The marine dissolved organic matter (DOM) reservoir is often conceptualized as a "capacitor" for carbon because of its potential for storage and exchange with the other active reservoirs. Yet, despite its size and potential importance, the dynamics of DOM have not been fully assessed, and therefore the ultimate role of this reservoir for the global and marine carbon and nitrogen cycle remains to be established. To resolve the dynamics of the complex DOM reservoir, a young investigator from the University of California San Diego will develop a spatially resolved time-series of fraction-specific d15N and D14C measurements of DOM. This time series will be placed within the California Current System which experiences physical/climate variability on a variety of timescales (e.g. seasonal, interannual, decadal). The climate-associated disturbances (deepening/shoaling thermocline or nutricline, changes in wind field) are expected to lead to measurable excursions in the isotopic composition of surface inorganic dissolved CO2 and NO3-pools. These excursions are ultimately recorded by organic matter fractions that are recently produced in surface waters; and when placed within the context of a time series, fraction-specific isotopic variability will, for the first time, directly identify DOM cycling on annual to multidecadal timescales. In addition, the time series will also help identify relative proportions of refractory and young DOM components in surface waters and delineate some of the physical and biological processes that control the size and age of the DOM reservoir.
As part of this CAREER award, a new, inquiry based, undergraduate course will be created to place the theoretical coupling between large scale phenomena such as climate-ocean-atmosphere and smaller scale phenomena such as ocean physics-chemistry-biology within a tangible, region-specific, real-world framework. A fundamental goal of this course will be for students to learn experientially how science "works"; students will design class projects that use archived time-series data to explore the above connections and collect oceanographic data on a 4-7 day educational cruise to the California Current System.
Among the broader impacts, this study will be one of the most comprehensive, long-term investigations of the interrelation between climate and biogeochemistry. The infrastructure of the larger, ongoing scientific programs in the region will provide context, and enhance the dissemination of scientific data collected as part of this proposal to a broad audience through both web-based tools and written and verbal communication. Through the four specific goals of the Education and Outreach plan, the PI will teach and mentor ~100 undergraduates, mentor ~8 high school students, empower both student groups to be more critical, better environmental decision-makers, provide ~8 educators with valuable scientific experience to take back to their own classrooms and so reach even a larger number of students, and also provide ~100,000 citizens with a valuable window into the marine ecosystem in their own backyards. In addition, the high school students that will be targeted represent some of the most under-served youths in San Diego County.
The amount of carbon dioxide (CO2) accumulating in the atmosphere is closely controlled by the physics, chemistry and biology of the ocean on short and long timescales. The ocean contains two major carbon reservoirs – dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC). Atmospheric carbon and DIC interact directly through physics and chemistry, and that interaction is well understood and known to be critical for maintaining the current CO2 content of the atmosphere. Atmospheric carbon and DOC interact indirectly through biology, and the timescales over which these interactions take place are very poorly understood. Based on "carbon dating" methods it is hypothesized that DOC interacts with CO2 on millennial timescales. The results of our research showed that dissolved carbohydrates, the major chemical family identified in DOC to date, interact with CO2 on sub-decadal timescales. This is much faster than what the "dating" methods show for the average DOC reservoir. This relatively fast turnover time for dissolved carbohydrates suggests that they play an important role in upper ocean ecosystems. Based on these results we hypothesize that dissolved carbohydrates act as a conduit for carbon transfer between autotrophic and heterotrophic microorganisms in the surface ocean. Climate change, for example, that impacts ecosystem structure and function, will affect carbohydrate inventory and thus, carbon partitioning into DOC and DIC. Our results also confirmed the importance of dissolved carbohydrate structure in controlling its rate of turnover. For example, carbohydrates containing glucose appeared to cycle independently in coastal environments. In addition, our data showed that lipid-like chemicals within DOC can have slow turnover rates, and that some of these compounds contribute to the storage of carbon within DOC on millennial timescales. Taken together, our results help to elucidate the role that the second largest reservoir of carbon in the ocean – DOC - plays in the global carbon cycle. Delineating processes that impact carbon cycling in the ocean and examining rates of carbon transformation are important components of illuminating the impact of future climate change on ocean-atmosphere interactions and upper ocean ecosystems. As part of this investigation we also developed some analytical methods that will find application in a variety of fields.
