We examined the molecular weight distribution of the fluorescence intensity of FDOMM and dissolved organic carbon (DOC) concentrations in the subtropical western North Pacific, in order to characterize photoreactivity of FDOMM and understand the relationship between FDOMM 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. FDOMM in a hydrophobic fraction (O-FDOMM) accounted for 78 and 37% of FDOMM in the surface and 1000-m depth seawater, respectively. Molecular weight distribution of O-FDOMM 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-FDOMM 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-FDOMM 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-FDOMM remains as photobleached FDOMM (no-fluorescence DOM) after photobleaching.
2015 by The Geochemical Society of Japan