My dissertation research described the influence of planktonic bacteria on upper ocean organic matter dynamics in the North Pacific Subtropical Gyre (NPSG). Analyses of dissolved organic matter (DOM) temporal dynamics were coupled to studies investigating the influence of heterotrophic bacterial production (HBP) on upper ocean organic carbon 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 on organic matter dynamics 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. Upper ocean HBP increased with light intensity at low light fluxes (<0.200 mmol 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 DOM inventories between 1988 and 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, the rate of DOP accumulation declined, coincident with significant accumulations of DOC and DON. Analyses of bacterial population dynamics between 1992 and 1999 revealed an apparent shift in the abundance of Prochlorococcus during the period of observation. Increasing abundance of Prochlorococcus coincident with accumulated inventories of DOM suggests that prokaryote population structure directly influences the cycling of organic matter in this ecosystem.