Intellectual Merit: Diatoms and dinoflagellates are both diverse and biogeochemically significant groups of plankton. The former have fast growth rates, dominate blooms and their mineral silicate components help drive significant export. The latter encompass diverse trophic strategies including mixotrophy and play a complex role in the cycling of carbon and nutrients. Current marine ecosystem models typically represent a single diatom "functional type" and do not explicitly reflect dinoflagellates or mixotrophy at all. This project will investigate and model the regional biogeography, seasonal succession, and inter-annual variability of the assemblages of diatoms and dinoflagellates in the subpolar North Atlantic, where the Continuous Plankton Recorder (CPR) survey has documented the abundance of more than 100 species of these organisms over several decades. The PIs will characterize the temporal and spatial variations of diatom and dinoflagellate assemblages and their key traits, including cell size and trophic strategy, in the CPR survey. They will use cluster analysis deduce biogeographic provinces in the subpolar North Atlantic based on the variations in species and trait assemblages of these two key groups of primary producers. To interpret how the interplay of environment and physiology regulates these spatial and temporal trends, they will develop and employ numerical simulations of the supolar North Atlantic which includes an ecosystem component where hundreds of potentially viable microbial physiologies are seeded and "survival of the fittest" in silico organizes ecosystem structure and function. Simulations will be used to seek mechanistic explanations for the previously noted interannual and decadal shifts in diatom and dinoflagellate abundance in the North Atlantic.
Broader Impacts: The proposed work is fundamentally interdisciplinary in nature. It fosters international collaborations through the EU/North American BASIN program. The tools developed will have potential applications in models of the global change and the carbon cycle, as well as studies of climate change and microbial evolution rooted in the paleo-record of these two groups. The project involves the development and training of a graduate student and post-doctoral scientist.
Motivation and background Phytoplankton are microscopic plants of the ocean: they combine energy from sunlight with dissolved carbon dioxide and nutrients to reproduce, creating new organic molecules. Some of this organic material is consumed by other plankton, which are in turn consumed by fish, fuelling an important resource for humans. A fraction of the organic matter sinks to the deep ocean where it leads to a store ofdissolved inorganic carbon that keeps atmospheric CO2 relatively low and the climate cooler than it otherwise would be. There is a very high diversity of phytoplankton, with very different lifestyles and implications for fisheries and ocean carbon storage. As a community, we are currently seeking to map out where these different types live (their biogeography) and to understand why they live in those particular environments. The "traits" of the plankton are characteristics of individuals which affect their interaction with the environment. Intellectual Merit We have examined the relationships between specific traits (for example cell size) and the biogeography of particular plankton species. We have developed mathematical models which encapsulate key traits and which can simulate their distribution in a virtual ocean. The success of such a model indicates a degree of qualitative and quantitative understanding of the real system. In this way, we have developed models which can predict the observed seasonal and regional distributions of two groups of phytoplankton, diatoms and dinoflagellates in the Atlantic Ocean, as well as the distribution of cell-sizes within those populations. By combining these simulations with analysis of observed populations, we have shown how a combination of nutrient supply and predation set the size structure of marine plankton populations. We have, for the first time, indicated that mixotrophy (combining photosynthesis and predation) leads to larger plankton and enhanced storage of carbon on the global scale. Broader Impacts A key outreach effort associated with this project has been to illustrate to the general public how plankton populations in the ocean (like trees on land) are arranged geographically into "biomes" (i.e. regions dominated by similar types). To this end we have worked with Dr. Jen Frazier and colleagues at the San Francisco Exploratorium to produce a hands-on exhibit for their Living Liquids display. The table-top exhibit shows animations of plankton populations in our global simulations. Visitors use a special maginifying glass throug which the exhibit reveals something of the diversity and cell size of the phytoplankton populations at the location where the user places it. See a video about the exhibit at www.exploratorium.edu/explore/videos/plankton-populations-exhibit
