As aquatic scientists have recognized the diversity of processes controlled by or dependent upon aquatic humus, it has become important to learn more about the genesis, chemical properties, and concentration of humic substances in aquatic ecosystems. There are three classes of aquatic humus (fulvic acids, humic acids, and humin), all of which share the characteristics of being heterogeneous biomolecules which are yellow to brown or black in color, high to moderate molecular weight, and biologically recalcitrant. Fulvic acids are organic acids which are soluble at any pH; humic acids are soluble above pH 2; and humin is insoluble under the full range of pH. Aquatic humus occurs in both dissolved and solid phases, with molecular weights ranging from about 500 D for dissolved fulvic acid to greater than 100,000 D for humic acids in sediments. Although the heterogeneity of these humic fractions makes rigorous chemical studies challenging, there are sufficient analytical methods at hand to make progress toward understanding the sources, formation pathways, and fate of aquatic humus.
