How Spiders Can Help Us Connect Aquatic Mercury Contamination to Terrestrial Food Webs
Just in time for Halloween, this "spooky science" study focuses on how spiders can help us connect aquatic mercury contamination to terrestrial food webs.
Mercury is a global pollutant, meaning it is distributed across the globe and can result in high mercury levels in fish and wildlife. Over the course of
many decades, mercury has entered aquatic environments through different means including atmospheric delivery, historical industrial discharges from point sources, and non-point source runoff. This myriad of sources has made it difficult for scientists to ascertain mercury cycling and food web contributions within both aquatic and terrestrial ecosystems.
Scientists have been measuring how much mercury is in spiders to assess how much pollution is in the water they live next to. Many spiders are shoreline specialists that live next to water on trees and docks and feed mainly on adult aquatic insects. Contaminants in the water can ‘hitch a ride’ on these insects when the go through their metamorphosis and fly to land to breed and complete their life cycle. These spider biosentinels have been critical for assessing mercury exposure because they are a bridge between aquatic and terrestrial food webs. Spiders eat lots of adult aquatic insects and are themselves key prey items for many terrestrial predators like birds (see Figure 1). While we have been able to track mercury movement and exposure through these pathways, we have not yet been able to determine the source of mercury in the aquatic environment, which is critical for assessing environmental injury within sites that have historical industrial contamination.
Within this study, we examined one of the biggest industrially impacted sites within the Great Lakes. The St. Louis River, located between Wisconsin and Minnesota, is polluted with mercury as well as other contaminants. While elevated mercury is a known problem within the St. Louis River, it was unclear if the mercury entering the food web was related to past industrial contamination or to atmospheric mercury deposition. To assess these sources and their influence on both the aquatic and terrestrial food webs, we used mercury stable isotope tracers. These tracers are effectively mercury fingerprints that can provide us with insight on where the mercury came from and the different processes it underwent in the environment. Using this tool, we have previously been able to identify the relative contributions of different mercury sources to sediments in the St. Louis River (see Figure 2). However, we also wanted to see if these industrial mercury sources were moving from sediments to aquatic and terrestrial food webs.
We used spider biosentinels to determine if legacy contamination in sediments within the St. Louis River was making it to aquatic insects (e.g., dragonfly larvae) and spiders (e.g., long jawed spiders) across a range of different habitat types. Our results astoundingly showed that the mercury fingerprint found in industrially contaminated sediments was preserved in aquatic insects during metamorphosis and could be measured in spiders feeding on the emergent adult insects. This indicated that historical legacy mercury was still actively contributing to mercury exposure within both the aquatic and terrestrial food webs of the St. Louis River ecosystem. Effectively, our results demonstrate that you can collect a spider living in a bush next to the river and it is capable of telling you if the mercury in that river came from industrial discharge or atmospheric deposition based on its mercury isotope signature. With these findings we hope to further apply spider bioindicators at other mercury-contaminated sites to inform sources of pollution, and to see if clean-up efforts are working.
Sarah Janssen is a research chemist in the Upper Midwest Water Science Center and the lead for the Mercury Research Laboratory. Her work examines mercury sources and cycling in freshwater ecosystems including the Great Lakes and the Florida Everglades. Recent work from her group has leveraged mercury stable isotopes to track industrial mercury contamination at sites around the country.
Christopher Kotalik is a research ecologist at the Columbia Environmental Research Center. His research focuses on evaluating the fate and effects of contaminants in aquatic food webs, including the transfer of aquatic contaminants to terrestrial ecosystems. Chris’ more recent work is modelling the biogeochemical drivers of mercury risk in aquatic ecosystems across the United States.
Collin Eagles-Smith is a supervisory research ecologist at the Forest and Rangeland Ecosystem Science Center. His work focuses on the ecological factors regulating contaminant bioaccumulation and trophic transfer, with an emphasis on contaminant cycling through food webs.
David Walters is a supervisory research ecologist at the Columbia Environmental Research Center. His current research topics include food webs and contaminant flux, aquatic-riparian linkages, stream fish ecology, land use and climate change, and invasive species.