Methylmercury (MeHg) is a neurotoxicant that is known to biomagnify in food webs and can result in fish consumption advisories. In aquatic invertebrates, MeHg concentrations are known to be highly variable (up to 30-fold) within and among freshwater systems and are a key predictor of mercury in fish, as diet is the main route of exposure. As such, there is global interest in understanding why some systems and invertebrate species accumulate more MeHg than others. The factors driving MeHg variability in aquatic invertebrates are not fully understood, but appear to correlate in part with diet and the physical and chemical parameters of the system. Since MeHg is primarily bound to the amino acid cysteine in proteins, my hypotheses were that cysteine content will be species-specific and that it will explain the within and among taxa variability in MeHg. In 2013 and 2014, food web taxa (benthic invertebrates, zooplankton, and fish) were collected from six lakes within Kejimkujik National Park, Nova Scotia, Canada that are known to vary in physical and chemical characteristics and biotic MeHg. Accurate detection and quantification of amino acids (especially thiols) are needed before they can be applied to the understanding of MeHg variability. I developed and validated a UPLC method to analyze amino acids in aquatic invertebrates, biofilm, and fish muscle. Overall, performance characteristics were reproducible, accurate, and comparable to other methods and tissues. The amino acid compositions of aquatic invertebrates, an understudied group, were significantly different based largely on their percentages of cysteine, histidine, proline, and aspartic acid + asparagine. Cysteine content differed among invertebrate taxa, with the lowest concentrations in Limnephilidae caddisflies (4.67 ± 0.58 nmol mg-1 tissue) and the highest concentrations in zooplankton (7.82 ± 2.88 nmol mg-1 tissue), but not among lakes (pH range 5.5-6.2). Primary consumers feeding on a benthic carbon source (Limnephilidae and Heptageniidae) had less cysteine and less MeHg than the pelagic primary consumers (bulk zooplankton). MeHg was significantly and positively related to cysteine content in five of the seven taxa. Despite this, more of the within-taxa variability was explained by δ15N, although this does not hold true when isotope data were adjusted for among-system differences in basal δ15N. This research generated new knowledge on 1) the importance of phylogenetics, diet, and lake characteristics in determining the amino acid composition of aquatic invertebrates and 2) on cysteine as a predictor of MeHg concentrations within aquatic invertebrates.
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