Mercury and methylmercury incidence and bioaccumulation in plankton from the central Pacific Ocean

Open ocean samples of phytoplankton and zooplankton were collected from the central Pacific on board the R/V Kilo Moana in October of 2011. The cruise traveled from Hawaii to Samoa, progressing through a High Nutrient Low Chlorophyll (HNLC) zone, and an equatorial upwelling region. Phytoplankton samples were size fractioned into 0.2-5 μm, 5-20 μm, and > 20 μm samples. Methylmercury concentrations were 2.91 ± 2.58 pmol g− 1 (wet weight) for the overall < 200 μm size fractions, and highest around the HNLC region. Phytoplankton bioconcentration factors (logBCFs) averaged to 5.69 ± 0.98 and were higher than the values found for coastal regions. Both %MeHg ([MeHg]/[Hg]) and logBCF values indicated that the lowest size fraction had the largest fraction of the HgT as MeHg, signifying enhanced accumulation of MeHg into smaller organisms. Zooplankton vertical net tows were completed from depths of 200 m up to ~ 10 m. Zooplankton samples were analyzed for carbon, nitrogen and sulfur in addition to Hg and MeHg at size fractionations of 0.2-0.5 mm, 0.5-1.0 mm, 1.0-2.0 mm and occasionally > 2.0 mm. Zooplankton abundance and MeHg concentrations both peaked at Stations 3 and 5 (upwelling region). The %MeHg in the organisms was highest in the > 2.0 mm size class, displaying MeHg bioaccumulation for increasing zooplankton sizes. Separate day and night net tows were collected at Stations 3 and 5 in order to investigate differences due to diurnal migration of zooplankton. There were higher concentrations of MeHg for all sizes in night collections of zooplankton at Station 5, but no discrepancy for Station 3. These results represent some of the few measurements of Hg and MeHg at the base of the open ocean food chain, and are significant as they represent the instigation of bioconcentration into the base of marine food webs.