Transfer of mercury across ecosystem boundaries in arid streams

Background/Question/Methods

Stream and river ecosystems are intimately connected to adjacent terrestrial habitats through the exchange of organic matter, nutrients, and organisms.  Traditionally, it was thought that the dominant directionality of the movement of inorganic and organic matter (OM) was from terrestrial to stream habitats; however, there has been a recent focus on the importance of aquatic-derived resources for adjacent riparian terrestrial habitats.  The reliance on aquatic-derived resources may be particularly relevant in arid landscapes because perennially-flowing rivers and streams can have relatively high in situ productivity which will lead to OM “spill out” to the surrounding terrestrial landscape, providing an allochthonously-derived resource subsidy to riparian consumers (i.e. insectivorous bats, birds, lizards, and spiders) in the form of emerging insects. Ecotoxicologists and environmental scientists have recently expanded the concept of cross-ecosystem resource subsidies to include the transfer of bioaccumulating contaminants. 

Mercury (Hg) is a toxic element with no known biological function found in a variety of environments around the world.  Through our activities, humans have greatly altered the global Hg cycle and subsequently released large amounts of Hg to the environment.  The input of Hg into aquatic systems occurs largely through atmospheric deposition of inorganic Hg, but the movement of aquatically-derived Hg to terrestrial environments is likely biologically mediated via the movement of organisms crossing ecosystems boundaries (e.g., movement of organisms from aquatic to terrestrial environments).  In this study, we assessed the importance of Hg transport from lotic ecosystems via aquatic insect emergences to terrestrial consumers.  We examined the Hg concentration in in-stream food web components (fishes and aquatic invertebrates) and in terrestrial consumers which forage in riparian zones, such as bats, birds and spiders.  The study sites included two “High Hg” stream ecosystems which have known geological Hg and a past history of Hg mining in the watersheds, and one “Low Hg” stream with little geological Hg and no Hg mining in its watershed. 

Results/Conclusions

Fishes and aquatic invertebrates at the “High Hg” sites generally exhibited higher Hg concentrations than at the “Low Hg” site.  We found significantly higher Hg concentrations in feathers of both resident and migrant birds at the “High Hg” sites.  However, we did not detect significant differences in Hg concentrations in insectivorous bats and terrestrial invertebrates among the “High” and “Low” sites.  This study supports a growing body of evidence demonstrating the role that aquatic organisms play in movement of toxicants to terrestrial ecosystems.