Measurements of DOC and DON in the Southern California Bight using oxidation by high temperature combustion

Abstract

Water column profiles of DOC and DON were determined using the high-temperature catalytic oxidation method of Suzuki et al. (Deep-Sea Research, 39, 185–198, 1992) during three cruises to Santa Monica Basin in the Southern California Bight. DOC concentrations ranged from a minimum of 62 μM at 600 m to a maximum of 103 μM at 30 m. Most near-surface concentrations were in the range of 90–98 μM. These concentrations were only 20–30 μM greater than previous estimates using wet oxidation techniques, the difference perhaps in part due to an unquantified system blank. DON concentrations ranged from 1.0 to 9.6 μM with little evidence of a gradient with depth. This concentration range overlaps existing estimates for the region using photo-oxidation methods. Mean atomic C:N ratios in the dissolved organic pool for the entire water column ranged from 16 to 38, the higher value likely an overestimate due to anomalously low DON values at depth in July. There was no apparent trend in the ratio with depth. During a diel study in July, changes in the concentrations of DOC in the surface 50 m occurred mostly within the precision of the analysis. DON concentrations did not vary above the nutricline but large changes occurred at the top of the nutricline. Significantly, these changes did not covary with changes in inorganic nitrogen, implying a non-conservative nature for DON in that portion of the water column. No correlation was evident between DOC and AOU concentrations, but a correlation did exist between salinity and AOU, suggesting that mixing is the dominant mechanism controlling the distribution of AOU in Santa Monica Basin. These results do not support extending the conclusions of Suzuki et al. (Marine Chemistry, 16, 83–97, 1985) and Sugimura and Suzuki ( Marine Chemistry, 24, 105–131, 1988) (i.e. DOC concentrations 2–3 times higher than previously estimated, strong correlation between DOC and AOU, near-surface TDN concentrations of ∼40 μM) to the Southern California Bight.