Aquatic ecosystems around the world are contaminated with a wide range of anthropogenic chemicals, including metals and organic pollutants, that originate from point and nonpoint sources. Many of these chemical contaminants have complex environmental cycles, are persistent and bioavailable, can be incorporated into aquatic food webs, and pose a threat to the health of wildlife and humans. Identifying appropriate sentinels that reflect bioavailability is critical to assessing and managing aquatic ecosystems impacted by contaminants. The objective of the present study is to review research on riparian spiders as sentinels of persistent and bioavailable chemical contaminants in aquatic ecosystems. Our review of the literature on riparian spiders as sentinels suggests that significant progress has been made during the last two decades of research. We identified 55 published studies conducted around the world in which riparian spiders (primarily of the families Tetragnathidae, Araneidae, Lycosidae, and Pisauridae) were used as sentinels of chemical contamination of lotic, lentic, and estuarine systems. For several contaminants, such as polychlorinated biphenyls (PCBs), Hg, and Se, it is now clear that riparian spiders are appropriate sentinels. However, many contaminants and factors that could impact chemical concentrations in riparian spiders have not been well characterized. Further study of riparian spiders and their potential role as sentinels is critical because it would allow for development of national‐scale programs that utilize riparian spiders as sentinels to monitor chemical contaminants in aquatic ecosystems. A riparian spider sentinel program in the United States would be complementary to existing national sentinel programs, including those for fish and immature dragonflies. Environ Toxicol Chem 2022;41:499–514. © 2021 SETAC

Mercury is a global pollutant whose supply to the environment has greatly increased due to human activities resulting in adverse effects on human and wildlife health. In response, the United States has substantially reduced mercury emissions over the last two decades, largely through air quality management; however, questions remain on the effectiveness of these emission controls in reducing mercury inputs to local ecosystems, due to a disproportional influence of global pools over local sources. Here, we show that trends in mercury concentrations in air and precipitation are largely decreasing across monitoring sites in the United States, coincident with decreases in stationary mercury emissions. Significant decreasing trends primarily occurred in the eastern United States, while trends in the western United States were more often insignificant or increasing, despite no clear increases in local, stationary emissions. For the eastern United States, local emissions appear to have had larger impacts on atmospheric and precipitation mercury concentrations than has been suggested from simulations using atmospheric models, suggesting that reductions in local mercury emissions can confer meaningful benefits to mercury-contaminated areas. These findings highlight the success of air management policies in the United States and are especially timely, as the United States Environmental Protection Agency has decided to roll back the Mercury and Air Toxics Standards.

There is growing concern over the health of North American birds, with evidence suggesting substantial population declines. Spiders are prominent dietary items for many bird species and mediate the transfer of contaminants to arachnivorous birds that consume them. Few studies have investigated the potential risk the spider exposure pathway poses to these birds because most studies have focused on piscivores. In the present study, we developed new chronic and acute As, Cd, Cu, Pb, Ni, Se, Zn, and MeHg spider-based avian wildlife values (SBAWVs) for multiple adult and nestling birds (primarily passerines) and then used the newly generated SBAWVs to characterize the risk to birds across 2 study areas: 1) 5 reaches in the southern Appalachian Mountains, an area with substantial mercury deposition but minimal anthropogenic impact, and 2) 4 reaches adjacent to the Emory River, an area impacted by the largest fly coal-ash spill in US history. We identified MeHg and Cu, Pb, Se, and Zn as contaminants of potential concern (COPC) at the Appalachian Mountain and Emory River study areas, respectively, based on dietary exposure of aquatic contaminants via riparian spiders. The identification of COPC at both study areas due to dietary spider exposure is notable not only because the spider exposure pathway has largely been uninvestigated at these sites but also because the aquatic systems in both areas have been studied extensively. Significant differences in MeHg concentrations were detected among spider taxa and suggest that the selection of spider taxa can impact risk characterization. These results indicate that the spider exposure pathway is important to consider when assessing potential risk, particularly for passerine birds.

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