Mercury Dynamics in the Gulf of Mexico: Insights from Biogeochemical Studies

Coastal waters, such as the Gulf of Mexico, are highly productive fisheries for both the local and global community. However, accumulation of methylmercury (MeHg) in fish is a growing health concern. Coastal zones can be impacted by large-scale extreme physical events (hurricanes) and by inputs from the terrestrial environment. For example, excess nutrients, organic matter and sediment inputs exacerbate oxygen depletion in coastal waters worldwide. Such events and changes impact Hg cycling and the bioaccumulation of MeHg in coastal fisheries, but this has not been well studied and was the overall impetus for studies of Hg fate, transport, and bioaccumulation reported here. Mercury dynamics in the eutrophic coastal region of the Gulf of Mexico were examined through sampling that occurred between July 2005 and July 2006, covering the period before and after Hurricanes Katrina and Rita, thus also allowing an examination of the impact of hurricanes on Hg dynamics and fate. Total Hg (THg), MeHg, and ancillary variables were measured, and on the latter cruises, rates of methylation (k_m) and demethylation (k_d) were also estimated using stable isotopes. The studies tested the following hypotheses: 1) that the highest net MeHg formation occurs at sites most affected by hypoxia; and 2) that the hurricanes stimulated MeHg formation. Most of the variance (60%) in July 2006 MeHg concentrations can be explained using a multivariate regression against OC (+), km (+), and THg (-). When looking across multiple sampling events, the influence of hurricanes is a dominant factor, with elevated %MeHg associated with fine hurricane deposits. The presentation will provide an overview of the dynamics of Hg and net MeHg formation and discuss the important variables that control MeHg formation and bioaccumulation in this important region.