Mercury may be moving from aquatic to land food webs in a much greater quantity than researchers once thought. Scientists have discovered that when stream insects are consumed by spiders, mercury enters the food web on land.
Mercury concentrations in streams, rivers and other aquatic environments have increased drastically globally. With the increase in mercury, populations of fish and other aquatic species have been exposed to these levels which are often present in their tissues.
Most research on the transfer of methylmercury, which is the toxic and bioaccumulating form of mercury, between aquatic and terrestrial food webs has focused on land carnivores that largely eat fish. An example of this would be bears who eat salmon. Before now, though, researchers haven't really looked at other terrestrial predators that consume aquatic insects.
Dissolved organic carbon can bind tightly to methylmercury. This, in turn, affects its transport and availability in aquatic food webs. The researchers believed that it was possible that dissolved organic carbon affects methylmercury transfer from stream food webs to terrestrial predators feeding on emerging adult insects. To find out if this was true, the researchers collected data for two years from 10 streams.
So what did they find? It turns out that there is a transfer of methylmecury from aquatic systems to terrestrial systems when insects from streams emerge and are caught and consumed by spiders. In addition, the researchers found that the amount of methylmercury that's bioavailable, which means usable by organisms, is largely dependent on the amount of dissolved organic carbon in the water. Streams with intermediate dissolved organic carbon had the highest levels of methylmercury concentrations.
"Our paper is important because the role of dissolved organic carbon in mediating the bioavailability of methylmercury is still poorly understood, and the reach of methylmercury from aquatic systems where it is produced to terrestrial systems is broader than previously recognized," said Ramsa Chaves-Ulloa, lead author who conducted the study as part of her Ph.D. at Dartmouth's Department of Biological Sciences.
The findings reveal a bit more about these aquatic food webs and how methylmercury is passed from streams to land. This is particularly important to note in areas where there are higher concentrations of mercury due to industrial practices.
The findings are published in the March 2016 journal Ecological Applications.
