Water Column Methylation in Estuaries

Abstract:

Methylmercury (MeHg) is a neurotoxin that bioaccumulates in aquatic food webs and affects humans and wildlife through fish consumption. Many studies have measured active methylation/demethylation in ocean margin sediments but few have reported similar rates for the marine water column. This presentation will review available evidence for water column methylation in estuaries, including new experimental measurements of methylation/demethylation rates from a deep subarctic fjord in Labrador Canada collected in Spring and Fall of 2012-2013. We used these and other data to construct a mass budget for MeHg in the estuary and show that water column methylation (with rates ranging from 1.5 to 2.8 % day^-1), is the largest contributor, followed by inputs from rivers (4.9 mol year^-1), to the in situ pool of MeHg available for uptake by biota. By contrast, the sediment in this system is a net sink for MeHg (-1.5 mol year^-1). We discuss the relationship between observed MeHg and other ancillary environmental factors (organic carbon, sulfur and nutrients) as well as implications for the response time of fish to future changes in mercury inputs.