Award: OCE-1830029

Increasing concentrations of atmospheric carbon dioxide are causing ocean acidification of the surface oceans, and this trend is expected to continue over the next century. Ocean acidification has many implications for the organisms that live there, with perhaps one of the more important issues being how decreasing pH may affect marine phytoplanktonthe base of the food web. Whilst the projected changes in pH may have only small effects on phytoplankton metabolism, these changes have the potential to significantly alter the availability of essential nutrients dissolved in seawater that are needed for phytoplankton to flourish. The trace element iron is needed by phytoplankton in greater quantities than any other trace element but is present at very low concentrations in seawater. Iron has been shown to limit phytoplankton biomass in upwards of 40% of the surface ocean, including large offshore regions as well as some nearshore and coastal waters. The chemical forms of iron in seawater determine how accessible this essential element is to phytoplankton, and changes in pH are expected to directly impact these chemical forms. The challenge then is to understand how projected and progressive increases in ocean acidification will influence the ability of phytoplankton to acquire this essential element and whether ocean acidification will lead to higher or lower primary production in the future ocean. This project investigated how decreases in pH (ocean acidification) altered the chemistry and the biological availability of the trace element iron in nearshore and offshore sea waters. Most iron in seawater is bound by organic molecules, some of which are degradation products of marine life such as humic-like substances, and some are biomolecules produced by bacteria specifically to compete with phytoplankton and other microbes for this essential but scarce element. A limited number of laboratory and modeling experiments have suggested that decreases in pH would affect iron availability differently depending on which type of iron-binding agents were present. The questions then are whether changes in pH alters iron availability to phytoplankton the same everywhere or if the effect is different in different regions, and how does this impact relate to the types of iron complexing molecules present. We sought to answer these questions by measuring the effect of pH on iron chemistry and availability to phytoplankton in three regions representative of much of the surface ocean: a nearshore upwelling region off the California coast, the central, nutrient poor subtropical gyre, and in biologically rich subarctic Pacific waters where insufficient iron availability prevents more phytoplankton growth. Shipboard incubation experiments were conducted on the Research Vessel Sikuliaq where surface waters containing phytoplankton were incubated over several days under different pH conditions. A wide range of biological measurements were applied to replicate experiments in each region to establish the response of phytoplankton and the changes in nutrient and trace metal chemistry that resulted from changes in ocean pH. The shipboard experiments showed that decreasing pH impacted phytoplankton production differently across ocean regimes, increasing the apparent bioavailability of iron in the near shore upwelling regions but decreasing iron availability in offshore waters. These distinctions in trace metal bioavailability with changing pH across different ocean regimes was also not confined to iron measurements of nickel chemistry, another essential trace element, also revealed diverging responses of nickel bioavailability with pH in the different incubations. The outcomes of this study are important because they provide insight into how climate-driven changes in the surface ocean may alter trace metal bioavailability and primary productivity in nearshore and offshore seawaters, and the importance of characterizing metal chemistry in these different regions to better predict the responses of ocean productivity to climate change. Project results have been shared broadly through conference presentations in the U.S. and Japan, in invited seminars in an undergraduate chemistry department, in the published dissertations of project graduate students, and in publicly accessible database submissions. Research activities were incorporated into an Art and Science outreach activity for the public in Florida, in hands-on laboratory activities on pH and iron-binding organic ligands for high school girls as part of the USF Oceanography Camp for Girls, and in REU program activities at OSU. During the pandemic period of the award, girls from this camp were mentored virtually through one-on-one meetings with project participants. Shipboard videos of the setup and sampling for trace metals at sea were shared in research blogs by a postdoctoral researcher and graduate student and were shared with the UNOLS marine technician group to engage the broader technical support community in trace metal fieldwork. Broader impacts of this project have also included the training and mentoring of undergraduate students, graduate students, postdoctoral researchers and early career researchers in ocean biogeochemistry, trace metal chemistry, and chemical oceanography field work.Finally, data from the project have been made publicly available in BCO-DMO. Last Modified: 01/19/2025 Submitted by: MarkLWells