Biogeochemistry of phosphorus in the North Pacific Subtropical Gyre

D. M. Karl

Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822

Abstract

Biosystems are built around phosphorus (P). It is a vital macronutrient that is used for energy transduction (ATP), information storage (DNA), metabolic regulation (RNA) and other key cellular functions. P-deficiency in microorganisms leads to predictable, coordinated changes in metabolism, biosynthesis and triggers specific genetic strategies for species survival. Ecosystems are also built around P. It is a vital macronutrient that controls primary production, community metabolism, N₂ fixation and carbon sequestration. P-deficiency in ecosystems leads to predictable, coordinated changes in metabolism, biosynthesis and triggers specific natural selection processes for biome survival. These anticipated changes, from genomes to biomes, in response to fluctuations in oceanic P availability emphasize the key role that P exerts on plankton community structure and function and, ultimately, the habitability of our planet. Despite the well-recognized importance of P in cellular and ecosystem processes, comprehensive P-cycle investigations in the world ocean are relatively rare. The Hawaii Ocean Time-series (HOT) program was established in October 1988 as an oceanic outpost for studies of ocean biogeochemistry, including P dynamics. During the past two decades systematic measurements have been made of dissolved inorganic (DIP) and organic P (DOP) pool inventories, particulate P pool concentrations and C-N-P composition, partial chemical characterization of the DOP pool, P-uptake and assimilation, P-export and mesopelagic zone remineralization, and the distribution of key P regulatory genes to name a few key parameters and processes. This field-based research effort has been fully integrated into the more conventional C and N cycles, and conducted with consideration of contemporary physical and climate forcing of this habitat. Selected results include: (1) detection of seasonal changes in concentrations and in the C:N:P stoichiometries of dissolved and particulate matter pools, (2) detection of a sustained 17-year drawdown of soluble reactive P and particulate P inventories, both consistent with enhanced new production by N₂ fixation, (3) systematic increases in the C:P and N:P ratios of exported organic matter that are also consistent with the hypothesized decadal-scale enhancement of N₂ fixation, (4) detection of planktonic resource partitioning between DIP and DOP, and (5) detection of active phosphonate cycling in the near-surface waters. Taken together, these experimental observations and long-term data sets confirm that P is the staff of life at Station ALOHA.