Tidal flux of dissolved organic carbon, total mercury, and methylmercury from a mangrove marsh

We determined the flux of DOC, total mercury (THg) and methyl mercury (MeHg) from a mangrove-dominated area of southwest Florida, USA, using a novel method for assessing tidal pumping. Samples were collected over 2 days in the wet season (September 2010) and dry season (April 2011) while discharge and fluorescent dissolved organic matter (FDOM) was continuously measured in situ over a ten and fourteen day period, respectively. We observed strong relationships between measured concentrations from discrete samples and values of FDOM measured in situ for DOC (r ≥ 0.95), filtered total Hg (r ≥ 0.88), and filtered MeHg (r ≥ 0.95). These relationships were used to develop continuous time series records of concentration with which we calculated flux and yield. We observed strong seasonality in both the concentrations and fluxes. The measured areal yield of DOC (180 ± 12.6 grams per meter squared per year) is in the middle of the range of previously reported values, but areal yields of dissolved Hg species - 28 ± 4.5 micrograms per meter squared per year for dissolved total Hg and 3.1 ± 0.4 micrograms per meter squared per year for dissolved MeHg - were 5 to 100 times greater than values previously published for non-tidal wetlands. Mangrove tidal fluxes were on the same order as atmospheric deposition for THg, and as potential benthic fluxes for MeHg, indicating flux from tidal wetlands may contribute to mercury accumulating in coastal food chains and should be included in regional mercury budgets and assessments.