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

Abstract

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 O-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.