DOES MERCURY BIOACCUMULATE IN AMPHIBIANS DEVELOPING IN VERNAL POOLS?

Vernal pools (short-hydroperiod seasonal wetlands) are a wetland type at tremendous risk due to habitat degradation and loss, triggering recent legislation in Maine aimed at vernal pool conservation. Vernal pools host a diversity of biota (e.g., wood frogs [Rana sylvatica], ambystomatid salamanders [Ambystoma spp.], fairy shrimp [Eubranchipus spp.]) adapted to larval development in temporary waters. Functions of these wetlands are compromised by the subtle threat of non-point source pollution. We studied total mercury (Hg) concentrations in 4 vernal pools in Acadia National Park (ANP), Maine, where elevated atmospheric deposition of Hg to the landscape is well-documented, to determine if mercury is bioaccumulated by developing wood frogs. We characterized the chemical environment in pool water, upland and pool litter, and sediment to 2 cm depth in 4 short-hydroperiod vernal pools from April snowmelt (weekly within one week of wood frog egg deposition), immediately before pool dry-down in late May-June (encompassing larval Gosner Stages 24-37), and during pool refill in early October. Total Hg concentration in pool water increased steadily during April-June. During October pool-refill, total Hg concentrations in water in 2 pools were comparable to those in June, whereas, concentrations dropped in the other two pools to levels measured soon after April ice melt. There were no strong temporal patterns in sediment, upland and within-wetland leaf litter total Hg; however, concentrations of total Hg in upland litter collected within 3 m of the pool edge were consistently 2-4 times greater than in litter collected within the pool. Total mercury masses for embryos within one week of egg-laying were below the lower limit for ultralow calibration, whereas, total Hg concentrations in larvae ranged 15.4-44.1 ppb 3 weeks post-hatch, comparable to concentrations previously measured in ANP in (Rana clamitans) and bullfrog (R. catesbeiana) larvae that were two-years old. While our study pools dried before larval metamorphosis, the relatively rapid bioaccumulation of total mercury suggests that juvenile wood frogs could be potential vectors of mercury bioaccumulated in pools retaining water long enough for successful metamorphosis and subsequent emigration and dispersal into the surrounding upland environment.