Relationship between the seasonal change in fluorescent dissolved organic matter and mixed layer depth in the subtropical western North Pacific

Spatial and temporal distributions of marine humic-like fluorescent dissolved organic matter (FDOMM) were determined in the subtropical western North Pacific to evaluate the controlling factors of FDOMM behaviors. The observations were conducted at 4 stations (15-30 degrees N) along 137 E in a subtropical area between January 2006 and April 2007. The florescence intensity of FDOMM was low (0.14-0.25 quinine sulfate units (QSU)) in the surface layer probably due to photodegradation, and increased with depth (0.90-1.10 QSU at 1000 m), irrespective of season and station. In the surface layer, the thickness of the water mass with low fluorescence intensity (<0.3 QSU) showed the seasonal change by being deeper in winter and shallower in summer, depending on the mixed layer depth (MLD). The average fluorescence intensity within the mixed layer also varied seasonally at midlatitudes; the intensity in summer was 40.8-53.8% of that in winter. Since the MLD was very much shallower in summer than in winter, FDOMM in the mixed layer would be kept within a shallow depth during the summer where intensive photodegradation could occur. The concentration of total organic carbon (TOC) was at its maximum at the water surface and decreased with depth, being adverse to FDOMM. Thus, the ratio of fluorescence intensity to TOC concentration was lowest (0.002-0.003) in surface water, which implies that FDOMM is not quantitatively important to the dissolved organic carbon pool. However, considering the possible difference in the stabilities of FDOMM against photochemical and microbial degradation, it is conceivable that photobleached FDOMM is one of the important organic groups constituting marine dissolved organic matter.