Stable carbon and nitrogen isotopes explain methylmercury concentrations in stream food webs of Lake George, New York (USA).

Abstract

Mercury has been studied extensively in lakes due to health risks associated with the consumption of contaminated fish, while stream ecosystems have received less attention. To better understand mercury bioavailability in the lower food web of streams, we collected macroinvertebrates (predators and detritivore) along with autochthonous (epilithic algae) and allochthonous (leaf litter) basal resources in eight streams entering Lake George. Samples were analyzed for methylmercury (MeHg), total mercury, and carbon and nitrogen isotopes (δ13C & δ15N) to determine how mercury concentrations in basal resources, biomagnification rates, and environmental factors (watershed characteristics and water chemistry) effected MeHg concentrations in predatory macroinvertebrates. While biomagnification rates, calculated as trophic magnification slope, explained between 68% and 98% of MeHg variability within a stream food web, the range was small (0.310-0.387) resulting in the biotic components following a consistent pattern of increasing MeHg among streams. The stream order was negatively related to basin slope for all biotic components and explained 70% of MeHg variability in predatory macroinvertebrates. Methylmercury concentrations were significantly and negatively related to δ13C in predators, epilithic algae, and leaf litter. We believe the biofilms on leaf litter utilized bacterial-respired carbon dioxide decreasing δ13C (<-28‰) and increasing MeHg while epilithic algal δ13C increased due to enhanced primary production resulting in biodilution of MeHg. Methylmercury in basal resources responded to δ13C similarly but through different processes. Our findings show shallow slopes elevate MeHg in basal resources and explain most of the predator MeHg variation among streams with little influence of biomagnification rates.