The Isolation and Partial Characterization of the Dissolved Organic Matter in Seawater

The significance of dissolved organic matter (DOM) in the biological and chemical processes of the sea has been a much debated subject. Isolation of the trace amount of DOM in large volumes of seawater would permit further characterization of its pertinent chemical properties and an assessment of its role in marine processes, such as the speciation of trace metals, A variety of techniques have been used to isolate as large a representative portion of DOM as possible without alteration or contamination. A number of DOM samples have been isolated using adsorption and filtration techniques and partially characterized, but they have generally been limited in number or geographical distribution. In this study, DOM was adsorbed onto activated charcoal and then extracted in three different fractions with a series of solvents. Performance of the procedure was followed by analysis for dissolved organic carbon (DOC). This procedure recovered about 50% of oceanic DOC, 23-63% of estuarine DOC, and 18-60% of other types of natural DOC. The representativeness of charcoal-isolated organic matter (OM) was determined for several terrigenous humic and humic-like substances by analyses before and after isolation, Variations of about 20% occurred in spectroscopic characteristics and copper associati.ng capacities following isolation. Highly variable recoveries of estuarine DOM and low recoveries of the recently generated DOM in laboratory experiments suggests that a disproportionate portion of DOM in biologically dynamic environments may not be recovered by activated charcoal. In addition to analyses of spectroscopic characteristics, apparent molecular weights, and stable carbon isotope compositions, the capacity of isolated OM to interact with copper was determined for each fractionated sample. This was done by determining the ability of an OM solution to increase the solubility of copper sulfide. DOM isolated from a wide variety of oceanic environments (intermediate depths, high and low productivity surface waters) displayed a generally narrow range of characteristics, though OM from different water masses could be distinguished on the basis of spectroscopic and isotopic characteristics. The isolated copper capacity per liter of seawater (mg isolated organic carbon x capacity/mg OC) was determined as much by the amount of isolated organic carbon (OC) as by the capacity per mg of that OC. Both factors apparently combined to increase the isolated capacity of seawater advected away from the Peruvian coastal upwelling area. The quality and quantity of OM isolated from estuarine seawater over the period of a diatom bloom varied little. The nature of isolated estuarine OM can be considered, to a first approximation, to result from the conservative mixing of six parts seawater and one part freshwater DOM, The impact of highly variable biological activity was limited to a brief increase in the absorptivity and an alteration of the stable carbon isotope composition immediately following the bloom crash. Isolated oceanic OM had absorptivities about 10 times lower than those of isolated estuarine OM and did not exhibit the pattern of 13C depletion found in isolated river water OM. It was not amenable to derivatization and identification using common techniques successfully applied to terrigenous OM. Typical oceanic material did resemble estuarine OM in isolated copper capacity per mg OC (only about 25% lower), but its isolated capacity per liter of seawater 0as less than half that of estuarine seawater