Search

Forest harvesting affects dissolved organic matter (DOM) and aqueous mercury inputs as well as the food web structure in small‐headwater streams, but how these upstream changes manifest downstream is unclear. To address this uncertainty, we examined DOM quality, autochthony in the caddisfly Hydropsychidae (using δ2H), and methylmercury (MeHg) concentrations in stream water and the caddisfly along a longitudinal gradient (first‐ to fourth‐order streams, subcatchments of 50–1900 ha) in paired partially harvested and reference catchments in central Ontario, Canada. Although measures of DOM quality (specific ultraviolet absorbance at 254 nm 2.20–11.62) and autochthony in caddisflies (4.9%–34.0%) varied among sites, no upstream‐to‐downstream differences in these measures were observed between the paired harvested and reference catchments. In contrast, MeHg levels in stream water (0.06–0.35 ng/L) and caddisflies (29.7–192 µg/kg dry wt) were significantly higher in the upstream sites but not the farthest downstream sites in the harvested catchments compared to the reference catchments. This suggests that while current mitigation measures used by forestry companies did not prevent elevated MeHg in water and invertebrates at smaller spatial scales (subcatchments of 50–400 ha), these upstream impacts did not manifest at larger spatial scales (subcatchments of 800–1900 ha). The present study advances our understanding of spatially cumulative impacts within harvested catchments, which is critical to help forest managers maintain healthy forest streams and their provisioning of aquatic ecosystem services. Environ Toxicol Chem 2022;41:1490–1507. © 2022 SETAC [ABSTRACT FROM AUTHOR]

Mercury contamination is a growing concern for freshwater food webs in ecosystems without point sources of mercury. Methylmercury (MeHg) is of particular concern, as this is the form of mercury that crosses the blood-brain barrier and is neurotoxic to organisms. Wetlands and benthic sediments have high organic content and low oxygen availability. Anaerobic bacteria drive the metabolic function in these ecosystems and subsequently can methylate mercury. The bioavailability of MeHg is controlled by physicochemical characteristics such as pH, binding affinities, and dissolved organic matter (DOM). Similarly, photodemethylation is influenced by similar characteristics and thereby the two processes should be studied in tandem. The degradation of MeHg through photochemistry is an effective destruction mechanism in freshwater lakes. This review will highlight the uncertainties and known effects of DOM on subsequent photoreactions that lead to the occurrence of mercury photodemethylation and reduction in mercury bioavailability in freshwater ecosystems. [ABSTRACT FROM AUTHOR]

The present study examined potential effects of seasonal variations in photoreactive dissolved organic matter (DOM) on methylmercury (MeHg) photodemethylation rates in freshwaters. A series of controlled experiments was carried out using natural and photochemically preconditioned DOM in water collected from 1 lake in June, August, and October. Natural DOM concentrations doubled between June and August (10.2-21.2 mg C L−1) and then remained stable into October (19.4 mg C L−1). Correspondingly, MeHg concentrations peaked in August (0.42 ng L−1), along with absorbances at 350 nm and 254 nm. Up to 70% of MeHg was photodemethylated in the short 48-h irradiation experiments, with June having significantly higher rates than the other sampling months ( p < 0.001). Photodemethylation rate constants were not affected by photoreactive DOM, nor were they affected by initial MeHg concentrations ( p > 0.10). However, MeHg photodemethylation efficiencies (quantified in moles MeHg lost/moles photon absorbed) were higher in treatments with less photoreactive DOM. Congruently, MeHg photodemethylation efficiencies also decreased over summer by up to 10 times across treatments in association with increased photoreactive DOM, and were negatively correlated with DOM concentration. These results suggest that an important driver of MeHg photodemethylation is the interplay between MeHg and DOM, with greater potential for photodemethylation in freshwaters with more photobleached DOM and lower DOM content. Environ Toxicol Chem 2017;36:1493-1502. © 2016 SETAC [ABSTRACT FROM AUTHOR]

Abstract: Water samples were collected from 20 wetland, river and lake sites across Eastern Ontario and Western Quebec to investigate the distribution of methylmercury (MeHg) associated with various size fractions of dissolved organic matter (DOM). Tangential Flow UltraFiltration (TUF) was used to fractionate DOM by nominal molecular size (<0.2μm, <300kDa, <30kDa, <5kDa and <1kDa). DOM fluorescence (DOM FL) and absorbance (DOC Abs) were used to quantify DOM photoreactivity and aromaticity in each sample. Significant differences in the size-associated distribution of MeHg, Dissolved Organic Carbon (DOC), DOM FL, and DOM Abs were observed between wetlands, rivers, and lakes. The low molecular weight (LMW) fraction (<5kDa) in wetlands contained the majority of MeHg (70.0±13.8%), DOC (56.1±9.4%), and DOM FL (77.4±7.5%). DOM FL was also high in the LMW fraction for rivers (60.6±25%) and lakes (75.2±16.9%). Mean MeHg concentrations in the LMW fraction of lakes (41±26pgL−1) and rivers (32±19pgL−1) were substantial but much lower than wetlands. Rivers had the highest percentage of methylmercury (38.0±23.5%) in the particulate (>0.2µm) fraction. This research highlights the importance of low molecular weight dissolved organic matter in methylmercury fate. For example, a large proportion of MeHg was found in the LMW weight fractions (mean=47.3±25.4%) of the wetlands, rivers, and lakes in this study. [Copyright &y& Elsevier]

Daytime volatilization of gaseous elemental mercury (Hg(0) aq) is a significant mechanism for mercury removal from aquatic systems and potentially limits the production and bioaccumulation of methylmercury. Changes in incoming solar radiation (in the ultraviolet range), dissolved organic matter, salinity, and total suspended particles were investigated concurrently with several mercury species (Hg(0) aq , dissolved total mercury (THg), easily reducible mercury (ERM), and mercury associated with total suspended solids (THg TSS)) during daylight hours near the mouth of a hypertidal river. There were no predictable temporal changes observed for Hg(0) aq in unfiltered surface water. Hg(0) aq ranged from 0 to 12 pg L−1, THg ranged from 0 to 492 pg L−1, ERM ranged from 13 to 381 pg L−1, and THg TSS ranged from <1.58 ng g−1 to 261.32 ng g−1. The range of Hg(0) aq predicted by the empirical model was similar to measured ERM concentrations, but it was shown that ERM did not significantly predict in-situ photoreducible Hg(II) (Hg(II) RED). Production of Hg(0) aq appears to largely be suppressed by suspended solids, which limits ultraviolet radiation transmission through surface water. Comparison of these results to an empirical model developed for this site to predict Hg(0) aq indicates that significantly more mercury is available for photoreduction near the mouth of the tidal river, and that Hg(II) will likely photoreduce quickly when TSS levels decrease with ocean mixing. [Display omitted] • Divalent mercury photoreduction lessens mercury concentrations in ecosystems. • Estuarine water samples were taken every 0.5 h from sunrise to sunset. • Empirical model predicts higher mercury photoreduction than measured in the field. • High suspended solids limited mercury photoreduction. [ABSTRACT FROM AUTHOR]

Quantifying methylmercury (MeHg) concentrations and uptake at the base of the food web is useful for assessing mercury exposure risk to higher trophic level organisms. Higher MeHg concentrations near the base of the food web may result in more MeHg exposure and accumulation in higher trophic organisms. Here, we analyze MeHg in caddisflies, mayflies, lake water, and sediment collected from two temperate lakes and one brook in Kejimkujik National Park, Nova Scotia, Canada. Overall, caddisfly larvae MeHg (15.38–276.96 ng/g; n = 29) was not significantly correlated with water chemistry. Whereas mayfly naiads MeHg (14.28–166.82 ng/g; n = 31) was positively correlated with water MeHg (r s = 0.43), negatively correlated with pH (r s = −0.49), and positively correlated with dissolved organic carbon (DOC; r s = 0.48). Of the mercury in insect tissues, the %MeHg ranged from 56 to 75 % in caddisfly larvae and 38–47 % in mayfly naiads. MeHg bioaccumulation factors (BAF) varied greatly (water to tissue BAFs = 0.145 × 106–1.054 × 106; sediment to tissue BAFs = 0.017 × 106–0.541 × 106). This study highlights the importance of quantifying variations in MeHg bioaccumulation and BAFs of common aquatic insect bioindicators at the base of complex food webs. [Display omitted] • Mayfly MeHg was more influenced by water than sediment. • Caddisfly larvae MeHg may be more influenced by sediment than water. • Each invertebrate type had wide %MeHg ranges likely due to feeding ecology. • Caddisflies and mayflies are good indicators of methylmercury loading to food webs. • Adult caddisflies and mayflies efficiently transfer MeHg to terrestrial food webs. [ABSTRACT FROM AUTHOR]