2. Dissolved Organic Matter

The ocean is a salty and very diluted broth of organic matter that contains about 680 Pg C. More than 97% of this organic matter is in the dissolved form. This huge amount of reduced carbon, comparable to the CO₂ accumulated in the atmosphere, results in an average concentration of 0.48 ppm (or 40.3 μmol kg⁻¹) when divided by the world ocean volume. Comparatively, salts add up to about 35,000 ppm. Low concentrations, extreme complexity of an organic matrix formed by myriads of compounds and vast interferences with the salt matrix are the main challenges associated with the accurate determination of dissolved organic matter (DOM) in the marine environment. In this chapter we aim to provide a useful overview of the procedures to effectively sample (“clean lab” protocols), process (filtration, ultrafiltration, solid-phase extraction), preserve (acidification, freezing, freeze-drying) and analyse the bulk DOM pool. Analytical methods for the characterization of its elemental (C, N, P), optical (absorption and fluorescence spectroscopy) and emerging molecular property (FT-ICR-MS, NMR) composition will be described. Out of the scope of this chapter is the determination of individual compounds (e.g. amino acids, carbohydrates) as well as the isotope characterization of DOM.