Phosphorus Dynamics at Station ALOHA

D. M. Karl, G. Tien and K. Yanagi

Abstract

The phosphorus (P) cycle in the oligotrophic waters of the North Pacific Ocean gyre is poorly understood and, therefore, largely ignored in ecological studies. Because selected procaryotic microorganisms (e.g., Trichodesmium) can fix N₂ and thereby relieve the system of "fixed-N" starvation, the availability of P or possibly some trace element (e.g., Fe or Mo) might ultimately control new production in the oligotrophic portions of the world ocean. A simple and straightforward method has been developed for the precise determination of nanomolar concentrations of soluble reactive phosphorus (SRP) dissolved in seawater. In contrast to the uniformly low and generally depth-invariant distributions of [NO₃+NO₂] in the upper 100 m of the water column at Station ALOHA, SRP vertical profiles are dynamic and variable, with mixed-layer concentrations ranging from < 30 nM to > 150 nM. Frequently, we observed near surface enrichments of SRP, features that we presently interpret to be the result of atmospheric deposition or subeuphotic zone scavenging by rising particles. During the 1991-1992 "quiescent" period, SRP pools approached our analytical detection level of approximately 1-2 nM. This efficient removal of SRP is entirely consistent with the hypothesized occurrence of N₂-fixation during this period (see abstracts by R. Letelier and by D. Karl et al.). The dissolved arsenic-to-SRP concentration ratios of > 10 during these N₂-fixation periods probably results in an, as yet, poorly quantified metabolic stress on the resident microbial populations. Throughout our observations at Station ALOHA dissolved organic P (DOP) is present at concentrations >= 3 times that of SRP. However, we know little about either the molecular structure or the bioavailability of the DOP pool. A research prospectus designed to improve our overall understanding of P-pool dynamics will be presented and discussed.