|English Abstract: |
Mercury is a heavy metal that is toxic to both humans and animals. It is emitted to the atmosphere through industrial processes and thereafter globally distributed; local pollution problems can also occur. Mercury bioaccumulates and under wet, suboxic conditions is chemically altered into methylmercury (MeHg), which is more biologically available. For this reason, traditionally it has been thought that large, top predators in aquatic ecosystems are most at risk; less is known about terrestrial ecosystems. Emissions are particularly high in Asia, and the route of exposure for humans in agricultural areas may be through rice, rather than fish. However, how mercury flows through natural wild ecosystems near rice paddies has not been fully explored in Asia, nor have the concentrations of mercury in organisms in such ecosystems been measured, to understand how their survival and reproduction might be affected.
We tested the mercury levels of free-ranging animal community nearby rice paddies in the Wanshan Mercury District, Guizhou Province, China, one of Asia’s largest mercury mines until its closure in 2001, and at a control site 200 km away, also in Guizhou, but with no history of mercury mining. We measured total mercury (THg) concentrations, and the percent methylated, in soil, rice, and a range of organisms, including earthworms, invertebrate rice pests, carnivorous invertebrates and frogs, snakes, fish, song birds, owls, kingfishers and humans. The sampling schemes for different taxa varied, but we attempted to sample organisms at a varying distances form the mines in Wanshan when possible. Most sampling occurred in 2014, but in 2015 we collected invertebrates at two different seasons: before the rice growing season, and close to harvest. At the most contaminated areas in Wanshan, we also collected samples for stable isotope analysis.
In the first project that focused on earthworms, THg and MeHg concentrations declined in soil and earthworms as distance increased from the mines, but the percentage of THg that was MeHg, and bioaccumulation factors in earthworms, increased over this gradient. This escalation in methylation and the incursion of MeHg into earthworms may be influenced by more acidic soil conditions and higher organic content further from the mines. In areas where the source of mercury is deposition, especially in water-logged and acidic rice paddy soil, earthworms may biomagnify MeHg more than was previously reported.
The second project focused on spiders and grasshoppers. Using the 2014 data, we saw a similar pattern as in earthworms, with the THg and MeHg concentrations in spiders and grasshoppers decreasing with increasing distance from the abandoned mines, and this pattern was statistically significant for all species. Patterns in the percentage of THg that was MeHg were complicated, with most species having a higher percentage methylation farther from the mines, IV
but grasshoppers showing a similar percentage of MeHg everywhere. There were strong correlations between soil THg/MeHg levels and invertebrate MeHg levels. Among the spiders, there was a tendency for those at higher trophic levels, as judged by stable isotopes, to have higher MeHg concentrations. For the 2015 two season data, THg concentrations in soil were stable during two seasons, but soil MeHg concentrations were significantly higher when rice was close to being mature. For all invertebrate species, and all sites, there was a strong seasonal increase in MeHg.
The third project had the widest taxonomic sampling, covering organisms at all trophic levels in this ecosystem. We measured THg and MeHg in soil, rice, 38 animal species (27 for MeHg), and humans. Our results confirm bioaccumulation of THg, and particularly MeHg, exhibiting exceptionally high trophic magnification slopes. Invertivorous songbirds accumulated the highest THg concentrations; at mining sites this was 15x the concentration reported to significantly impair their reproduction, and at non-mining sites it was 3x this level. High concentrations in specialist rice pests and in granivorous birds, as high as in piscivorous birds, suggest rice is a primary source of exposure.
Our findings suggest there could be significant population level health effects and consequent biodiversity loss in sensitive ecosystems, like agricultural wetlands, across Asia. Multinational biomonitoring with invertivorous songbirds is necessary for assessing this risk and for successful implementation of the Minamata Convention on Mercury.