Developing a food web-based transfer factor of radiocesium for fish, whitespotted char (Salvelinus leucomaenis) in headwater streams

Abstract

We developed a food web-based transfer factor (TF_web) to study contaminant movements from multiple prey items to a predator based on the dietary contributions of prey items with their respective contamination levels. TF_web was used to evaluate the transfer of ¹³⁷Cs into whitespotted char (Salvelinus leucomaenis) from the trophic structure of a stream-riparian ecosystem in headwater streams draining a Japanese cedar forest. We also examined the applicability of this method by comparing sites with different contamination levels but similar surrounding environments in Fukushima and Gunma. All samples were collected from August 2012 to May 2013. The dietary contributions from both aquatic and terrestrial prey items to whitespotted char were analyzed using stable carbon and nitrogen isotope ratios. ¹³⁷Cs activity concentrations in char ranged from 704 to 6082 Bq kg^-1-dry in Fukushima and from 193 to 618 Bq kg^-1-dry in Gunma. Dominant prey taxa such as mayflies (Ephemera japonica), spider crickets (Rhaphidosphoridae gen. spp.), and freshwater crabs (Geothelphusa dehaani), each of them accounted for 3-12% of the fish diet, based on lower and upper estimates, respectively. TF_web ranged from 1.12 to 3.79 in Fukushima and from 1.30 to 4.30 in Gunma, which suggested bioaccumulation from prey items to predator. Widely used ecological parameters TFs by media-char and TTF by single prey-char showed high variability with both dilution and accumulation. TF_web is applicable for ¹³⁷Cs transfer in predator-prey systems with complex food web structures of stream-riparian ecosystems.

Keywords: Detrital food web; Freshwater salmonid; Radiocesium; Stable isotope; Transfer factor.