Trophic structure and biomagnification of mercury in an assemblage of deepwater chondrichthyans from southeastern Australia

Stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) were used to evaluate trophic structure and mercury biomagnification in an assemblage of 16 deepwater chondrichthyans (primarily, 2 squaliformes: Centroselachus crepidater and Etmopterus baxteri) collected from continental waters off southeastern Australia from 2004 to 2006. In all species, mean trophic position (TP; quantified by δ¹⁵N) ranged from 3.5 to 4.7 (indicative of tertiary consumers). Minor variation in δ¹³C enrichment was observed between species (−18.7 to −17.1‰) with the exception of Squalus acanthias (−19.3 ± 0.1‰). Total mercury (THg) levels ranged from 0.3 to 4.5 mg kg⁻¹ (wet mass, wm) with the highest concentrations correlated with increasing individual size and TP. Using published (TP and THg) data on low-mid trophic prey groups collected from the study area, THg biomagnification factors between selected predator−prey associations and trophic magnification factors (TMF) within various assemblage and community groupings were calculated. As an assemblage, deepwater elasmobranchs demonstrated moderate rates of THg biomagnification, as indicated by the regression slope (0.69 TP giving a TMF of 4.8) while higher rates were reported in the extended continental shelf/slope community (1.13 TP giving a TMF of 13.4). Among-system differences in TMF were found between low-mid and mid-high order food chains as well as between shelf/upper-slope, mid-slope, and benthic food webs, signifying that bioaccumulation pathways are closely related to physical-chemical (bathome affinity) and community (presumably species composition and food chain length) structure.