Abstract
Marine dissolved organic carbon (DOC) represents a large and dynamic reservoir of reduced carbon (662 x 1015g C) comparable in size to the Earth’s reservoir of atmospheric CO2. Our knowledge on the transformation and removal of DOC through various sinks and processes is often obscured by a lack of observable and quantifiable mechanisms. For example, self-assembling microgels, transitional in size between dissolved and particulate matter, have been suggested to play a key intermediary role in organic matter bioreactivity. Microgel formation is the first step in creating a particulate sink for dissolved organic matter, with up to 10% of DOC in the ocean (70 x 1015g C) predicted to exist in the gel phase. Although they are considered macrogels due to their larger size and greater stickiness than microgels, transparent exopolymer particles (TEP) also abiotically assemble from dissolved organic precursors (namely, polysaccharides). As microgels and TEP span the dissolved-to-particulate size continuum of organic matter, it is important to understand how polymer gel dynamics influence the total organic carbon (TOC) pool. Analytical challenges limit our ability to directly quantify the organic carbon present in these gels, or even to measure DOC removal through gel formation. In regions of the ocean where DOC concentrations are as low 35 µM, the potential 10% thermodynamic yield of gel formation is close to the analytical uncertainty of DOC measurement (1-3 µM; CV ~3%). This uncertainty also makes pursuit of other studies on DOC dynamics, such as uptake and respiration, difficult. This dissertation uses an analytical chemistry approach to enhance understanding of sources and sinks of marine organic carbon in the ocean through field observations of dynamics in addition to improvements of DOC and gel methodologies. In Chapter 2, the abundance of TEP was observed across a surface ocean gradient of TOC, from a phytoplankton bloom region in the western North Atlantic to oligotrophic waters in the Sargasso Sea, including a coastal region sampled near Cape Cod. Results from this chapter support the hypothesis that TEP aggregation with non-gel particulate organic carbon (POC) enhances ballasting, thereby mutually facilitating export and subsequent sedimentation of both TEP and POC, and increasing the efficiency of the biological pump. Chapter 3 investigated the fluorescence quenching assay previously developed by Ding et al. (2007) to estimate the carbon content of microgels in natural seawater samples, because results obtained by applying the method are often inconclusive. Sensitivity to pH was identified as the primary driver for the variability, suggesting that the method should be discontinued in its present form. To improve the precision of the DOC measurement, Chapter 4 described the development of a prototype flow-through TOC/DOC analyzer system that combines UV and wet chemical oxidation and is capable of sub-micromolar precision. While the prototype instrument shows superior analytical skill relative to the standard HTC technique, application is currently limited to freshwater analyses due to oxidation interference by chloride ion.