Seasonality of odonate‐mediated methylmercury flux from permanent and semipermanent ponds and potential risk to red‐winged blackbirds (Agelaius phoeniceus)

Abstract

Methylmercury (MeHg) is an aquatic contaminant that can be transferred to terrestrial predators by emergent aquatic insects such as odonates (damselflies and dragonflies). We assessed the effects of month and pond permanence on odonate-mediated MeHg flux (calculated as emergent odonate biomass × MeHg concentration) in 10 experimental ponds and the potential risk to nestling red-winged blackbirds (Agelaius phoeniceus) posed by consuming MeHg-contaminated odonates. Emergent odonates were collected weekly from permanent ponds with bluegill (Lepomis macrochirus; n = 5) and semipermanent ponds without fish (n = 5) over an 8-mo period (January-August 2015). The MeHg flux from damselflies, aeshnid dragonflies, and libellulid dragonflies began in March and peaked in April, May, and June, respectively, and then declined throughout the rest of the summer. Odonate-mediated MeHg flux from semipermanent ponds without fish was greater than that from permanent ponds with fish. Nesting of red-winged blackbirds overlapped with peak odonate emergence and odonate-mediated MeHg flux. Because their diet can be dominated by damselflies and dragonflies, we tested the hypothesis that MeHg-contaminated odonates may pose a health risk to nestling red-winged blackbirds. Concentrations of MeHg in odonates exceeded wildlife values (the minimum odonate MeHg concentrations causing physiologically significant doses in consumers) for nestlings, suggesting that MeHg-contaminated odonates can pose a health risk to nestling red-winged blackbirds. Environ Toxicol Chem 2017;36:2833-2837. © 2017 SETAC.

Mercury exposure to red-winged blackbirds (Agelaius phoeniceus) and dragonfly (Odonata: Aeshnidae) nymphs in Prairie Pothole wetlands

“…However, studies show that odonates contribute a large proportion of MeHg flux from ponds in the Great Plains (Chumchal and Drenner 2015). This flux can be greater in fishless ponds, such as in our study (compared with fish consuming invertebrates; Chumchal et al 2017), which may result in some risk to more vulnerable organisms such as passerine nestlings (Williams et al 2017).…”

“…As migrating birds require high protein intake to sustain the energetic costs of migration and breeding, feeding on organisms originating from the wetlands represents MeHg transfer from aquatic systems to the terrestrial food web and poses a risk to these populations. In systems too small to support permanent fish populations, dragonfly nymphs (Odonata: Anisoptera: Aeshnidae) can be responsible for a large flux of MeHg from aquatic to terrestrial food webs (Haro et al 2013;Chumchal and Drenner 2015;Chumchal et al 2017) and therefore may impact the health of invertivorous songbirds (Williams et al 2017). Given that PPR wetlands and their passerine residents will likely experience further decline with persistent habitat destruction, wetland drainage, and climate change (Steen et al 2016), it is important to assess Hg exposure to inform strategies to mitigate multiple environmental stressors on these populations.…”

Seasonality of dipteran‐mediated methylmercury flux from ponds

“…Type II errors (accepting a false null hypothesis) can be controlled and power can be increased by increasing alpha (Keppel and Wickens 2004). In the present study, we reduced the chance of making a type II error and increased power by increasing alpha to p 0.10 as we have done in previous pond experiments (Drenner et al 1998;Chumchal et al 2005;Tweedy et al 2013;Williams et al 2017).…”

“…The present study is phase 2 of the experiment in which we examined the seasonality of insect‐mediated MeHg flux from permanent fish ponds and semipermanent fishless ponds from winter through summer 2015 (Table ). We have previously published a paper examining the seasonality of odonate‐mediated MeHg flux from that experiment (Williams et al ). The present study describes the seasonality of dipteran‐mediated MeHg flux.…”

“…Methylmercury is transported to terrestrial ecosystems by MeHg-contaminated insects emerging from aquatic ecosystems (referred to as insectmediated MeHg flux; Chumchal and Drenner 2015). Terrestrial consumers, like songbirds, bats, and spiders are exposed to MeHg by consuming MeHg-contaminated emergent insects (Gerrard and St. Louis 2001;Tweedy et al 2013;Gann et al 2015;Schulwitz et al 2015;Korstian et al 2018;Williams et al 2017;Becker et al 2018). Previous research has suggested that insect-mediated MeHg flux can be high enough to pose a health risk to songbirds (Williams et al 2017).…”