The "Forgotten" Open Ocean P-Cycle

David Karl, Georgia Tien, Karin Björkman, Katsumi Yanagi, Ricardo Letelier, Albert Colman and Angie Thomson

School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822

Abstract

Nutrient dynamics in open ocean ecosystems have traditionally focused on nitrogen (N) as the presumptive production rate limiting macronutrient. Only in recent years have the ecological roles of trace elements (e.g., Fe and Zn) and phosphorus (P) been given serious consideration. The coincident bioavailability of P and certain trace elements is required to support significant rates of dinitrogen (N₂) fixation, a microbiological process that can cause an alternation from N to P limitation with numerous attendant biogeochemical consequences. Despite this potential role of P, few studies in open ocean ecosystems have focused on pool inventories or dynamics.

Since 1988 we have made observations and conducted experiments collectively designed to improve our understanding of the role of P in the productivity of the subtropical North Pacific Ocean. These studies include: (1) the laboratory development and field testing of a new high-sensitivity assay system for soluble reactive phosphorus (SRP) concentrations in seawater (MAGIC), (2) a 7-year time-series of measurements of dissolved (SRP and total dissolved P [TDP]) and particulate P pools, and particulate P export rates, (3) the direct measurement of the bioavailability of selected dissolved organic and inorganic compounds using an isotope dilution radiotracer technique, (4) partial chemical characterization of the dissolved organic P (DOP) pool using a new, low-power UV-photosensitivity analytical procedure, (5) large volume collections (> 1000 l) of inorganic and organic P compounds for natural radiogenic (³²P/³³P) and ³¹P-NMR chemical characterizations, (6) estimates of upward P flux by vertically migrating populations of Trichodesmium, (7) direct measurements of rates of phosphate uptake and DOP production using regeneration ³²PO₄ radiotracer procedures and (8) laboratory development and field application of a novel technique designed to provide direct measurements of PO₄ (as opposed to SRP measurement) and DOP (as opposed to [TDP- SRP] measurement) in seawater. Other process-oriented studies (e.g., measurements of activities of selected enzymes involved in P transformations, organic P photohydrolysis in nature, atmospheric P deposition rates and potential role of volcanogenic P flux) are also planned. While this is largely a progress report in an ongoing investigation, our results to date have already provided us with a revised view of P dynamics in the sea. Foremost among our interim conclusions is the observation, supported by several independent lines of evidence, that contemporary productivity is both P-limited and non steady-state.