Influence of Dissolved Organic Carbon on Methylmercury Bioavailability across Minnesota Stream Ecosystems

Stream ecosystems are widely contaminated by mercury (Hg) via atmospheric transport and deposition in watersheds. Dissolved organic carbon (DOC) is well-known to be the dominant ligand for aqueous methylmercury (MeHg), the bioaccumulative form of Hg in aquatic food webs. However, it is less clear if and how the concentration and character (e.g., aromaticity) of DOC influences the availability of dissolved MeHg to stream food webs. In this work, we analyzed total-Hg and/or MeHg concentrations in water, seston, and macroinvertebrates (filter-feeding hydropsychid caddisflies), and other physiochemical properties in 30 streams along a south–north geographic gradient in eastern Minnesota that corresponds to substantial changes in dominant land cover (i.e., agriculture, urban, wetland, and forest). In general, MeHg concentrations in seston and hydropsychids were higher in watersheds with more forest and wetland coverage, and increased with dissolved MeHg concentration. However, we found that the efficiency of MeHg incorporation into the stream food webs (i.e., bioconcentration factors of MeHg in both seston and hydropsychids, BCFMeHg = solid MeHg ÷ dissolved MeHg) decreased significantly with DOC concentration and aromaticity, suggesting that MeHg bioavailability to the base of food webs was attenuated at higher levels of terrestrial DOC. Therefore, our findings suggest that there is a dual role of DOC on MeHg cycling in streams: terrestrial DOC acts as the primary carrier ligand of dissolved MeHg for transport into surface waters, yet this aromatic DOC also attenuates dissolved MeHg uptake by aquatic food webs. Thus, consideration of MeHg bioavailability and its environmental regulation could help improve predictive models of MeHg bioaccumulation in stream ecosystems

Appendix A. Macroinvertebrate guild assignments and length–biomass regressions.

Macroinvertebrate guild assignments and length–biomass regressions

Appendix A. Nitrate concentration and stable isotope data for rivers sampled during 2005.

Nitrate concentration and stable isotope data for rivers sampled during 2005

Appendix A. Photographs showing Jackrabbit Spring pool and representative native vegetation reaches, saltcedar reaches, and cleared reaches during and after removal of saltcedar.

Photographs showing Jackrabbit Spring pool and representative native vegetation reaches, saltcedar reaches, and cleared reaches during and after removal of saltcedar

Mercury Bioaccumulation in a Stream Network

Mercury (Hg) contamination is common in stream and river ecosystems, but factors mediating Hg cycling in the flowing waters are much less understood than in the lakes and wetlands. In this study, we examined the spatial patterns of methylmercury (MeHg) concentrations in the dominant groups of aquatic insect larvae across a network of streams (drainage area ranging from 0.5 to 150 km2) in northern California during summer baseflow conditions. We found that, with the exception of water striders, all invertebrate groups showed significant (p −1) while most of the upstream tributaries had aqueous MeHg concentrations close to or below the established detection limits (0.02 ng L−1). A filamentous alga abundant in South Fork Eel River (Cladophora glomerata) had an exceptionally high fraction of total-Hg as MeHg (i.e., %MeHg from 50−100%). Since other potential hotspots of in-stream Hg methylation (e.g., surface sediment and deep pools) had %MeHg lower than or similar to surface water (∼14%), we hypothesize that Cladophora and possibly other autotrophs may serve as hotspots of in-stream MeHg production in this bedrock-dominated stream. Recent studies in other regions concluded that wetland abundance in the watershed is the predominant factor in governing Hg concentrations of stream biota. However, our results show that, in the absence of wetlands, substantial spatial variation of Hg bioaccumulation can arise in stream networks due to the influence of in-stream processes

Effects of Stream Water Chemistry and Tree Species on Release and Methylation of Mercury during Litter Decomposition

Foliage of terrestrial plants provides an important energy and nutrient source to aquatic ecosystems but also represents a potential source of contaminants, such as mercury (Hg). In this study, we examined how different stream water types and terrestrial tree species influenced the release of Hg from senesced litter to the water and its subsequent methylation during hypoxic litter decomposition. After laboratory incubations of maple leaf litter for 66 days, we observed 10-fold differences in dissolved Hg (DHg, a concentrations (e.g., eutrophic streams draining agricultural land) are associated with higher Hg release and methylation compared to more pristine sites (e.g., clear waters from coldwater trout stream). Across six tree species collected at the same site and incubated with the same source water, litter from slower decomposing species (e.g., cedar and pine) yielded higher DHg concentrations than those with more labile carbon (e.g., maple and birch). Percent MeHg, however, was relatively similar among different leaf species (i.e., 61−86%). Our study is the first to demonstrate that stream water chemistry and terrestrial plant litter characteristics are important factors determining Hg release and methylation during hypoxic litter decomposition. These results suggest that certain watershed and aquatic ecosystem properties can determine the levels of MeHg inputs during litterfall events

Impacts on excretion of simulated tradeoffs between predator biomasses.

<p>Estimated total recycled N (NH₄) and P (SRP) excretion (g·day⁻¹) in Fox Cr. due to simulated changes in the relative abundance (by biomass) of <i>O. mykiss</i> and <i>D. tenebrosus</i>. Simulations assumed a fixed total biomass of predators (6275 g) within the study reach, and estimated total excretion rates (left y-axis) and ratios (right y-axis) by bootstrapped re-sampling of surveyed individuals. Predator relative abundance (x-axis) varies by 10% increments from 100% <i>O. mykiss</i> composition to 100% <i>D. tenebrosus</i> composition, expressed as the proportion of predator biomass (salamander:fish). Vertical line indicates the observed ratio of predators in Fox Cr.</p

In Situ Production of Methylmercury within a Stream Channel in Northern California

Natural stream ecosystems throughout the world are contaminated by methylmercury, a highly toxic compound that bioaccumulates and biomagnifies in aquatic food webs. Wetlands are widely recognized as hotspots for the production of methylmercury and are often assumed to be the main sources of this neurotoxin in downstream ecosystems. However, many streams lacking wetlands in their drainage basins (e.g., montane and semiarid regions in the western United States) have significant methylmercury contamination, and the sources of methylmercury in these streams remain largely unknown. In this study, we observed substantial production of methylmercury within a highly productive stream channel in northern California (South Fork Eel River) within a drainage basin lacking wetlands. We found that in situ methylmercury production is positively related to phosphorus removal and water temperature within the stream channel, supporting hypothesized biological mediation of in situ mercury transformation. Moreover, our data suggest that epiphytic microbial communities on a dominant filamentous alga (Cladophora glomerata) could play a role in in situ methylmercury production. Because peak in situ methylmercury production coincides with the period of the highest biological productivity during summer baseflow, methylmercury produced internally may be efficiently routed into local stream food webs. Our study provides strong evidence that stream channels, especially those associated with high primary productivity, can be important for regulating the bioavailability and toxicity of this global contaminant

Density and biomass (abundance survey) and average wet weight elemental composition of diets (diet survey) for <i>O. mykiss</i> and <i>D. tenebrosus</i> in Fox Creek, California.

<p>Values calculated using total area of the study reach.</p

Excretion rates of <i>D. tenebrosus</i>.

<p>Nitrogen (NH₄) and phosphorus (SRP) nutrient excretion rates (ug·min⁻¹) of <i>D. tenebrosus</i>. Lines represent the fit of the top model selected by AICc for P (<i>log₁₀</i>[µg_P·min⁻¹] = −3.12+1.60(<i>log</i>₁₀[mass]), r² = 0.31, P = 0.01), and N excretion rates (<i>log₁₀</i>[µg_N·min⁻¹] = −2.04+1.41(<i>log</i>₁₀[mass]), r² = 0.79, P<<0.001).</p