Project: Composition of the plankton community and its contribution to particle flux in the Sargasso Sea

The overall objective of this proposal is to investigate linkages between the presence of different key groups of phytoplankton in the euphotic zone and their contribution to particle flux at the subtropical North Atlantic time-series station BATS (Bermuda Atlantic Time-series Study) by applying a range of traditional and novel molecular techniques.

The 'biological pump', the photosynthetically mediated transformation of dissolved inorganic carbon into particulate and dissolved organic carbon in surface ocean waters and its subsequent export to deep water, is a significant driver of the atmospheric carbon uptake by the oceans. But this "biologically pumped" production, inasmuch as it depends on the composition and activity of planktonic organisms, is susceptible to long-term climatic changes in surface ocean properties such as increased temperature and changes in nutrient supply, especially in subtropical gyres. The sub-tropical gyres and the transition zones at their boundaries play an important role in the global carbon cycle because of their vast size and generally high per area export production. As evidenced in recent studies, the biological mechanisms driving regional to basin scale variability in carbon export in these biomes is far from understood, thus limiting our ability to mechanistically explain the biological pump and to predict its possible responses in the face of environmental change. In an effort to improve this situation with an accurate assessment of the contribution of different plankton groups to overall fluxes, the investigators will test the following two specific hypotheses: 1. The long held notion that large cells and those with mineral tests are major contributors to downward particle flux needs to be re-evaluated. We hypothesize that pico and nanoplankton (also those without mineral tests) are generally important contributors to downward particle flux at BATS. Consequently, the diversity of taxonomic groups contributing to particle flux is greater than previously expected. 2. The relative contribution of taxonomic groups to downward particle flux is a function of physical forcing. We hypothesize that episodic events (e.g., winter storms and eddies) lead to a reduction in diversity of sedimenting phytoplankton (e.g., dominance by a single group such as diatoms) compared to periods marked by more stable conditions in the water column. The broader impacts include furthering knowledge of the diversity and biology of phytoplankton groups that have a significant impact on the carbon export in subtropical gyres, thereby advancing our understanding of regional to basin scale variability in the biogeochemistry of these biomes. The project provides new opportunities for undergraduate and graduate education, as well as offer research opportunities to local high school students and teachers as part of the "Ask-a-Biologist" initiative. The project also includes an international component through collaboration with a molecular ecology group in Barcelona, Spain.