ABSTRACT

We examined the molecular weight distribution of the fluorescence intensity of FDOM_M and dissolved organic carbon (DOC) concentrations in the subtropical western North Pacific, in order to characterize photoreactivity of FDOM_M and understand the relationship between FDOM_M and DOC pool. DOC concentrations in a hydrophobic fraction (O-DOC),which is extracted by solid phase extraction, accounted for 34 and 38% of bulk organic carbon in the surface and deep layer. FDOM_M in a hydrophobic fraction (O-FDOM_M) accounted for 78 and 37% of FDOM_M in the surface and 1000-m depth seawater, respectively. Molecular weight distribution of O-FDOM_M and O-DOC showed that ratio of the fluorescence intensity to the DOC concentration of each MW fraction increased with depth. This suggests that the contribution of O-FDOM_M to O-DOC in the deeper seawater is higher than that in the surface layer. When deep-seawater samples were irradiated by natural sunlight for 5 days, the fluorescence intensity of O-FDOM_M decreased to almost half and its molecular weight shifted to lower one, whereas the O-DOC concentration did not decrease. It implies that a part of O-FDOM_M remains as photobleached FDOM_M (no-fluorescence DOM) after photobleaching.