PhD defense on mercury cycling by Stephen G. Kohler

On the 15th of September Stephen G. Kohler successfully defended his PhD entitled “Seasonal biogeochemical cycling of mercury on the Arctic Ocean shelf” at the Department of Chemistry at NTNU.

Stephen G. Kohler defending his PhD entitled “Seasonal biogeochemical cycling of mercury on the Arctic Ocean shelf” at the department of Chemistry at NTNU

During his PhD, Stephen found seasonal variations in the Arctic waters total mercury (THg) concentration, with an increase in summer and a surprising decrease in winter, and that this might be related to particle scavenging. He also found seasonal variations in toxic methylated mercury (MeHg) concentrations, with lowest concentrations observed during spring.


Mercury is a toxic pollutant that enters the Arctic marine environment via anthropogenic activity, river runoff, land erosion, deposition from the atmosphere, and melting permafrost. Once in the marine environment, mercury can accumulate in the marine food web and humans can become exposed to this pollutant through the consumption of seafood. Although efforts have been made to reduce the anthropogenic emission of mercury, its biogeochemical cycling in the marine environment is expected to change due to climate change. Climate change is particularly noticeable in the Arctic, and the indigenous people living in this part of the world have one of the highest mercury exposures on Earth. Despite intense efforts, seasonal studies on mercury in the Arctic are limited to the ice-free and sunlit summer months. In his thesis, however, Stephen Kohler investigated the full seasonal cycle of mercury in an Arctic shelf sea in both seawater and sediment and was thereby the first-ever to report measurements on mercury in Arctic Ocean waters during winter.


He found that the total mercury concentration in the northern Barents Sea was approximately 33% lower in winter than in summer. This decline was suggested to be due to seasonal particle scavenging, which is the removal of a particle (mercury) in the water column through downward transport (i.e., precipitation and sinking). He found that the losses of mercury may be linked with the scavenging of lead and manganese, and that precipitation of manganese oxides could scavenge mercury to deeper waters and sediment. The greatest declines of THg concentrations were exhibited in surface waters.


Stephen Kohler further observed a seasonal variation in the more toxic MeHg concentrations in the Barents Sea. MeHg production can be produced by bacteria when consuming organic carbon in both subsurface waters and sediment. Dr. Kohler observed that MeHg concentrations were lower in spring, after the polar night, and higher in autumn and winter. The decrease in toxic MeHg concentrations suggested a demethylation process, or destruction of MeHg, which coincides with both the seasonal input of inorganic mercury and the Arctic spring bloom. A potential biological demethylation mechanism during the spring bloom is an important observation since it may help mitigate mercury uptake into the food web, and thereby the exposure to mercury in humans. 


Dr. Stephen G. Kohler’s work suggests that the vertical distribution of mercury in the northern Barents Sea shows a distinct seasonal variation that is coupled to major physical, chemical, and biological changes in the Arctic Ocean. His work is an important contribution to closing the knowledge gap on the accumulation of mercury in the marine food web and the exposure of humans to this pollutant, which is of particular importance in the Arctic.