Ecosystem Controls on Methylmercury Production by Periphyton Biofilms in a Contaminated Stream: Implications for Predictive Modeling.

Periphyton biofilms produce a substantial fraction of the overall monomethylmercury (MMHg) flux in East Fork Poplar Creek, an industrially contaminated, freshwater creek in Oak Ridge, Tennessee. We examined periphyton MMHg production across seasons, locations, and light conditions using mercury stable isotopes. Methylation and demethylation rate potentials (k and k , respectively) were calculated using a transient availability kinetic model. Light exposure and season were significant predictors of k , with greater values in full light exposure and in the summer. Season, light exposure, and location were significant predictors of k , which was highest in dark conditions, in the spring, and at the upstream location. Light exposure was the controlling factor for net MMHg production, with positive production for periphyton grown under full light exposure and net demethylation for periphyton grown in the dark. Ambient MMHg and k were significantly correlated. Transient availability rate potentials were 15 times higher for k and 9 times higher for k compared to full availability rate potentials (k and k ) calculated at 1 d. No significant model for the prediction of k or k could be constructed using light, season, and location. In addition, there were no significant differences among treatments for the full availability k , k , or net MMHg calculated using the full availability rate potentials. k was not correlated with ambient MMHg concentrations. The present results underscore the importance of applying transient availability kinetics to MMHg production data when estimating MMHg production potential and flux. Environ Toxicol Chem 2019;38:2426-2435. © 2019 SETAC.