Abstract

Riverine mercury biomagnification is most frequently studied via metrics such as the trophic magnification factor (TMF), the biomagnification factor (BMF) and the trophic magnification slope (TMS). However, these metrics prove problematic in lotic systems dominated by benthic macroinvertebrates because short food chains, the frequent lack of fish, patchy basal production and marked spatial heterogeneity obscure the signal that they are meant to extract. We argue that stable isotopes of carbon and nitrogen (δ13C, δ15N) are not merely optional enhancements but are a prerequisite for a mechanistic understanding of the transfer of methylmercury (MeHg). Nitrogen isotopes distinguish among trophic positions not as categories, but as gradients, whilst carbon isotopes reflect the underlying pathways increasingly seen as defining how MeHg enters food webs. Invertebrate studies, published more recently, report systematically lower magnification levels compared to community-based studies and become far more comprehensible when carbon source and trophic pathways are made explicit, with Hg isotopes (δ202Hg, Δ199Hg) providing supplementary data about mercury sources and processing. To be able to carry out reliable river monitoring, such frameworks require clarity in choices of background value, discrimination factors and statistical methods.

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