Abstract
Tadpoles are important prey items for many aquatic organisms and often represent the largest vertebrate biomass in many fishless wetland ecosystems. Neurotoxic mercury (Hg) can, at elevated levels, decrease growth, lower survival, and cause developmental instability in amphibians. We compared total Hg (THg) body burden and concentration in boreal chorus frog ( Pseudacris maculata) and wood frog ( Rana sylvatica) tadpoles. Overall, body burden and concentration were lower in boreal chorus frog tadpoles than wood frog tadpoles, as expected, because boreal chorus frog tadpoles consume at lower trophic levels. The variables species, stage, and mass explained 21% of total variation for body burden in our models but had negligible predictive ability for THg concentration. The vast majority of the remaining variation in both body burden and THg concentration was attributable to differences among ponds; tadpoles from ponds in three areas had considerably higher THg body burden and concentration. The pond-to-pond differences were not related to any water chemistry or physical parameter measured, and we assumed that differences in wetland geomorphology likely played an important role in determining Hg levels in tadpoles. This is the first report of Hg in frog tadpoles in the Prairie Pothole Region of North America.
Highlights
The elevated transport and deposition of mercury (Hg) to remote pristine areas, such as the prairie region of North America, occurs via anthropogenic Hg emissions (Schuster et al 2002)
The Prairie Pothole Region (PPR) of North America is an area of abundant isolated wetland ponds that are usually small, shallow, and productive water bodies surrounded by riparian zones that are usually dominated by cattails (Typha latifolia), bulrushes (Schoenoplectus spp.), sedges (Carex spp. and Scirpus spp.), and willows (Salix spp.) (Batzer and Sharitz 2006; Johnson et al 2010; Hayashi et al 2016)
Hg cycling in the wetland ponds of the PPR is vastly understudied
Summary
The elevated transport and deposition of mercury (Hg) to remote pristine areas, such as the prairie region of North America, occurs via anthropogenic Hg emissions (Schuster et al 2002). The Prairie Pothole Region (PPR) of North America is an area of abundant isolated wetland ponds that are usually small, shallow, and productive water bodies surrounded by riparian zones that are usually dominated by cattails (Typha latifolia), bulrushes (Schoenoplectus spp.), sedges (Carex spp. and Scirpus spp.), and willows (Salix spp.) (Batzer and Sharitz 2006; Johnson et al 2010; Hayashi et al 2016) These wetlands provide many ecosystem services including groundwater recharge, flood control, water quality, and carbon and nutrient sequestration (van der Kamp et al 2003; Hayashi et al 2016). Deposited Hg may be readily methylated (Hoggarth et al 2015) and accumulated (Sando et al 2007; Hall et al 2009) in these environments because prairie wetlands have potentially high dissolved organic carbon (DOC) concentration, a warm temperature in the summer months, and low dissolved oxygen; all variables which lead to anoxic conditions in which the main methylating organisms (sulfate and iron reducing bacteria and methanogens (Gilmour et al 2013; Parks et al 2013)) thrive (Paranjape and Hall 2017)
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