Evaluating the Potential Efficacy of Mercury Total Maximum Daily Loads on Aqueous Methylmercury Levels in Four Coastal Watersheds Sarah E. Rothenberg Richard F. Ambrose Jennifer A. Jay 10.1021/es702819f.s001 https://acs.figshare.com/articles/journal_contribution/Evaluating_the_Potential_Efficacy_of_Mercury_Total_Maximum_Daily_Loads_on_Aqueous_Methylmercury_Levels_in_Four_Coastal_Watersheds/2923912 Of the ∼780 U.S. EPA approved mercury total maximum daily loads (TMDLs), most specify a reduction in total mercury (Hg<sub>T</sub>) loads to reduce methylmercury levels in fish tissue, assuming a 1:1 correspondence. However, mercury methylation is more complex, and therefore, proposed load reductions may not be adequate. Using multiple regression with microlevel and macrolevel variables, the potential efficacy of mercury TMDLs on decreasing aqueous methylmercury levels was investigated in four coastal watersheds: Mugu Lagoon (CA), San Francisco Bay Estuary, Long Island Sound, and south Florida. Hg<sub>T</sub> and methylmercury levels were positively correlated in all watersheds except in Long Island Sound, where spatial differences explained over 40% of the variability in methylmercury levels. A mercury TMDL would be least effective in Long Island Sound due to spatial heterogeneity but most effective in south Florida, where the ratio between aqueous Hg<sub>T</sub> and methylmercury levels was close to 1 and the 95% confidence interval was narrow, indicating a probable reduction in aqueous methylmercury levels if Hg<sub>T</sub> loads were reduced. 2008-08-01 00:00:00 load methylmercury levels EPA U.S Long Island Sound HgT CA TMDL Maximum Daily Loads Aqueous Methylmercury Levels