Of estuarine humic substances

See, J. H., and Bronk, D. A. (2005). Changes in molecular weight distributions, C N ratios, and chemical strucmres of estuarine humic substances with respect to season and age. Mar. Chem. 97, 334-346. 

On the basis of few data from a limited range of geographical areas, it appears that humic substances in estuarine zones exhibit many attributes of the transitional nature of the environment. Aquatic humic substances show concentrations and chemical characteristics intermediate between those found in river and ocean waters, indicating relatively little in situ production, consumption, or chemical change. Sedimentary humic substance concentrations are somewhat higher than are usually found in the ocean, reflecting the high primary productivity and shallow water depths, but are chemically similar to either riverine or oceanic endmembers. The actual nature of estuarine humic substances is poorly known, but this problem is no worse than for humic substances from most environments. 

Although mechanisms of DOM removal by physical/chemical processes in the mixing zone of estuaries are not well understood, they are believed to be important processes affecting the composition of riverine/estuarine DOM. Some of the earliest work on DOM removal processes noted that iron is important in the initial steps of flocculating humic substances across an estuarine salinity gradient and that much of this humic material was composed of humic acids (Swanson and Palacas, 1965 Eckert and Sholkovitz, 1976 ... 

Sholkovitz, E. R., E. A. Boyle, and N. B. Price. 1978. The removal of dissolved humic substances and iron during estuarine mixing." Earth Planet. Sci. Lett. 40 130-136. 

Allusion has been made above to changes that occur in the humic substances introduced to estuaries by the riverine source. This section reviews the chemistry of these changes, and considers their effect on the delivery of riverine humic substances to the oceans. Because riverine humic substances derive from zones of low ionic strength, the rapid increase in both the types and concentration of dissolved salts upon estuarine mixing should have important effects on their ion-exchange properties, their conformations in solution, and their solubility. 

Aggregation of dissolved humic substances can also occur with particulate materials in the estuarine water column. Preston and Riley (1982) showed that the adsorption of riverine humic substances onto kaolinite, montmorillonite, and illite increased with increasing salinity and dissolved humic substance concentration. Adsorption increased in the order kaolinite < illite < montmorillonite, which they ascribed to increasing cation-exchange capacity of the clays. They found considerable quantitative differences between the extent of adsorption of riverine versus extracted sedimentary humic substances, indicating the importance of using materials of proper origin in experiments of this type. 

RETENTION OF RIVERINE HUMIC SUBSTANCES IN ESTUARINE SEDIMENTS... 

While the bulk of riverine humic substances pass through estuaries with little retention, it is clear that some terrigenous organic matter is retained in the sediments. However, neither the nature of the retained organic material nor its mechanism of delivery to the sediments is understood. Little, if any, metabolism of riverine humic substances appears to occur during the estuarine passage. 

Calculated equilibrium speciation of (a) mercury and (b) copper during estuarine mixing of hypothetical river water with seawater. Hum, humic substance. Note logarithmic scale on y-axis. Source. From Mantoura, R. F. C., et al. (1978). Estuarine and Coastal Marine Science 6, 387 08. 

Raspor, B., Nurnberg, H.W., Valenta, P. and Branica, M., 1984. Studies in seawater and lakewater on interactions of trace metals with humic substances isolated from marine and estuarine sediments. 11. Voltammetric investigations on trace metal complex formation in the dissolved phase. Mar. Chem., 15 231-249. 

Esham, E. C., Ye, W., and Moran, M. A. (2000). Identification and characterization of humic substances-degrading bacterial isolates from an estuarine environment. FEMS Microbiol. Ecology 34,103-111. 

Often the study of humic substances in estuaries has been undertaken because estuaries are more easily sampled than the open ocean, and not because of the unique aspects of estuaries per se. It is only in the past few years that systematic studies of humic materials, traversing the salinity gradient, have been carried out. Techniques used in most studies have been those borrowed from the classical fields of soil humus studies it seems likely that in time the techniques will evolve in response to the unique chemical processes that humic materials undergo in the estuarine zone. Quantitatively considered, the literature on humic materials in estuaries lacks the extent of the geographical or topical coverage of the soils literature. This paucity of data causes many conclusions drawn so far to be quite tentative and in need of corroboration. 

The isolation of humic substances from estuarine waters has usually been performed by acidification of the water sample followed by either filtration, to yield humic acids only, or adsorption onto a resin such as XAD-2, which appears to allow recovery of both the humic and fulvic acid fractions (Stuer-mer and Harvey, 1977a). Table 1 provides a listing of determinations of humic and fulvic acids in estuarine waters by a variety of investigators, along with the techniques employed. The concentrations of dissolved humic and fulvic acids found typically range from undetectable to less than 2 mg C/L, with most values in the tens of fig C/L for the humic acid fraction and hundreds of fig C/L for the fulvic fraction. Higher values are usually found at lower salinities. However, it must be stressed that few analyses are available most of these are from estuaries of the northeastern United States and a different isolation technique has been used in virtually every study cited. 

The ratios of humic to fulvic acids in estuarine and coastal sediments range from 0.4 to 3.4, the higher values being associated with areas or sediments having a terrestrial influence (Palacas et al., 1968 Brown et al., 1972 Hue and Durand, 1973 Pelet and Debyser, 1977 MacFarlane, 1978). These values are also consistent with those from other marine and terrestrial environments (Ishiwatari, 1966 Kononova, 1975 Stuermer et al., 1978 Cronin and Morris, 1982). Other parameters measured on coastal humic substances, such as elemental composition, spectral properties, organic components, stable isotope ratios, or C ages (Pelet and Debyser, 1977 Stuermer et al., 1978 Benoit et al., 1979 Nissenbaum, 1979) are consistent with terrestrial or marine humic compounds, or a mixture of these two endmembers. 

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