Mercury cycling in Australian estuaries and near shore coastal ecosystems: Triggers for management

Abstract

Mercury (Hg) sources to estuaries (natural and anthropogenic) as well as Hg concentrations in Australian nearshore marine environment fish are reviewed herein. The question of whether Australian estuaries have a Hg contamination problem is addressed. The Hg concentrations in fish, excluding sharks, tuna, barramundi and some stingrays, in estuaries and near-shore ecosystems with no discernable pollution sources are typically below 0.5 mg/kg wet weight, the level of health concern. There is no relationship of Hg concentration with fish size or age nor any evidence of biomagnification. In locations with historic large discrete Hg input sources (e.g. Derwent Estuary, Tasmania, Princess Royal Harbour WA, Port Phillip Bay Vic, Sydney sewage outfalls NSW), Hg concentrations in some sediment-dwelling fish such as flatheads exceed the health limit of 0.5 mg/kg. In this paper, we also review, within an Australian context, the biogeochemical processes controlling the formation and accumulation of methyl mercury (MeHg). On entering waterways, Hg rapidly partitions to particulate matter and deposits into sediments. The remobilisation of Hg from sediment is dependent on the formation of MeHg by bacteria and ultimately the interplay of S, Fe and Se cycling. Fish species that move and feed in different areas have Hg concentrations that do not reflect the sediment Hg concentrations where they are caught, i.e. there is an uncoupling of diet and potential Hg exposure. Concluding remarks focus on management interventions: source reduction, preventing eutrophication and promoting system biodiversity and biodiverse diets to mediate the accumulation of Hg in marine organisms and limit the intake of Hg by humans when consuming fish.