Humic Substances as a Natural Factor Lowering Ecological Risk in Estuaries

Humic substances are considered to be a necessary component of terrestrial and aquatic ecosystems and the most persistent part of the decomposing organic matter. In marine environments about 90% of the dissolved carbon, as well as a major part of the sedimentary organic carbon, is humic matter. In the salt marsh estuaries, in addition to the humic matter brought about by rivers, a predominant autochthonous vascular plant Spartina alterniflora serves as a source of humic substances. Estuarine humic substances consist of C, N, and O, containing functional groups that can interact with inorganic elements and organic compounds. In this way humic substances apparently affect the bioavailability and toxicity of different pollutants. Although they could be used and structurally transformed by natural assemblages of microorganisms in some way, a considerable part of estuarine humic substances persist, apparently because of their adsorption on mineral components of estuarine sediments such as clay minerals. Due to their specific characteristics, including natural longevity in ecosystems, humic substances represent a natural factor apparently lowering ecological risk in salt marsh estuaries.