DIURNAL TRENDS IN METHYLMERCURY CONCENTRATION AND ORGANIC MATTER PHOTO-REACTIVITY IN AGRICULTURAL WETLANDS OF THE YOLO BYPASS, CALIFORNIA

Aqueous concentrations of methylmercury (MeHg) are known to vary temporally and spatially due to multiple, concurrent mechanisms of production and loss, as well as variations in hydrologic connectivity between the methylating substrate (most commonly the benthos) and the overlying water. Diurnal trends in MeHg production, bacterial demethylation, photo-demethylation, and transport processes (including diffusion and advection) have been identified in some wetlands, however, the magnitude and relative importance of each process in mediating surface-water MeHg concentrations are not well known. Temporal variation in aqueous MeHg concentration likely impacts the bioaccumulation of MeHg into the base of the aquatic food chain, and may challenge regulatory efforts designed to mitigate MeHg exports from point and non-point sources. Surface-water MeHg concentration was monitored in two agricultural wetland settings (wild-rice and white-rice fields) over a 24-hour period within the Yolo Bypass Wildlife Area, California using a combination of in situ optical sensors and surface-water samples. In the wild-rice field (sampled at 12 cm depth of 25 cm water column), MeHg concentrations doubled from ~1 ng/L to ~2 ng/L over the nighttime hours and returned to ~1 ng/L during the daylight hours, however in the white-rice field (sampled at 10 cm depth of 20 cm water column) MeHg concentration (0.73 ^± 0.08 ng/L) did not vary significantly during the 24-hour period. Similar trends were observed when MeHg data were expressed as a percentage of total Hg; both wetland habitats exhibited similar levels (~20% MeHg) following the nighttime period but the wild-rice field declined to ~10% in the early evening. Field parameters measured in situ (solar radiation, pH, dissolved oxygen, and temperature) exhibited large diurnal trends in both wetlands, whereas optical proxies for dissolved organic matter (DOM) composition mirrored the fluctuations in MeHg concentration and %MeHg, with a strong diurnal trend in the wild-rice wetland and no apparent trend in the white-rice wetland. These results suggest a unique link between DOM photo-reactivity and photo-demethylation in these wetlands. Monitoring efforts designed to characterize daily and longer-term loads of MeHg within and from agricultural wetlands such as rice fields should include a diurnal component.