2011/703 Accumulation of mercury in estuarine food webs: biogeochemical and ecological considerations

By Hugh Jones

Estuarine systems that are exposed to industrial pollutants often retain a high loading of contaminants, including mercury (Hg), due to prevailing physical, chemical and biological conditions. Estuarine biota are principally exposed to Hg through dietary uptake, which can lead to higher order species bioaccumulating significant concentrations that can also be harmful to human health if consumed. Methylmercury (MeHg) production, bioaccumulation, and biomagnification in estuarine food webs are broadly understood but our knowledge of Hg food pathways and selenium’s (Se) interaction with Hg is lacking. Current observations show poor correlation between bioaccumulation and environmental loadings, indicating that food web uptake and transfer of Hg are not straightforward. Understanding the mechanisms that underpin this variability is critical to quantifying and managing Hg exposure risks, and for developing appropriate management actions. The studies within this thesis examined the bioavailability, trophic magnification and bioaccumulation of Hg within a contaminated estuary to provide better capacity to manage the ecosystem and human health concerns.

This study has shown that Hg contamination is a significant issue in the Derwent Estuary, and that bioavailability, trophic magnification and association with Se are complex functions of ecosystem processes and biological response. The large pool of Hg that is already present within the Derwent ecosystem, principally in the sediments, remains the most significant Hg source, and it is largely this pool that is converted to MeHg and accumulated in the local food webs. Unfortunately, the decrease in external Hg inputs from industrial sources over the last 30–40 years through improved industrial practices has not reduced MeHg in the local fish species. This study suggests that the current pool of Hg principally in the sediments is sufficient to allow methylation to take place for many decades or centuries to come.