Partial characterization of the dissolved organic phosphorus pool in the oligotrophic North Pacific Ocean

David M. Karl¹ & Katsumi Yanagi²

¹Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822

²Faculty of International Study of Culture, Kyushu Sangyo University, Matsukadai 2-3-1, Higashiku, Fukuoka 813, Japan

Abstract

The combined application of two independent methods for phosphorus (P) determination in seawater, continuous-flow UV photodecomposition and magnesium-induced coprecipitation (MAGIC), was used to provide a partial characterization of the dissolved P pools at Sta. ALOHA (22 45'N, 158'W) in the oligotrophic subtropical North Pacific Ocean. Comprehensive laboratory analyses of the UV light-induced photodecomposition of a variety of specific organic P compounds dissolved in seawater confirmed that submicromolar concentrations of monophosphate compounds could be analytically separated from nucleotide di- and triphosphates based upon a previously described temperature-controlled low-pressure, mercury vapor UV irradiation treatment. When combined with a recently described high-precision P-detection system, the total dissolved phosphorus (TDP) in seawater could be reproducibly subdivided into three chemically distinct pools: soluble reactive phosphorus (SRP; presumably dominated by orthophosphate), UV-labile phosphorus (P_UV-L; containing primarily monophosphate esters), and UV-stable phosphorus (P_UV-S; containing primarily nucteotide di- and triphosphates, nucleic acids, and other compounds that are resistant to the UV treatment). Field application of these procedures to samples collected at Sta. ALOHA during the period September 1991 to March 1992 (HOT-30 to HOT-35) revealed the presence of all three operationally defined pools. In the upper portion of the water column (0-100 m) the TDP pool (29.26 ± 2.32 mmol P m^-2) contained, on average, 23% SRP, 26% P_UV-S, and 51% P_UV-L. With increasing depth, the concentration of P_UV-L decreased while that of the P_UV-S pool increased; the P_UV-L: P_UV-S ratios decreased from values of 2-5 in the upper water column to <= 1.0 at 200 m.