Vertical change in the composition of marine humic-like fluorescent dissolved organic matter in the subtropical western North Pacific and its relation to photoreactivity

The vertical change in the composition of marine humic-like fluorescent dissolved organic matter (FDOMM) and its controlling factors were examined in the subtropical western North Pacific. Fluorescence intensity and molecular weight distribution were measured on the hydrophobic fraction of FDOMM (O-FDOMM) fractionated with ODS solid phase extraction. Although the same vertical trends with low fluorescence intensity in surface waters and high in deep waters were observed both for bulk FDOMM and O-FDOMM, the contribution of O-FDOMM to bulk FDOMM decreased with depth. A sunlight irradiation experiment of deep seawater revealed that the fluorescence intensity of O-FDOMM as well as bulk FDOMM tended to decrease, whereas the contribution of O-FDOMM to FDOMM increased from 42% to 62% throughout the irradiation experiment. This suggests that the hydrophobic fraction is more refractory to photoirradiation than a hydrophilic one and that the difference of photo-lability results in the vertical change of the contribution of O-FDOMM to FDOMM. Molecular weight (MW) distributions of O-FDOMM were measured by high-performance size exclusion chromatography. O-FDOMM was composed of three MW fractions throughout the samples examined and the compositional change was noted with an increase in the high MW fraction with depth. The photoirradiation experiment of deep seawater sample clearly showed that a high MW fraction was more susceptible than a low MW fraction of O-FDOMM. The present study strongly suggests that FDOMM is composed of some fractions with different photoreactivity, and that photoirradiation is the main factor controlling the compositional change in FDOMM with depth. The fate of each fraction of FDOMM after photoirradiation is presumably one of the key processes in the biogeochemical cycle of marine DOM.