Coupling eutrophication to neurotoxic MeHg
In a recently published paper researchers from BNI have, together with a team of international colleagues, investigated the implications of eutrophication for production and bioaccumulation of neurotoxic monomethylmercury (MeHg).
This neurotoxin bioaccumulates in aquatic food webs, and as eutrophication is expanding in many areas, it could potentially affect MeHg production and thus levels in food webs.
New model includes Hg chemistry
The researchers adapted the existing biogeochemical model previously developed at BNI, BALTSEM, to also include Hg chemistry.
The new model could thus investigate the impact of eutrophication by calculating the bioaccumulation of MeHg into phytoplankton.
For model evaluation, they measured total methylated Hg (MeHgT) in the Baltic Sea and found low concentrations (39 ± 16 fM) above the halocline, and high concentrations in anoxic waters (1249 ± 369 fM).
To close the Baltic Sea MeHgT budget, the researchers inferred an average normoxic water column HgII methylation rate constant of 2 × 10–4 d–1.
Comparing different scenarios
The researchers compared the present day (2005-2014) eutrophic state of the Baltic Sea to an oligo/mesotrophic scenario.
They found that eutrophication increases primary production and export of organic matter and associated Hg to the sediment. This effectively removes Hg from the active biogeochemical cycle and results in a 27% lower present-day water column Hg reservoir.
However, increase in organic matter production and remineralization stimulates microbial Hg methylation, resulting in a seasonal increase in both water and phytoplankton MeHg reservoirs above the halocline.
Impacts of eutrophication of MeHg
Previous studies of systems dominated by external MeHg sources or benthic production, found eutrophication to decrease MeHg levels in plankton.
This new Baltic Sea study shows that in systems with MeHg production in the normoxic water column, eutrophication can increase phytoplankton MeHg content.
The new study has increased our understanding of how eutrophication and its effects on the mercury cycle on an ecosystem level, showing that eutrophication has the potential to enhance the transfer of MeHg not only to phytoplankton, but also to higher trophic levels.