No such thing as a free meal: organotin transfer across the freshwater–terrestrial interface

Summary Emergent aquatic insects can represent an important subsidy to terrestrial ecosystems but may also transport accumulated contaminants across ecosystem boundaries when larvae develop in contaminated sediments. We sampled tetragnathid spiders (terrestrial predators), larval chironomids (spider prey of aquatic origin) and terrestrial insects (terrestrial prey) from two contaminated and two control sites in the Norfolk Broads (U.K.) to determine whether the organotin compound tributyltin (TBT) is transferred by emergent aquatic insects. TBT, a biocide in antifoulant paints, was prohibited in the U.K. in 1987 and globally since 2008 but persists in sediments for decades. Combining stable-isotope analyses commonly used in ecology with ecotoxicological methods enabled us to test whether aquatic subsidies could transport organotin to terrestrial predators. Stable-isotope mixing models (δ15N and δ13C) indicated that chironomids contributed 31–98% to spider biomass. Subsequent organotin analyses revealed consistent, low-level butyltin (dibutyltin; DBT) contamination of chironomids from the most contaminated site but not from the other three sites. Spiders from the most contaminated site had DBT concentrations similar to those of their chironomid prey. To assess bioaccumulation, we used δ15N values as a proxy for trophic position of sediments, chironomids and spiders, and correlated these values with the respective DBT concentrations. Notwithstanding indications of 15N-enrichment along this short food chain, chironomid DBT concentrations were significantly greater than those of their spider predators. Biota sediment accumulation factors (sediments to chironomids) and biomagnification factors (chironomids to spiders) were below the thresholds defining the occurrence of bioaccumulation and biomagnification. Although biomagnification was not detected, it is of concern that butyltins are still present in freshwater food webs c. 25 years since last known TBT use in the U.K., and continue to be transferred to terrestrial consumers.