Dissolved organic carbon measurements at Station ALOHA measured by high temperature catalytic oxidation: Characteristics and variation in the water column

L.M. Tupas, B.N. Popp, D.V. Hebel, G. Tien and D.M. Karl

Abstract

High temperature catalytic oxidation was used to analyze the dissolved organic carbon (DOC) content of water column samples from the US-JGOFS Station ALOHA. Shipboard measurements of freshly collected samples and laboratory analysis of frozen preserved samples showed no significant difference. Oligotrophic seawater samples for DOC analysis were found to be best preserved unmodified and frozen and were stable for at least one year if stored properly. Monthly measurements of DOC at Station ALOHA showed a large variation in surface waters, ranging from 80 to 125 µM carbon. DOC content decreased from these surface values to about 65 µM carbon at 250 m depth even through what is considered to be the "mixed layer." Temporal variation of DOC in waters below 250 m is relatively small. Mesoscale variations in DOC content of surface waters were also observed with almost the same magnitude as monthly observations. Integrated values of DOC (0-45 m) showed a positive correlation with dissolved organic nitrogen content but not with dissolved organic phosphorus. Molecular size fractionation of seawater samples from 0 to 1000 m depth showed that up to 70% of the DOC in the water column was less than 1000 daltons, 15% was between 1000 and 10,000 daltons, 3% between 10,000 and 30,000 and the remainder greater than 30,000 daltons.