Mercury (Hg) is a globally distributed contaminant that is currently impairing wildlife in many lakes, streams, and wetlands. The effectiveness of current methods to reduce Hg toxicity (emissions reductions, consumption advisories, and ecosystem management alternatives) in any ecosystem is predicated on having a good understanding of those factors that control Hg cycling in the environment. Photochemical reactions have long been known to affect the phase, redox state, partitioning, transport, and fate of many metals in aquatic environments. Probably no metal is more affected by exposure to sunlight (photosensitivity) in terms of its environmental behavior than Hg. Sunlight drives two important photochemical processes involving mercury: photochemical reduction and photochemical demethylation. Photochemical Hg reduction involves both redox and phase changes, from ionic Hg (II) to dissolved gaseous Hg (0). Dissolved gaseous mercury (DGM) is sparingly soluble in water, and will seek to evade from the water to the atmosphere, thereby removing Hg from the aquatic environment. Methylmercury (MeHg), the most bioaccumulative form of Hg, is also photosensitive and can undergo photochemical degradation to Hg (II), and then possibly reduced to DGM. This two step process serves two important roles, demethylation of MeHg to a less toxic form of Hg, and then possibly elimination of Hg from the aquatic ecosystem by evasion.

The Aquatic Cycling of Mercury of the Everglades (ACME) project has been investigating the role of photochemistry in the mercury cycle of the Everglades, and what environmental factors control these reactions. Results to date have shown that DGM production and evasion, re-oxidation of Hg (0), and MeHg degradation are all occurring in the Everglades. The quantity and quality of dissolved organic carbon (DOC) plays a major role in regulating the rates and locations of these processes within the water column of the Everglades. Dissolved organic carbon serves as an effective filter of ultraviolet light (the most important wavelengths for these photochemical reactions) and can substantially limit the water depths and yield rates of many photosensitive reactions. On the other hand, the interactions of DOC with incident sunlight results in the production of several chemical oxidants, which can drive the reverse reaction and re-oxidize DGM to Hg (II). This ionic form of mercury could then become methylated if transported to a methylation site (for example, the sediment-water interface).

Field and laboratory data from samples collected across the Everglades shows there is a systematic increasing trend from north to south in gross DGM production rates, ranging from about 0.1 to 1 ng per liter per day. A corresponding north-to-south decreasing trend in DOC concentrations is believed to be the primary controlling factor of the DGM production rates. At lower DOC levels, DGM production and potential mercury removal to volatilization is maximized. The southern most sites in Water Conservation Area 3 and Everglades National Park showed the highest gross DGM production rates, which were similar in magnitude to rates of Hg deposition from rainfall. Measured net DGM production rates (gross DGM production minus DGM oxidation) also show a strong north-to-south trend. Net DGM production rates are significantly less than gross production rates, and across all our sampling sites an average of only about 15 percent of gross DGM produced is available for volatilization.

Methylmercury degradation rates also show a strong north-to-south increasing trend in the Everglades (ranging from 2 to 15 percent per day), and are also thought to be controlled by light penetration limitations of DOC. Maximal photodemethylation rates measured in Everglades National Park may be a principal reason why MeHg levels are not higher in resident aquatic biota. These rates of demethylation rival or exceed the microbial demethylation rates measured in sediments, emphasizing the need to consider this detoxification pathway when investigating Hg cycling processes. Current Everglades restoration decisions regarding water flow rates, sources, and depths will all have affects on the concentrations of DOC throughout this ecosystem, and will likewise have an affect on the photochemical processes that influence mercury toxicity in this sensitive environment.

Because the same set of mercury cycling processes operate in most aquatic ecosystems, our findings have applicability beyond the Everglades. In many aquatic environments, photochemical processes may be the most effective Hg detoxification mechanisms available to reduce the impacts of MeHg on local food webs (including humans). Strategies for managing aquatic ecosystems need to consider how management actions will influence factors important for photochemical reactions, and in turn, how these changes might affect Hg toxicity.

(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)