Do non-native terrestrial isopods transfer marine-derived mercury to threatened steelhead trout (O. mykiss) in a coastal California stream?

Abstract:

Steelhead trout (O. mykiss) populations in the Big Sur region of coastal California are listed as threatened under the Endangered Species Act and are potentially vulnerable to the impacts of climate change, non-native species, and accumulation of toxins. Recent field sampling of O. mykiss in Big Creek, California, revealed that non-native terrestrial isopods (P. scaber and A. vulgare) made up about 40% of the energy value in the stomach contents of the fish. These terrestrial isopods are known to be effective bioindicators of the amount of mercury (Hg) an area receives from wet deposition, in addition to their having generally elevated fractions of methylmercury (MeHg) due to the capability of these organisms to methylate Hg²⁺ in their gut. This coastal area of California is also well known for its heavy summertime marine fog, which has been shown to result in elevated levels of MeHg in wet deposition. Thus, we measured total Hg (THg) and MeHg concentrations in O. mykiss and their associated aquatic and terrestrial invertebrate prey to determine if there could be evidence of marine-derived mercury in this linked stream-riparian food web. Samples of O. mykiss from Big Creek contained on average 997 ppb THg (dw) which corresponds to about 260 ppb THg (ww), a relatively elevated concentration near to that of the average for tuna from the US EPA. Terrestrial isopods had an average of 187 ppb and 145 ppb for THg and MeHg (dw), respectively, resulting in a mean MeHg fraction of 0.78. Terrestrial predators contained the highest levels of Hg across all organisms with centipedes and spiders containing 859 and 652 ppb of THg, respectively, and with MeHg fractions of > 0.9. Terrestrial invertebrates in the riparian zone contained on average 301 ppb of THg while aquatic invertebrates in the stream contained on average 52 ppb of THg. In general, terrestrial invertebrates from this study had higher THg and MeHg concentrations compared with another similar study in northern California, whereas aquatic invertebrates had similar THg and MeHg concentrations between studies. This suggests that the consumption of terrestrial prey by O. mykiss in Big Creek could result in elevated Hg loadings in these fish, however, the contribution of marine-derived Hg to terrestrial invertebrates in this ecosystem is still an open question.