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

摘要

The vertical change in the composition of marine humic-like fluorescent dissolved organic matter (FDOM_M) 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 FDOM_M (O-FDOM_M) 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 FDOM_M and O-FDOM_M, the contribution of O-FDOM_M to bulk FDOM_M decreased with depth. A sunlight irradiation experiment of deep seawater revealed that the fluorescence intensity of O-FDOM_M as well as bulk FDOM_M tended to decrease, whereas the contribution of O-FDOM_M to FDOM_M 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 0-FDOM_M to FDOM_M. Molecular weight (MW) distributions of O-FDOM_M were measured by high-performance size exclusion chromatography. O-FDOM_M 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-FDOM_M. The present study strongly suggests that FDOM_M is composed of some fractions with different photoreactivity, and that photoirradiation is the main factor controlling the compositional change in FDOM_M with depth. The fate of each fraction of FDOM_M after photoirradiation is presumably one of the key processes in the biogeochemical cycle of marine DOM.