Influence of hydroperiod and canopy cover on mercury accumulation and methylation in Carolina bay wetland sediments in the Southeastern United States

Isolated wetlands are crucial to the global mercury (Hg) cycle due to their widespread distribution and variable hydrology. To quantify atmospheric Hg deposition, sediment accumulation, and Hg methylation, we collected surface sediments from 10 Carolina bay wetlands-including short- and long-hydroperiod bays during wet and dry seasons for total Hg (THg) and methylmercury (MeHg) analysis. THg and MeHg concentrations ranged from 23.2 to 224.8 ng/g and 0.4-5.5 ng/g, respectively, and %MeHg ranged from 0.4 to 3.1 %. Short-hydroperiod bays had higher THg concentrations (116.4 ± 46.5 ng/g) than long-hydroperiod bays (72.7 ± 48.1 ng/g) under flooded conditions (P = 0.001). MeHg concentrations were influenced by THg (P < 0.0001) and were highest in long-hydroperiod bay center locations (P = 0.002). The methylation potential (%MeHg) was inversely related to THg and covarying tree canopy cover (P = 0.004). Our results indicate that short-hydroperiod bays tend to sequester higher concentrations of atmospheric Hg under flooded conditions than long-hydroperiod bays, however, long hydroperiods tend to enhance Hg methylation and MeHg production. Canopy cover shows a significant impact on Hg cycling within the Carolina bay ecosystem. Our study demonstrates that Hg accumulation and methylation in Carolina bay wetlands contribute to the biogeochemical cycling of Hg, as well as anaerobic Hg methylation under long hydroperiods and flooded conditions, which advances our understanding of Hg cycling across different wetland types.