Coastal locations and environmental influence on uptake of mercury and methylmercury into phytoplankton

Methylation and demethylation rates of Hg have been measured in sediment, and fluxes to the water column have been estimated. However, direct values of mercury (Hg) species transfer into algae from the sediment-water interface have not previously been measured. Experiments were preformed measuring Hg and methylmercury (MeHg) uptake into phytoplankton directly from sediment where speciation transformations were quantified with stable Hg isotopes. In order to investigate effects of different near shore environments and temperatures, 3 different sites were selected for experiments in: Maine, Connecticut and Delaware. Aspects of the sediment such as organic matter content, Hg and MeHg levels, and methylation and demethylation rates were measured for each site. A coastal diatom was culture and gently transferred into individual core tubes above the water-sediment interface. Each individual sediment core was spiked at set depth intervals with a different stable isotope treatment prior to algae addition in order to evaluate the rate of transfer into the biota for Hg and MeHg. Five treatments were investigated using a separate isotopic scheme for each core: (1) Inorganic ²⁰⁰Hg only, (2) Methyl ¹⁹⁹Hg isotope only, (3) Both the inorganic ²⁰⁰Hg and methyl ¹⁹⁹Hg isotopes, (4) Both inorganic ²⁰⁰Hg and methyl ¹⁹⁹Hg isotopes into the water column (not sediment), (5) no Hg isotope spikes (control). Sediment cores incubated for 3 days. After incubation the diatoms were collected onto 5 µm polycarbonate filters for analysis. Mercury variations were associated with unique methylation potentials for different coastal environments, and dependent on organic matter and Hg and MeHg concentrations in sediment. By tracing stable isotope dynamics, transformations occurring in the sediment and influences of sedimentary methylation and demethylation potentials on phytoplankton uptake were quantified for each environment.