Award: OCE-1433710

Project Outcomes Report Award Title: Collaborative Research: Isotopic insights to mercury in marine food webs and how it varies with ocean biogeochemistry Federal Award ID: 1433710 Report Submission Period: 08/01/2021 to 07/31/2022 Mercury is a pervasive element that exists in the marine environment in several different chemical forms. The Monomethylmercury (MMHg) form of mercury is an organic, carbon-containing, neuorotoxin. It bioaccumulates in commonly harvested marine organisms, which can pose a human health threat. Knowledge of where and how mercury changes its chemical form allows us to understand the impacts of mercury on ocean food webs. Previous work examined mercury concentrations in nine species of marine fishes from the North Pacific Ocean and found that MMHg appears to enter open ocean food webs at waters depths of 200—600 m. At these depths, dissolved oxygen concentrations are much lower than in surface waters. Thus, we hypothesized that oxygen influences the depth where mercury transformations occur. To study how mercury enters the marine food chain, we analyzed mercury in rainfall, marine organic particles, zooplankton and micronekton (small fishes, squid and shrimp) from the surface ocean to 1500 meters depth on four research expeditions in the Pacific Ocean. Intellectual Merit Mercury deposited to the surface ocean from the atmosphere is quickly adsorbed onto organic particles, which then settle through the ocean and are eventually eaten by organisms. We found distinct nitrogen isotope values of individual amino acid compounds in small (<53 micrometers) and large particles (>53 micrometers). Our analyses suggest that these distinct values are due to a high proportion of amino acids that derive from microbes clinging to the particles or from microbes breaking down particles for their growth. The proportion of amino acids with distinct nitrogen isotope values also increases with depth. This is important to the mercury cycle because the transformation of elemental mercury to toxic MMHg has been associated with microbial metabolism. We can also trace these distinct amino acid nitrogen isotope values into zooplankton and micronekton—indicating whether small or large particles form the base of the food chain at depth. Our study also revealed seasonal and spatial changes in the rates at which small and large particles, and the mercury within those particles, settle out of the surface ocean. Mercury isotopic variations can provide insight into a wide variety of environmental processes. Mass-dependent processes, which affect mercury isotopic variations, are associated with most biotic and abiotic chemical reactions. On the other hand, mass-independent processes, which occur during photochemical reactions, drive isotopic variations of mercury at the surface where there is abundant sunlight. The unusual combination of mass-dependent and mass-independent processes provided information on reaction pathways and the biogeochemical history of mercury. Our mercury isotopic analyses indicate that the MMHg bioaccumulated in fish is derived primarily from elemental mercury deposited to the surface ocean by rainfall. Our results also indicate that marine particles host the majority of mercury available for the formation and degradation of MMHg, which we found can happen throughout the water column. These particles further transport mercury to deeper waters where it can enter deeper residing fish. Our mercury isotope results indicate that photochemical reactions that break down MMHg are responsible for the low concentrations of mercury in fish living in well-lit surface waters. Combined, these findings highlight the critical need to understand how MMHg levels in marine biota will respond to changes in atmospheric mercury emissions, deposition of inorganic mercury to the surface ocean, and hypothesized future expansion of low oxygen zones in the ocean as the Earth warms. Broader Impacts This project trained three graduate students and three undergraduate students, all women and one of native Hawaiian ancestry. We also provided at sea experiences for an additional 6 undergraduate and 7 graduate students exposing them to a diversity of sampling techniques and learning opportunities. This research provided early career opportunities to coPI Hannides. CoPI Duncan has generated substantial public outreach for the project. We produced eight television episodes of the Telly Award winning series Voice of the Sea. These episodes aired widely in Hawaii and in U.S. Pacific territories and affiliated Pacific regions. In Hawaii alone, our viewership averages 10,000-20,000 viewers per episode (Nielsen Ratings and Marshall Marketing surveys). Audience feedback shows that Voice of the Sea episodes have been effective at increasing viewer knowledge and interest as well as viewer connection to scientists across age groups, from middle school, to college, to adult populations. We presented our research results at numerous seminars and conferences. We published results of this research and outreach in 10 peer-reviewed papers. We have recently submitted for publication two more manuscripts and have at least three additional manuscripts in preparation for submission. Our data is publicly available through the Biological and Chemical Oceanography Data Management Office (http://www.bco-dmo.org/project/560580). Last Modified: 09/29/2020 Submitted by: Joel D Blum