MARINE CHEMISTRY, 45 (1994) 207-216

Dissolved organic carbon in oligotrophic waters: experiments on sample preservation, storage and analysis

Luis M. Tupas, Brian N. Popp, David M. Karl

(Received March 9, 1993; revision accepted September 8, 1993)

Different methods of preservation and storage of samples for analysis of abundance of dissolved organic carbon (DOC) in oligotrophic waters were evaluated and compared to shipboard measurements. DOC concentrations in samples stored frozen (-20C) in acid-cleaned polypropylene tubes, high-density polyethylene bottles, and combusted glass ampoules, even for extended periods (up to 5 months after collection), were indistinguishable from those measured on ship at the time of collection. Addition of phosphoric acid (0.025% H₃PO₄ final concentration in seawater) was necessary to preserve samples at 4C. Filtration prior to storage was not necessary for the oligotrophic ocean samples analyzed in this study. Removal of dissolved inorganic carbon can be accomplished by bubbling with either high-purity nitrogen or oxygen with no effects on DOC abundance measurements. An estimate of the analytical blank was determined by injecting distilled water which was exposed to high-intensity ultraviolet light, acidified and purged with nitrogen to remove inorganic carbon. The analytical blank measured in this study was 18.7 ± 1.5 µM C for a 100 ul injection volume. This value was applied as the minimum correction to DOC abundance measurements of seawater. Using the methods described in this paper we observed DOC concentrations of approximately 90-115 µM C for the upper 50 m of the water column at the US-JGOFS Station ALOHA (22 45'N, 158W). DOC concentrations decreased with depth to concentrations of approximately 50 µM C at 500 m and remained relatively constant at greater depths.