Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Hugh Ducklow


The research presented in this dissertation describes the influence of planktonic bacterial growth on upper ocean organic matter dynamics in the North Pacific Subtropical Gyre (NPSG). Examination of the temporal dynamics in dissolved organic matter (DOM) was coupled with investigations that targeted the influence of heterotrophic bacterial production (HBP) on organic matter fluxes in the NPSG. Nine cruises to the Hawaii Ocean Time-series field site Station ALOHA revealed that HBP accounted for a large flux of organic carbon in the upper ocean of the NPSG. HBP was significantly enhanced by sunlight, with photoenhancement of HBP accounting for 3.2 mol C m-2 yr-1, equivalent to 21% of the annual photoautotrophic production in this ecosystem. These observations suggest that HBP in the upper ocean of the oligotrophic NPSG exerts a large influence over organic matter fluxes in this ecosystem, and that a large fraction of HBP depends on sunlight. Several experiments were conducted to asses the response of heterotrophic protein production to irradiance at Station ALOHA. The results of these experiments revealed that HBP responded to irradiance similar to the response of photosynthesis to irradiance in this ecosystem. Upper ocean HBP increased with light intensity at low light fluxes (<200 mumol quanta m-2 s -1), but saturated or declined with increasing irradiance. Experiments conducted in the upper and lower photic zone revealed significant photoinhibition of bacterial production in the lower photic zone. Overall, the heterotrophic response was similar to the photosynthetic response, suggesting light-driven HBP could result from mixotrophic growth by the photoautotrophic unicellular cyanobacteria Prochlorococcus. Analyses of dissolved organic matter (DOM) inventories from 1988 to 1999 revealed multiyear increases in the inventories of dissolved organic carbon, nitrogen, and phosphorous (DOC, DON, and DOP) in the upper ocean of the NPSG. During the latter half of the observation period, rates of DOP accumulation declined, coincident with significant DOC and DON accumulation. Analyses of bacterial population dynamics between 1992 and 1999 revealed an apparent shift in the abundance of Prochlorococcus during the period of observation. These results suggest that prokaryote population structure directly influences the cycling of organic matter in this ecosystem.



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