Mercury in the Mountains
Groundbreaking research just published by the Vermont Center of Ecostudies reports on a surprising finding. Toxic mercury is bioaccumulating in the terrestrial montane food web. The team of biologists, which included me, tracked mercury up the food web – from tree needles and leaves to insects and spiders to salamanders and songbirds, up to top predators such as hawks and owls.
From 2004 – 2007 we sampled leaf litter, foliage from three dominant tree species (balsam fir, heart-leaved paper birch, and mountain ash), arthropods, the terrestrial red-backed salamander, Bicknell’s Thrush, northern-saw-whet owls and sharp-shinned hawks.
Spiders had mercury levels higher than other invertebrates we sampled. This may help explain one puzzling finding from our thrush samples. We know that individual Bicknell’s Thrushes sampled early and late in a single breeding season invariably show declining blood mercury concentrations. We have long suspected that spiders comprise a large portion of the early season (late May – mid-June) diet, since few invertebrates are available at this time.
We know from many hours of video recording at nests that adults feed nestlings a steady diet of caterpillars and other late-emerging insects. The relatively high mercury concentrations in spiders, most of which are insectivorous and thus occupy a higher trophic level, may well be responsible for the higher early-season levels in Bicknell’s Thrush. The subsequent decline in thrush blood mercury may reflect a progressive diet switch to foliage consuming arthropods.
The two top predatory birds, sharp-shinned hawk and northern saw-whet owl, had much higher mercury in their blood than the thrushes. The hawks had nine times more mercury in their blood than the owls on average. These owls mostly feed on vegetation eating red-backed voles, while the hawks are feeding on songbirds that are a much higher trophic level. Unlike the thrushes, these birds did not show a change in mercury levels as the season progressed.
There were significant year effects in different ecosystem compartments indicating a possible connection between atmospheric mercury deposition, detrital-layer mercury concentrations, arthropod mercury concentrations, and passerine blood mercury concentrations. Most of the atmospheric mercury that falls on the mountains in the Northeast likely comes from coal fired power plants in the Midwest.
Our findings provide strong evidence that mercury bioaccumulates and biomagnifies in the montane forest biotic community. Our previous work has already demonstrated that these high-elevation coniferous forests are particularly vulnerable to climate change. The cumulative impacts of climate change, mercury contamination, acid precipitation and other stressors on these “sky islands” could threaten their unique flora and fauna.