Quantitation of methyl mercury photodegradation in lakes - the missing link to complete mercury cycling budgets in forest-wetland-lake ecosystems

Biomagnification of the neurotoxin methyl mercury (MeHg) in aquatic food-webs is a major threat to higher organisms in forest-wetland-lake ecosystems.

Surprisingly, the single most important process counteracting uptake in biota: MeHg photodegradation in surface waters, remains to be quantified.

Research on MeHg photolysis has so far been empiric in nature, making results very site-specific, and the few reported modeling approches do not comply with the theory of photchemistry.

In this project we take an apparent quantum yield (AQY) approach to improve and validate a preliminary model by which rates of MeHg photolysis can be predicted in surface waters.

The model is validated by in situ MeHg degradation experiments conducted along depth gradients in three contrasting lakes covering the seasonal variability in the major input parameters: sunlight, MeHg concentrations and absorbance of irradiance by chromophoric natural organic matter (CDOM). At ten additional lakes in situ experiments will be conducted once at one depth.

Experimental work will also be conducted to further improve the model module accunting for the quenching of photoreactions by CDOM.

The finally validated AQY model will provide robust estimates on rates of MeHg photodegradation in Swedish lakes completing budgets for forest-wetland-lake ecosystems.

This reaserch is useful for society facing the consequences of climate-driven changes in land-use and surface water brownification on MeHg biomagnification.