Award: PLR-1043748

The ocean surrounding Antarctica, known as the Southern Ocean, is one of the most biologically important ecosystems on our planet. It is home to a diverse assemblage of organisms, all of which are nourished either directly or indirectly by phytoplankton, the microscopic plants of the sea. The phytoplankton that live in the Southern Ocean are important not only because they form the base of the marine food web, but they also play a significant role in regulating our planetÆs climate. As phytoplankton photosynthesize, oxygen is released and carbon dioxide, a greenhouse gas, is consumed. Due to fossil fuel emissions, concentrations of carbon dioxide in the atmosphere are rising, which is causing atmospheric and surface ocean temperatures to rise. Elevated temperatures and higher concentrations of carbon dioxide are currently impacting the Southern Ocean ecosystem at an unprecedented rate. Increased concentrations of carbon dioxide are changing the carbon chemistry of the seawater, while higher temperatures are causing glaciers and sea ice to melt, which releases iron into the surrounding ocean. Iron is a micronutrient that is essential for phytoplankton growth, and its concentrations are typically very low in the Southern Ocean. Increasing temperature, carbon dioxide, and iron may create an environment that favors some species of phytoplankton over others, which may affect nutrient cycles and the rates at which the phytoplankton photosynthesize. The major goal of this research was to determine which types of phytoplankton will thrive in this changing environment, and how elevated temperature, carbon dioxide, and iron will impact the rates at which nutrients, such a carbon and nitrogen, are used by phytoplankton. To realize our objectives we conducted a series of experiments on phytoplankton communities collected from the Ross Sea, Antarctica over two separate Austral summer (December – January) seasons. The phytoplankton communities were transported from the ocean to a laboratory at McMurdo Station, a United States Antarctic Research Center, where they were subjected to changes in temperature, carbon dioxide, and iron. These variables were tested individually as well as in a combination and compared to present-day conditions. The phytoplankton were grown for several weeks under these conditions and changes to the phytoplankton community and the biogeochemistry of the seawater were measured every two to three days. The experiments conducted in Antarctica have provided us with a more comprehensive view of how phytoplankton will respond to environmental change in the future. We found that temperature is the main variable that promotes changes in the phytoplankton community and alters the rates at which they use nutrients. Of the three parameters tested (temperature, carbon dioxide and iron), increases in temperature had the largest impact on determining what phytoplankton species are present and how fast they grow. The addition of iron also increased phytoplankton growth and rates at which they use nitrogen and carbon, but this response was much less than the response of the same community to increased temperature. Higher concentrations of carbon dioxide had very little impact on the phytoplankton community. Through this study, we also found that higher temperatures favor some species more than others. For example, Pseudo-nitzschia spp., a potentially toxic phytoplankton species that also blooms off the coast of California, will likely become more abundant as temperatures increase in the Southern Ocean. The fact that Pseudo-nitzschia spp. may increase in the Southern Ocean has major implications for the future health of that ecosystem because the toxin that it produces can be transferred up the food web to potentially vulnerable species including seals and sea birds. While increasing the abundance of Pseudo-nitzschia spp may negatively impact the Southern Ocean ecosystem, we al...