Date of Completion

3-31-2016

Embargo Period

3-30-2017

Keywords

phytoplankton, zooplankton, mercury, methylmercury, bioaccumulation

Major Advisor

Robert P. Mason

Associate Advisor

Hans Dam

Associate Advisor

Craig Tobias

Associate Advisor

Penny Vlahos

Field of Study

Oceanography

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Methylmercury (MeHg) is bioconcentrated in phytoplankton and transferred via consumption to zooplankton, planktivorous fish, and eventually larger predators. This dissertation research investigated the transfer of Hg and MeHg from phytoplankton to zooplankton through laboratory experiments and field measurements for several different realms of the marine environment, including coastal, oceanic, and polar regions. Phytoplankton samples were size fractioned into 0.2-5 µm, 5-20 µm, and seston of >20 µm samples, and demonstrate distinctive regional and global variations. The MeHg bioconcentration factors (logBCF) for phytoplankton in the Pacific Ocean had logBCF values that averaged 5.7 ± 1.0 and were higher than the range for Long Island Sound and coastal regions (2.6-5). Zooplankton samples were analyzed for Hg and MeHg at size fractionations of 0.2-0.5 mm, 0.5-1.0 mm, 1.0-2.0 mm and >2.0 mm. The %MeHg in the organisms was typically highest in the largest size class, displaying MeHg bioaccumulation for increasing zooplankton sizes. The carbon (δ13C), nitrogen (δ15N) and sulfur (δ34S) isotopes were also evaluated, and demonstrate characteristic feeding sources and patterns for zooplankton Hg and MeHg accumulation. Feeding experiments were performed using stable isotopes of inorganic Hg (200Hg) and MeHg (CH3199Hg) in order to determine assimilation efficiencies (AE) and uptake and transfer of Hg and MeHg into marine diatoms and the copepod Acartia tonsa. The average AEs were much greater for CH3199Hg (~85%) than for 200Hg (~35%) demonstrating more efficient transfer of MeHg at the planktonic level. Furthermore, these experiments gave evidence for active uptake of MeHg into algae at low (~pM) concentrations. Coastal estuarine uptake experiments for Hg isotope species directly from sediment were also undertaken, and demonstrate that sedimentary sources have a limited and highly variable impact on coastal phytoplankton MeHg accumulation.

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