Accumulation of Methylmercury in the High-Altitude Lake Uru Uru (3686 m a.s.l, Bolivia) Controlled by Sediment Efflux and Photodegradation

Abstract

In shallow aquatic environments, sediment is a significant source of monomethylmercury (MMHg) for surface water (SW). High-altitude aquatic ecosystems are characterized by extreme hydro-climatic constraints (e.g., low oxygen and high UV radiation). We studied, during two seasons, the diel cycles of MMHg in SW and sediment porewaters (PW) of Lake Uru Uru (3686 m a.s.l, Bolivia) contaminated by urban and mining activities. Our results show that diel changes in SW MMHg concentrations (up to 1.8 ng L−1) overwhelm seasonal ones, with higher MMHg accumulation during the night-time and the dry season. The calculation of MMHg diffusive fluxes demonstrates that the sediment compartment was the primary source of MMHg to the SW. Most MMHg efflux occurred during the dry season (35.7 ± 17.4 ng m−2 day−1), when the lake was relatively shallow, more eutrophicated, and with the redoxcline located above the sediment–water interface (SWI). Changes in MMHg accumulation in the PWs were attributed to diel redox oscillations around the SWI driving both the bacterial sulfate reduction and bio-methylation. Finally, we highlight that although MMHg loading from the PW to the SW is large, MMHg photodegradation and demethylation by microorganisms control the net MMHg accumulation in the water column.

Mercury, organic matter, iron, and sulfur co-cycling in a ferruginous meromictic lake

“…No trace of dimethylmercury was found in the samples, consistently with previous results (Cossa et al, 1994), suggesting that DGM consists entirely of Hg 0 . The high DGM measured in surface waters suggests that Hg 0 is formed via Hg II photoreduction in the euphotic zone, and is partially reinjected in the atmosphere as already shown in numerous lakes (Amyot et al, 1994;Guédron et al, 2020a;Saiz-Lopez et al, 2018). In contrast, the presence of DGM peaks in the aphotic monimolimnion supports non-photochemical processes for DGM production, such as Hg II microbial reduction (Lamborg et al, 2013;Mason et al, 1995;Rolfhus and Fitzgerald, 2004) and/or natural OMmediated reduction of Hg II (Allard and Arsenie, 1991;Zheng et al, 2011).…”

In Situ Photochemical Transformation of Hg Species and Associated Isotopic Fractionation in the Water Column of High-Altitude Lakes from the Bolivian Altiplano

“…Methylation was however detected in waters from UU incubated with stable isotopes during the dry season (see below), consistent with previous observations in this shallow lake where strong redox shift and high concentrations of suspended particles favor the establishment of reducing microniches and restrict photodemethylation in the water column. 48,66 Regarding Lake Titicaca, several compartments can contribute to the pool of MMHg present in the water column as methylation was previously clearly demonstrated in benthic biofilms, green-algae periphyton, and carbonate-rich sediments 50,51 and also in the water column itself under anoxic conditions that developed after a severe algal bloom. 46 3.3.…”

Climatic Controls on a Holocene Mercury Stable Isotope Sediment Record of Lake Titicaca

“…40 Under the assumption of circum-zero Δ 200 Hg in geogenic sources and an insignificant contribution of litterfall-derived Hg, we suggest that the positive Δ 199 Hg exc values were produced after deposition or discharge to Lake Titicaca, most likely during Hg(II) photoreduction in the water column. 93 Experimental studies of photoreduction of Hg(II) to Hg(0) in the presence of natural organic matter (NOM) have demonstrated positive Δ 199 Hg anomalies in the residual Hg(II) fraction. 37,77 This suggests that positive Δ 199 Hg anomalies are caused by substantial photochemical reduction of Hg(II) and subsequent re-emission losses of Hg(0).…”