Spiders transfer mercury contamination from water to land animals

Many spiders near water bodies feed on insects like dragonflies. But when these insects inhabit mercury-contaminated waters, they might transfer this hazardous metal to spider species that feed on them. 

Thus, according to a new study led by the U.S. Geological Survey Upper Midwest Water Science Center, spiders with habitats close to the shoreline can move mercury contamination from riverbeds up the food chain to land animals.

Contaminant transfer

Adult aquatic insects provide subsidies to linked terrestrial ecosystems, supporting consumers such as birds, bats, amphibians, and spiders. 

However, contaminants that are accumulated by aquatic insect larvae and retained through metamorphosis can result in insect-mediated contaminant transfer (i.e., flux) to terrestrial biota.

“The transfer of contaminants from aquatic to terrestrial ecosystems represents an often overlooked exposure pathway that can elevate health risks to wildlife, highlighting the need for monitoring recipient terrestrial animals that eat aquatic insects emerging from contaminated waters,” the authors explained.

Methylmercury

A significant proportion of the mercury present in aquatic environments comes from industrial waste and other human-related activities (although there are natural sources of mercury too). 

When this mercury reaches the water, microorganisms transform it into methylmercury, a more toxic form that biomagnifies and increases in animals up the food chain. 

As a result, researchers have increasingly recognized spiders living on lakeshores and riverbeds as a potential link between aquatic contamination and predominantly terrestrial creatures such as birds, amphibians, and bats. 

How the research was conducted

To better understand this phenomenon, the experts assessed whether shoreline spiders’ tissue contained mercury from adjacent aquatic sources and examined how these animals connect mercury pollution in water and land animals.

The scientists collected long-jawed spiders found near two tributaries of Lake Superior, and gathered samples of sediments, dragonfly larvae, and yellow perch fish from these locations. 

Afterwards, they identified and measured the various sources of mercury contamination, including direct industrial discharge, rainwater, and land runoff. 

What the researchers learned

The analysis revealed that the origin of mercury in the sediments was identical up the aquatic food chain in wetlands, reservoir shorelines, and urban shorelines. 

For example, when sediment contained a higher level of industrial mercury, so did the dragonfly larvae, spider, and yellow perch tissues that they examined. According to this data, long-jawed spiders could indicate how mercury pollution moves from aquatic environments to land-based wildlife. 

Thus, such spiders could shed new light on the sources of mercury pollution and inform management decisions by providing a novel tool for the monitoring of remediation activities.

The researchers also collected and analyzed tissue samples from two additional types of arachnids from certain locations: the fishing spider and the orb-weaver spider. 

Study implications 

The data indicated variability in mercury sources across these different taxa. Such variability can be attributed to their distinctive feeding patterns: while fishing spiders predominantly hunt on land near water, orb-weavers consume both water and land-based insects. 

The long-jawed variety, however, mainly preys on adult water insects. Thus, these findings suggest that, although long-jawed spiders are reliable indicators of aquatic pollution, not every species living near the shoreline can be considered an effective mercury pollution sentinel.

Mercury contamination 

Mercury contamination is a serious environmental issue caused by the release of mercury into ecosystems. It can occur through industrial processes, mining, and natural sources. 

Mercury is toxic to humans and wildlife, and it can accumulate in organisms, leading to health problems, including harmful effects on the nervous, digestive and immune systems.

Efforts to reduce mercury contamination include stricter regulations on emissions and responsible disposal of mercury-containing products.

The study is published in the journal Environmental Sciences & Technology Letters.

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