The Dark Side of Subsidies: Adult Stream Insects Export Organic Contaminants to Riparian Predators

Abstract

Aquatic insects provide a critical energy subsidy to riparian food webs, yet their role as vectors of contaminants to terrestrial ecosystems is poorly understood. We investigated relationships between aquatic resource utilization and contaminant exposure for a riparian invertivore assemblage (spiders and herptiles) along a stream contaminated with polychlorinated biphenyls (PCBs). Stable carbon (delta13C) and nitrogen (delta15N) isotopes indicated that aquatic insect utilization varied among predators, with progressive enrichment of delta13C and depletion of delta15N as predators shifted from aquatic to terrestrial prey. PCB concentrations significantly increased along these isotopic gradients; delta13C and delta15N explained 65% and 15% of the variance in predator sigmaPCBs, respectively. PCBs in predators were high, exceeding 2000 ng/g wet mass (the human-health advisory prohibiting any consumption of fish tissue) in three species. Greater consideration should be given to streams as lateral exporters rather than simply as longitudinal conduits for contaminants. Persistent contaminants are underutilized for addressing landscape-level questions in subsidy research, but our results demonstrate they are an ideal in situ tracer of stream-derived energy because they label stream organic matter and invertebrates over large distances. Likewise, riparian predators such as tetragnathid spiders have great potential as biological monitors of stream condition and as an assessment tool for risk management of contaminated aquatic sediments.

In a state of flux: The energetic pathways that move contaminants from aquatic to terrestrial environments

“…We highlight the ways in which a new understanding of the aquatic-terrestrial interface has prompted an integrated view of cross-boundary contaminant flows within complex ecological networks. We pay particular attention to aquatic insects, which as an important source of energy for riparian consumers such as arthropods, birds, mammals, and reptiles, are especially likely to move contaminants into terrestrial ecosystems [4][5][6] (Fig. 1).…”

“…For example, using natural abundances of stable isotopes now allows scientists to track contaminant fluxes within and between ecosystems [13] and to predict exposure to riparian and terrestrial consumers [6]. Scientists also use the isotopic compositions of contaminants themselves to track the contribution of anthropogenic sources of contaminants in aquatic consumers.…”

Bottom‐up nutrient and top‐down fish impacts on insect‐mediated mercury flux from aquatic ecosystems

“…Emergent aquatic insects transport MeHg to nearby terrestrial habitats, potentially exposing terrestrial consumers to aquatic pollutants [3,4], the so called darkside of cross-system subsidies [5]. The flux of MeHg from aquatic environments to terrestrial environments is especially significant because MeHg is not normally produced in terrestrial ecosystems.…”

“…Because Hg concentrations are now elevated above baseline levels in all aquatic ecosystems due to widespread anthropogenic emissions and a dynamic global atmospheric cycle [6], it is important to understand the factors that control the emergence of aquatic insects and the movement of MeHg out of aquatic systems. Recent studies have demonstrated that aquatic insects biotransport MeHg and other contaminants to terrestrial consumers [e.g., 3,5,9], but the factors that regulate the flux of MeHg out of aquatic ecosystems via emergent insects have not been studied.…”

Levels and distribution of polybrominated diphenyl ethers in the aquatic and terrestrial environment around a wastewater treatment plant

“…However, whether PBDEs in the aquatic compartment can also be transferred to the surrounding terrestrial environment is unclear. The aquatic to terrestrial transfer could be an important transport mechanism for POPs since they can move through the food web in a similar manner as nutrients and energy, bioaccumulate through the food chain and thereby posing an increasing risk to animals at the top of the food chain (Walters et al 2008). Furthermore, analysis of stable isotopes of e.g., carbon and nitrogen can be used to elucidate the structure of local food webs and is a very useful complementary tool to investigate the flow of organic matter and contaminants in the ecosystem (Jardine et al 2006).…”