The dark side of subsidies: adult stream insects export organic contaminants to riparian predators.

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.