Coupled Intensification of Nitrogen and Phosphorus Cycles in the Subtropical North Pacific Ocean

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This project seeks fundamental information on the interactions between nitrogen (N) and phosphorus (P) in the oligotrophic North Pacific Ocean, and their relationships to rates and mechanisms of ecosystem production and export. The research is guided by the following general hypothesis: "Climate-related shifts in the structure of the epipelagic habitat have selected for N2-fixing bacteria, resulting in a coupled intensification of N and P cycling rates which has forced the system from N- to P-limitation with numerous ecological and biogeochemical consequences. This hypothesized shift from N- to P-limitation has affected community structure, rates of gross primary production and export, and the elemental (C, N, P) stoichiometry of dissolved and particulate matter pools. The investigators will conduct a series of field and laboratory experiments designed to evaluate several ecological predictions of this coupled intensification model of nutrient dynamics in the North Pacific Subtropical Gyre with a primary focus on P pool dynamics. The field research will be conducted at Sta. ALOHA (22 degrees 45'N, 158 degrees W) and will complement the ongoing Hawaii Ocean Time-series (HOT) core measurement program. These efforts will focus on: (1) dissolved inorganic P uptake and remineralization rates, (2) dissolved organic P pool production rates, chemical characterizations and bioavailability and (3) physiological and biochemical assessments of Trichodesmium and Prochlorococcus. Knowledge derived from this project will be used to refine our understanding of the oligotrophic North Pacific ecosystem and to predict the response of oceanic habitats to environmental variability and global climate change.