Fluorescence as a possible tool for studying the nature and water column distribution of DOC components

Seawater samples from the Hawaiian HOTS station (used in the DOC/DON intercalibration exercise), and from a depth profile from the Sargasso Sea were studied by fluorescence spectroscopy using excitation-emission matrices based on corrected spectra. Our results suggest that marine dissolved organic matter (DOM) fluorescence can be categorized into two distinct groups of fluorescing compounds. One group is distinguished by a protein-type fluorescence (excitation about 220 and 270 nm and emission about 300–350 nm) and the other group is characterized by a humic-type fluorescence (excitation about 230 and 310 nm and emission about 420–450 nm). Chemical evidence supports the interpretation that proteins and humic substances are mainly responsible for these two fluorescence types in seawater. At both the Atlantic and Pacific stations, protein-type fluorescence was dominant in the upper water column, while humic-type fluorescence was dominant in the deeper samples. These results suggest that fluorescence can be used to study the nature and distribution of two major DOM fractions, i.e. recent biochemical versus old humic substances, in the open ocean. The relevance of our results to the controversial 'new' or 'missing' DOC, as revealed by high temperature combustion oxidation DOC analyzers, is discussed.