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The elemental stoichiometry of dissolved and particulate matter in the sea, especially the nitrogen-tophosphorus ratio, is an important parameter for studies of the nutrient control of plankton growth and for modeling biogeochemical processes, including carbon sequestration. Nitrogen (N) and phosphorus (P) pools have been measured on approximately monthly intervals for a 9-yr period at a deep-ocean station in the North Paci"c subtropical gyre (Sta. ALOHA; 22345N, 1583W). These data sets reveal complex interactions between N and P pools, and several unexpected secular trends. Models based on steady-state assumptions will not capture these temporal variations, especially the apparently rapid response of the microbial assemblages to stochastic nutrient intrusion events and the time-varying (seasonal, interannual and decadal scale) changes in dissolved matter N : P ratios. Based on an analysis of these data, we hypothesize that the gyre is presently in a period of net "xed N sequestration and P control of plankton rate processes.

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Algal chlorophyll and carotenoid distributions were measured periodically in the euphotic zone of Sta. ALOHA (22"45'N, 158"OO'W) between February 1989 and October 1991 to document the variability in phytoplankton abundance and composition. The annual mean depth-integrated (O-200 db) concentration of Chl a displayed significant interannual variability. Seasonal patterns in Chl a concentration were found to be depth-dependent. Elevated Chl a in the mixed layer is the result of photoadaptation as the mixed layer deepens in winter. Increases in Chl a at the deep chlorophyll maximum layer (DCML) in spring are explained by increased nutrient availability caused by a deepening of the DCML relative to the bg = 24.25 density surface.An algorithm based on the ratios of Chl a to diagnostic pigments present in specific algal taxa was used to estimate the contribution to total Chl a by the major algal groups represented within the DCML. Results indicate the presence of a phytoplankton community at the DCML with the following mean composition: Prochlorococcus spp. (39%), cyanobacteria (24%), prymnesiophytes (22%),and chrysophytes (13%). No single taxon is responsible for the springtime increase in Chl a observed in this habitat. Results from size fractionation and normal-phase high performance liquid chromatography confirm that Prochforococcus spp. are the principal contributors of Chl a to the DCML.

Based on a long-term set of observations and measurements at a station in the subtropical North Pacific Ocean, it now appears that contemporaneous rates of primary production in low-nutrient open ocean regions and perhaps in the ocean as a whole may be greater than had been considered in field studies conducted in previous decades. Data collected at the Hawaii Ocean Time-Series (HOT) Station ALOHA from October 1988 to July 1997 indicate that daytime particulate organic carbon (POC) production, based on 12-h 14C in situ incubations, averages 472 mg C m-2 d-l (SD = 125 mg C m-2 d-l; n = 70). This carbon production rate is two-to three-fold greater than most of the pre-1980 estimates. We present evidence that particulate production rates may have been overestimated by up to 30% as a result of 14C-labeled dissolved organic carbon (14C-DOC) adsorption onto glass fiber filters. More importantly, when one considers the 14C-DOC that is produced but not adsorbed onto the filters, gross primary production rates (14C-POC plus 14C-DOC) in the subtropical North Pacific Ocean may approach 1 g C m-2 d-l. We hypothesize that the large flux of 14C-DOC may be a manifestation of decade-scale habitat changes resulting from variations in climate. The balance between POC and DOC production will ultimately influence the structure of the food web, especially the interactions of phytoplankton and heterotrophic bacterial populations, and the mechanisms and rates of carbon sequestration by the biological pump.