In-Reservoir Physical Processes Modulate Aqueous and Biological Methylmercury Export from a Seasonally Anoxic...

Area of Science:

• Environmental Chemistry
• Aquatic Ecology
• Mercury Biogeochemistry

Background:

• Anoxic conditions in reservoirs drive methylmercury (MeHg) production, a critical step in mercury's aquatic food web uptake.
• Understanding MeHg's biological uptake and export dynamics from reservoirs is crucial but remains incomplete.

Purpose of the Study:

• To investigate the relationship between reservoir stratification, anoxia, and MeHg concentrations and export loads.
• To differentiate MeHg export via aqueous versus biological compartments in reservoirs.

Main Methods:

• Analyzed MeHg concentrations in water, zooplankton, suspended particles, and detritus.
• Assessed MeHg export loads at reservoir outflows.
• Correlated MeHg dynamics with reservoir stratification and anoxia.

Main Results:

• MeHg levels in water, zooplankton, particles, and detritus rose during reservoir destratification.
• Zooplankton MeHg concentrations were strongly linked to MeHg in water during destratification.
• Reservoir anoxia was identified as a primary factor controlling MeHg export.
• Biological MeHg constituted only 5% of total MeHg export, with aqueous compartments dominating.

Conclusions:

• In-reservoir physical processes significantly influence MeHg incorporation at the base of aquatic food webs.
• Seasonal reservoir stratification and destratification impact MeHg dynamics and downstream release.
• Aqueous export is the dominant pathway for MeHg release from these reservoirs.