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

Stéphane Guédron,Stéfany Rocha-Lupa,Marizol Flores,Sylvain Bouchet,David Amouroux,Emmanuel Tessier,Carlos Heredia,Pablo Fernandez-Saavedra,Sarah Bureau,David Point,Israel Quino-Lima,Dario Achá

doi:10.3390/app10217936

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.

Highlights

Lake sediment is known to be an important source of mercury (Hg) species, including the neurotoxic monomethylmercury (MMHg) for overlying waters [1], through their diffusion and advection under dissolved and colloidal phases [2,3,4,5,6]
The water column was oxic during both seasons, with the lowest oxygen levels
Dissolved gaseous mercury (DGM = 11.2 ± 9.9 pg L−1 ) in surface water (SW) was mainly in the form of Hg(0) [19] and was overall low and never exceeded 1% of THgUNF for both seasons (Figure 2a), similar to previous studies in Lake Uru Uru [19] and Lake Titicaca [24] but in the lowest range of values reported for temperate freshwater lakes [36,37]

Summary

Introduction

Lake sediment is known to be an important source of mercury (Hg) species, including the neurotoxic monomethylmercury (MMHg) for overlying waters [1], through their diffusion and advection under dissolved and colloidal phases [2,3,4,5,6]. In shallow environments (e.g., shallow lakes, lagoons, ponds), both the shear stress and the bio-irrigation driven by tides, wind, and benthic organisms enhance the upward release of elements contained in the sediment porewater towards the water column [2,3,4,5,6,11,12,13] Such MMHg exchange is mainly regulated by the position of the redox front around the SWI where it is mainly produced, the sediment structure (e.g., porosity, tortuosity), and the abundance and type of (in)organic ligands [14,15] and colloids [16]

Methods

Results

Conclusion