Fluxes of particulate carbon from the surface ocean are greatly influenced by the size, taxonomic composition and trophic interactions of the resident planktonic community. Large and/or heavily-ballasted phytoplankton such as diatoms and coccolithophores are key contributors to carbon export due to their high sinking rates and direct routes of export through large zooplankton. The potential contributions of small, unballasted phytoplankton, through aggregation and/or trophic re-packaging, have been recognized more recently. This recognition comes as direct observations in the field show unexpected trends. In the Sargasso Sea, for example, shallow carbon export has increased in the last decade but the corresponding shift in phytoplankton community composition during this time has not been towards larger cells like diatoms. Instead, the abundance of the picoplanktonic cyanobacterium, Synechococccus, has increased significantly. The trophic pathways that link the increased abundance of Synechococcus to carbon export have not been characterized. These observations guided the investigators to their overarching research question, "How do plankton size, community composition and trophic interactions modify carbon export from the euphotic zone". Since small phytoplankton are responsible for the majority of primary production in oligotrophic subtropical gyres, the trophic interactions that include them must be characterized in order to achieve a mechanistic understanding of the function of the biological pump in the oligotrophic regions of the ocean. This requires a complete characterization of the major organisms and their rates of production and consumption. Accordingly, the research objectives are: 1) to characterize (qualitatively and quantitatively) trophic interactions between major plankton groups in the euphotic zone and rates of, and contributors to, carbon export and 2) to develop a constrained food web model, based on these data, that will allow us to better understand current and predict near-future patterns in export production in the Sargasso Sea. The investigators will use a combination of field-based process studies and food web modeling to quantify rates of carbon exchange between key components of the ecosystem at the Bermuda Atlantic Time-series Study (BATS) site. Measurements will include a novel, DNA-based approach to characterizing and quantifying planktonic contributors to carbon export. The well-documented seasonal variability at BATS and the occurrence of mesoscale eddies will be used as a natural laboratory in which to study ecosystems of different structure. This study is unique in that its aims to characterize multiple food web interactions and carbon export simultaneously and over similar time and space scales. A key strength of the proposed research is also the tight connection and feedback between the data collection and modeling components.
Broader Impacts Characterizing the complex interactions between the biological community and export production is critical for predicting changes in phytoplankton species dominance, trophic relationships and export production that might occur under scenarios of climate-related changes in ocean circulation and mixing. This research may also help to understand the biological mechanisms that drive current regional to basin scale variability in carbon export in oligotrophic gyres. This proposal will contribute to the education of undergraduate and graduate students through the inclusion of student support. Undergraduate students in this project will be partly supported through Arizona State University (ASU)'s School of Life Sciences Undergraduate Research Program which seeks to increase the participation of minorities in science. Web and classroom materials based on this research will be developed and distributed through a partnership with the award-winning ASU-sponsored Ask A Biologist K-12 Web site. Direct undergraduate involvement in research will also be enhanced at the University of South Carolina and the Bermuda Institute of Ocean Sciences.
The ocean is responsible for the sequestration of more than 25% of the carbon released by anthropogenic activities every year. A combination of different biological processes (biological carbon pump) play an essential role in the sequestration of the atmospheric carbon to the ocean interior. Briefly, the phytoplankton convert atmospheric CO2 through photosynthesis to particulate and dissolved organic matter. Most of this organic carbon is recycled through utilization by planktonic grazers and respired back into the atmosphere, but a small fraction is exported to deeper depths and removed from contact with the atmosphere for decades or centuries. The objective of this research was to investigate how different taxa of pico- and nano-phytoplankton and their trophic dynamics are linked to the carbon flux in Sargasso Sea near the open ocean times series station BATS (Bermuda Atlantic Time-series Station). This site was chosen because of its more than 25 yr record of ongoing investigations, but also because it is considered representative of subtropical gyre regions that are expected to expand in response to the continuing warming of the oceans. This research was a collaborative effort by researchers from four different US universities and institutes. We used a combination of experiments coupled with microscopy and DNA-based tools to determine composition and taxon specific growth and grazing rates of the resident phytoplankton communities in the water column, as well as the composition of the sinking particulate matter. We also investigated prey utilization and fecal pellet production by the resident zooplankton community. We collected seawater samples in the upper 100m of the water column and sinking particles using particle traps at 150m during the spring and the summer of 2011 and 2012 at the Bermuda Atlantic Time-series Study station and in the surrounding mesoscale eddies. Mesoscale eddies are large circular features several 10s of km in diameter that influence nutrient availability to the phytoplankton community. Our results show that primary productivity in the Sargasso Sea is linked to the close relationship between phytoplankton and its micro grazers, and there were no specific patterns in productivity and grazing related to the interaction between mesoscale features and seasonality. We also found that micro grazers tightly control the growth of the pico and nano phytoplankton community, impacting the export of carbon in different ways. In some cases this coupled relationship favors efficient recycling of carbon within the euphotic zone (depth where sufficient light is available for photosynthesis, usually around 100m in the open ocean). In other cases, despite the coupled relationship, we found specific taxa of pico and nano-phytoplankton in the trap material, suggesting that the export of pico and nano-phytoplankton to the deeper ocean involves other pathways that still need further clarification. In addition, we found that larger zooplankton help mediate the transport of specific taxa of pico- and nanoplankton out of the epipelagic via the consumption of cells and detrital aggregates and subsequent production of fecal pellets. The results of this study will establish a baseline that will enable us to better predict the consequences of a changing community on the biological carbon pump in a future ocean. The data have been submitted to the Biological and Chemical Oceanography Data Management Office. The research also supported the educational and scientific development of one PostDoc, two undergraduate students who completed theses, one Master of Science and one PhD student and one high school student. We participated in several international conferences and are in the process of preparing several publications for the peer-reviewed literature. We publicized this research and its importance in outreach material for the public and K-12 on ASU’s award winning Ask-a-Biologist web site.
