Some spiders that live near water bodies can transfer mercury contamination from aquatic insects to terrestrial predators, according to a new study. This finding reveals a previously overlooked exposure pathway that can pose health risks to wildlife and humans.
Mercury is a toxic metal that can enter waterways from various sources, such as industrial pollution, mining, volcanic eruptions, and forest fires. In water, bacteria can convert mercury into methylmercury, a more harmful form that can accumulate in the tissues of aquatic organisms. Methylmercury can biomagnify, or increase in concentration, as it moves up the food chain from small fish to large fish, and eventually to humans and other animals that consume fish.
Spiders as a Link Between Aquatic and Terrestrial Ecosystems
Many spiders that live near lakes and rivers feed on aquatic insects, such as dragonflies and mayflies. These insects can carry mercury from their larval stage in water to their adult stage on land, where they become prey for spiders. Spiders, in turn, can be eaten by birds, bats, amphibians, and other terrestrial animals. Thus, spiders can act as a link between aquatic and terrestrial mercury cycles, transferring the metal from water to land.
A Study of Shoreline Spiders and Mercury Sources
To better understand how spiders can move mercury contamination from riverbeds to land animals, a team of researchers from the U.S. Geological Survey and other institutions conducted a study along two tributaries of Lake Superior in the U.S. The researchers collected long-jawed spiders, a common shoreline spider species, as well as sediments, dragonfly larvae, and yellow perch fish from different sites. They analyzed the samples for mercury concentrations and isotopes, which are different forms of the same element that can indicate the source and transformation of mercury in the environment.
The researchers found that the origin of mercury in the sediments was the same as in the aquatic food chain in wetlands, reservoir shorelines, and urban shorelines. For example, when sediment contained a higher proportion of industrial mercury, so did the dragonfly larvae, spider, and yellow perch tissues that were collected. The researchers also found that the spiders had higher mercury levels than the fish, suggesting that they are more efficient at accumulating the metal from their prey.
Implications for Mercury Monitoring and Management
The study, which was published in the journal Environmental Science & Technology Letters, shows that shoreline spiders can reflect the sources and levels of mercury contamination in aquatic ecosystems. The researchers suggest that spiders could be used as indicators of mercury pollution and as tools for monitoring the effectiveness of remediation activities. The study also highlights the need for assessing the risks of mercury exposure to terrestrial animals that feed on aquatic insects and spiders, as well as to humans who consume these animals.