DISSOLVED ORGANIC-CARBON IN OLIGOTROPHIC WATERS - EXPERIMENTS ON SAMPLE PRESERVATION, STORAGE AND ANALYSIS

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 concentrati ons in samples stored frozen (-20-degrees-C) in acid-cleaned polypropy lene tubes, high-density polyethylene bottles, and combusted glass amp oules, even for extended periods (up to 5 months after collection), we re indistinguishable from those measured on ship at the time of collec tion. Addition of phosphoric acid (0.025% H3PO4 final concentration in seawater) was necessary to preserve samples at 4-degrees-C. Filtratio n prior to storage was not necessary for the oligotrophic ocean sample s analyzed in this study. Removal of dissolved inorganic carbon can be accomplished by bubbling with either high-purity nitrogen or oxygen w ith no effects on DOC abundance measurements. An estimate of the analy tical blank was determined by injecting distilled water which was expo sed to high-intensity ultraviolet light, acidified and purged with nit rogen to remove inorganic carbon. The analytical blank measured in thi s study was 18.7 +/- 1.5 mum C for a 100 mul injection volume. This va lue was applied as the minimum correction to DOC abundance measurement s of seawater. Using the methods described in this paper we observed D OC concentrations of approximately 90-115 muM C for the upper 50 m of the water column at the US-JGOFS Station ALOHA (22-degrees-45'N, 158-d egrees-W). DOC concentrations decreased with depth to concentrations o f approximately 50 muM C at 500 m and remained relatively constant at greater depths.