Dissolved Organic Carbon Dynamics in the Upper Water Column at Station ALOHA

Luis Tupas, Georgia Tien, Dale Hebel and David Karl

School of Ocean and Earth Science and Technology, University qf Hawaii, Honolulu, HI 96822

Abstract

The concentrations of dissolved organic carbon (DOC) at discrete depths in the upper 1000 m of the water column at Station ALOHA were measured using a high temperature combustion oxidation (HTCO) method on cruises from 1993 to 1995. Large variations in the monthly depth profiles were observed with near-surface values ranging from 65-120 µmol C kg^-1, decreasing to 45-65 µmol C kg^-1 at 250 m and about 40 µmol C kg^-1 below 500 m. A large gradient in DOC concentration in the upper 0-100 m was observed even when measurements of temperature and potential density showed a well-mixed upper water column. These results suggest contemporaneous production of DOC, most likely via photosynthetic processes. These HTCO concentrations are substantially higher than measurements of DOC made prior to 1993 using the persulphate wet-oxidation method. DOC variability within a cruise was also observed and this may be indicative of movement of various water masses through Station ALOHA or variability resulting from other mesoscale processes. In addition to these relatively high frequency variations, we have also observed low frequency (months to years) secular changes in DOC inventories. Depth-integrated values of DOC show an increase of about 0.43 µmol C m^-2 yr^-1 from 0-200 m but a decrease of about 1.5 µmol C m^-2 yr^-1 from 200-1000 m from 1993-1995. The results of molecular size fractionation showed that 60-70% of the DOC was less than 1000 Dalton, regardless of depth of the water sample. Measurements of dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) showed that the upper water column was depleted in phosphorus more than nitrogen and implies an uncoupling between the processes of the formation and utilization of dissolved organic matter. No significant relationship, however, was found between DOC and measurements of particulate biomass such as suspended particulate carbon and bacterial number.