Graduation date: 2007Experiments were designed to answer key questions about diatom-derived organic matter cycling (i.e., production and degradation) in coastal systems. Dissolved organic matter (DOM) production was examined in axenic batch cultures of five diatom species. Dissolved organic carbon (DOC) release rates varied between species, but were significantly higher for all species in exponential versus stationary growth. The DOM produced by several of the species adhered to filters and was measured as POM when cells were separated from the medium by fractionation. Thus, field measurements of POM and DOM may be biased by species-specific differences in the quality of the DOM. Experiments were also conducted to examine particulate organic matter (POM) and DOM degradation by coastal microbial assemblages. In each experiment, POM and DOM concentrations were elevated above those found in recently upwelled waters, indicative of bloom activity. Reductions in bacterial grazers resulted in significant increases in bacterial biomass in these experiments, suggesting that grazing is the primary control on bacteria. However, on two dates, there was no significant net loss of bacteria in treatments with grazers, indicating a close coupling between bacterial growth and mortality. DOC degradation was initially rapid and then slowed. After 3 d, 47 % of the DOC was degraded and reductions in grazing or nutrient additions did not significantly enhance the degradation. On average, 33 % of the POC was degraded after 3 d, although some portion was converted to DOC and not respired. Despite the DOC and POC degradation, there was little evidence of DON and PON degradation. These results suggest that at least half of the diatom-derived DOC is rapidly degraded, but that there is also a less labile fraction that may accumulate if retained in coastal waters. Additionally, the slow degradation time of the less labile fraction relative to potential offshelf transport mechanisms suggests that Oregon’s coastal waters may be a significant source of organic matter to adjacent offshore waters. Finally, because of its lability, the POC will contribute to shelf bottom water oxygen utilization as it decays.