Molecular Associations of Dissolved Organic Matter with Inorganic Mercury and Methylmercury in Canadian Boreal Streams

Abstract:

Boreal forests have long been identified as important hotspots for carbon cycling, but they also capture and sequester inorganic mercury (Hg²⁺) and the neurotoxin methylmercury (MeHg). The slow release of Hg²⁺ and MeHg stored in terrestrial soils and sediment can impact biota in small lakes and rivers, contributing to biomagnification and toxicity. Understanding hydrological connections and transport mechanisms is central to addressing Hg²⁺ and MeHg movement from terrestrial into aquatic systems. While fundamental relationships between dissolved organic matter (DOM) and mercury (Hg) exist based on both dissolved organic carbon (DOC) concentration and DOM aromaticity, we are slowly beginning to untangle how the diversity of DOM molecular composition can impact Hg transport. Here, we explore relationships between DOM composition and Hg²⁺ and MeHg concentrations in 14 first- and second-order boreal forest streams in northern Ontario, Canada. Using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) we identify specific DOM molecules with strong correlations to Hg²⁺ and MeHg concentrations in streams and how these associations vary across seasons. Forest streams with a higher proportion of aliphatic and hydrogenated compounds more effectively transport Hg²⁺ whereas strong associations between condensed aromatics, tannin molecules and MeHg were identified. Additionally, we detected unique DOM molecules that are significantly associated with Hg²⁺ and MeHg transport, with 5 key terrestrially derived molecules that can facilitate the transport of both Hg species from soils to water. In addition to this commonality, important differences were also observed, which help to explain previous observations wherein DOC-Hg²⁺ concentration relationships tend to be stronger than those for DOC-MeHg.