethylation in Kajimkujik lakes. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (12MB)

Abstract

Methylmercury (MeHg) concentration in surface waters is a key variable regulating mercury availability to food webs. Few studies have quantified the seasonal importance of photodemethylation reactions and the influence of chromophoric dissolved organic matter (DOM) properties on these relationships. To address this research gap we have used numerous controlled experiments that focused primarily on the quantification of the relationships between solar radiation exposures, DOM, and MeHg within six freshwater lakes in Kejimkujik National Park and National Historic Site in Nova Scotia. The concentration of DOM was found to strongly control the photoreactivity of DOM in these study lakes across sampling seasons (R²=0.94). The effect of DOM photoreactivity on MeHg photodemethylation was directly tested using photochemically manipulated water from one lake collected in three different months. Photodemethylation rate constants and efficiencies tended to be higher in water collected during June, when in-situ DOM concentration was lower, than in water collected in August and October. Experiments that included water from all six lakes in summer and fall showed that DOM concentration could explain 76% of variation in photodemethylation rate constants. The outcomes from this combination of studies and experiments provide insight for prediction of photodemethylation potential in our study system and for comparison with MeHg concentrations in corresponding food webs. Methylmercury is associated with DOM (DOM-MeHg) in complexes, however in high DOM waters the proportion of DOM that is associated with MeHg (DOM-MeHg) will decrease and this MeHg-free DOM may be critical in regulating photodemethylation reactions. Photodemethylation will still occur in high DOM waters but at a limited rate because a smaller proportion of the photoreactions will involve DOM-MeHg complexes. This is the first study to test and quantify a competitive interaction between MeHg photodemethylation and DOM phototransformations (both photomineralization and photobleaching) to support the conceptual idea that higher dissolved organic carbon systems will have slower rates of photodemethylation. Overall, this compiled body of work yielded a method for predicting seasonal and spatial changes to MeHg concentrations in surface waters depending on environmental and physicochemical factors.

Actions (login required)

View Item