Scripps Institution of Oceanography

This thesis provides chemical characterization data for ultrafiltered dissolved organic matter (UDOM) isolated from multiple depths in the western equatorial Atlantic and subtropical Pacific Oceans, the Southern California Bight, and the Benguela upwelling regime (n = 80). Multiple chemical characterization measurements were performed on this large set of UDOM samples including elemental analysis, stable C and N isotopic composition (d13C and d15N), radiocarbon analysis, 1H-NMR spectroscopy, monosaccharide composition, and novel application of several protein quantification methods. Most samples were collected as part of an extensive field program aimed at describing the biocomplexity of ocean ecosystems. Therefore, complimentary data collected as part of this field program enabled a uniquely comprehensive assessment of relationships between physical-biological variables and DOM composition.

Nitrogen (N) isotope dynamics are a common theme in all chapters and these data were used to study the flow of N within the microbial loop of the upper ocean and through the DOM reservoir. d15N-UDOM showed remarkable homogeneity over this vast spatial array when compared to the d15N dynamics of the short-lived particulate N reservoir suggesting that dissolved organic nitrogen (DON) contributed by contemporary processes does not accumulate in the upper ocean. Further isotopic analyses of planktonic nucleic acids (NA) and the protein component of DON provided evidence for the rapid recycling of DON contributed by diazotrophs in these regions characterized by extremely low concentrations of inorganic N. N isotope fractionation effects associated with the biosynthesis of NA by marine phytoplankton were also examined in one chapter to establish the validity of using d15N-NA of size-fractionated field assemblages to identify biological sinks for dissolved organic nitrogen (DON).

Basin-wide comparisons of chemical composition data revealed significant differences in d15N-UDON between the subtropical N. Atlantic (avg.±s.d. = 4.1±0.6 per mil) and Pacific Oceans (5.3±0.8 per mil and 15.0±1.4, p < 0.001). The Atlantic Ocean UDOM samples also exhibited higher C/N ratios (16.1±1.5) than those in the Pacific (15.0±1.4, p < 0.05), and significant differences in the monosaccharide composition of UDOM were observed both between and within these basins. Principal component analyses allowed preliminary identification of specific biological imprints on DOM composition particularly in regions dominated by diazotrophs. Furthermore, molecular level characterization combined with radiocarbon signatures of UDOM allowed for a novel assessment of UDOM reactivity, and together, these basin-wide observations provided new insights into the flow of C and N through the UDOM reservoir.

This thesis has begun to interpret the information recorded in the chemical composition of DOM as it relates to the environmental processing of C and N in the ocean. New observations made in each chapter are the product of analytical approaches that combine a variety of environmental and DOM composition data over large spatial expanses in the ocean. This focus on producing a more complete chemical portrait of DOM through the simultaneous measurement of multiple parameters enabled this study to identify several processes that impart compositional variability to UDOM.