Mechanisms controlling the production and fate of DOM during diatom blooms

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Intellectual Merit: The residence time of DOM in surface waters of marine systems is the main factor determining its contribution to the efficiency and magnitude of the biological carbon pump. There is growing evidence that the export of DOM from the surface ocean represents a highly efficient pathway for the sequestration of organic carbon in the deep sea in some ocean regions. Current estimates are that 20% of the carbon transported to depth by the biological pump globally occurs via the export of DOC. Effective export of DOM requires that the DOM produced by phytoplankton persist in surface waters until vertical exchange processes transport the material to depth. The mechanisms controlling the time scale for the accumulation and persistence of DOM in surface waters are dominated by biological processes that influence the amount and chemical character of the DOM produced and its consumption and decomposition by microbes. This project addresses these two coupled biological processes to examine controls on the accumulation of DOM during temperate diatom blooms. Diatom blooms are known to produce prodigious quantities of DOM upon entering nutrient stress with a chemical composition that varies with the type of nutrient limitation (Si or N). This variable composition likely influences the nutritional value of DOM to microbes driving species successions towards functional groups of heterotrophic prokaryotes that are best able to metabolize particular forms of DOM. To date each side of this coupled system of production/consumption has been examined independently. A few studies have examined how limitation by different limiting nutrients affects the chemical character of the DOM produced by phytoplankton, while others have focused on the fate of DOM without detailed understanding of the mechanisms influencing its initial chemical composition. This research project will examine both sides of this coupled process simultaneously to see how different forms of nutrient limitation drive the chemical character of DOM and the subsequent microbial response which together determine the fate of DOM produced during diatom blooms. The investigators will employ a combination of laboratory and field based approaches to: 1) Investigate how limitation by either N or Si impacts the chemical composition of the DOM released by diatom blooms. 2) Determine how differences in the composition of DOM produced by diatoms experiencing different nutrient stresses affects it susceptibility to heterotrophic microbial processing through changes in the productivity, growth efficiency and community structure of bacterioplankton. The research will focus on diatom blooms for two reasons. Diatom blooms are a regular feature in regions of the ocean where DOC export is known to be significant, i.e. such as the North Atlantic, making the fate of the DOM produced during blooms a potentially significant mechanism of C export. In addition, the direct release of DOM from phytoplankton is the best studied of numerous DOM production process providing the background for formulating hypotheses on how changes in DOM production and composition affect the bacterial response that drives its consumption. Preliminary data indicates that waters of the Santa Barbara Channel, California are an ideal model system for conducting this research because the spring diatom bloom is sufficiently predictable and amenable to the types of manipulations required for these studies, and ambient DOM concentrations are low for coastal waters allowing small changes in DOM concentrations to be resolved in both laboratory and field experiments.Broader Impacts: This research will contribute to the greater scientific goal of understanding the role of upper ocean food webs in carbon cycling. The findings will improve our basic understanding of DOM dynamics aiding modelers in the development of improved representations of key processes in ecosystem models. The project will enable the education of the next generation of biological oceanographers by training graduate students, and it will be leveraged to introduce oceanography into the curriculum of local K- 12 students to make them aware of the importance of the oceans to their lives and of the possibility of oceanography as a career choice.