Tracing DOM in the ocean with UV-visible spectroscopy

A fraction of dissolved organic matter (DOM) absorbs and fluoresces ultraviolet and visible light and is defined as colored DOM (CDOM) and fluorescent DOM (FDOM), respectively. These optically active compounds play important roles in marine elemental (carbon, nitrogen, phosphorus, and trace element) cycles and in regulating the underwater light environment. Although the chemical structure is not well constrained, CDOM/FDOM has been recognized as a substrate for heterotrophic microbes, a component of recalcitrant DOM, and a good tracer of water mass. In this chapter, the chemistry of CDOM and FDOM is firstly summarized, focusing on the chemical/molecular properties that contribute to the absorbance and fluorescence spectra of marine DOM (Section 4.2). A series of sources and sinks of marine CDOM/FDOM are defined and detailed (Section 4.3). The global distributions of CDOM/FDOM quantity and quality are addressed, and major factors shaping the distributions are identified (Section 4.4). The chapter briefly concludes with our recommendation regarding terminology and future research involving CDOM and FDOM as a tracer of physical, chemical, and biological processes in the ocean (Section 4.5).