Award: OCE-1638834

The major goals of the project were to determine how single-celled phytoplankton communities from different parts of the ocean (temperate vs. subtropical) respond to changing temperatures. We characterized the genetic, functional and taxonomic diversity of thermal responses of phytoplankton to predict phytoplankton community re-organization under different future scenarios. We hypothesized that an interaction of temperature and nutrients in both estuarine and open ocean environments would influence the functional, genetic and taxonomic diversity of phytoplankton. For example, we hypothesized that rising temperatures would lead to shifts in the phytoplankton community composition of the community and that we would see temperature and nutrients interact to influence phytoplankton growth rates. To test our hypotheses, we conducted two sets of experiments that examined the interaction of temperature and nutrients on phytoplankton growth and community composition. We collected seawater from NB in the winter (2 deg C) and summer (>20 deg C), and grew the phytoplankton either with nutrients amended or not and under higher, lower or in situ temperatures. We also isolated individual phytoplankton cells from the water during the winter and summer and put these cells through temperature "stress tests" to determine how they might respond to rising seawater temperatures. Finally, we generated a database consisting of the results of our phytoplankton stress tests along with all other published thermal response curves in the phytoplankton to obtain a global view of how different groups of phytoplankton respond to thermal stress. The main findings of our study included the observation that wintertime phytoplankton communities respond to increased nutrients with an increase in cell size and shift in community composition and the increase in temperature resulted in increased biomass, but only when nutrients were present. These results suggest a strong interactive effect of nutrients and temperature on phytoplankton communities. Through a series of phytoplankton stress tests, we identified a type of phytoplankton that is sensitive to light and temperature- highlighting another type of interactive effect driving the phytoplankton response to increasing sea surface temperatures. Finally, our global database of phytoplankton response to temperatures revealed significant differences in thermal response among the major functional groups of phytoplankton in the ocean today. The implication is that there is no generic phytoplankton response to temperature but instead that each major group of phytoplankton has a different response and thus there may be significant reshuffling in phytoplankton communities of the future. Last Modified: 01/13/2022 Submitted by: Tatiana Rynearson