COMPOSITION AND CYCLING OF COLLOIDS IN MARINE ENVIRONMENTS

Colloidal (COM) or macromolecular organic matter makes up a significan t portion of the bulk dissolved organic matter (DOM) pool in aquatic e nvironments. Because of their high specific surface areas and complexa tion capacities, marine colloids are of great importance not only in t he global carbon cycle but also in the biogeochemical cycling of many particle-reactive nuclides and trace elements in the ocean. However, t he colloidal pool as a whole is still poorly understood and largely un characterized. Recently, cross-flow ultrafiltration and other separati on techniques, which have been successfully used to isolate marine col loids, combined with a multitracer approach, have greatly advanced our understanding of the cycling of COM and its associated trace elements in marine environments. In this paper we focus on recent developments on isotopic and elemental composition of colloids which allow organic matter cycling in marine environments to be constrained. Major sectio ns review sampling techniques for aquatic colloids, concentrations and distribution of COM, biochemical and elemental (organic and inorganic ) characterization, and stable isotopic (C-13 and N-15) and radioisoto pic (C-14 and Th-234) characterization of marine colloids. We discuss sources and turnover rates of organic matter in the ocean, importance of benthic boundary layer processes in the cycling of DOM, changes in the paradigms of marine organic matter cycling, and research needs for a better understanding of the biogeochemistry of marine colloids.