Estuaries, humic substances

Mayer, L.M., Schick, L.L., Loder, T.C. 1999. Dissolved protein fluorescence in two main estuaries. Marine Chemistry, 64, 171-179. Miano, T.M., Senesi N., 1992. Synchronous excitation fluorescence spectroscopy applied to soil humic substances chemistry. Sci Total Environ, 117/118, 41-51. 

Bushaw-Newton and Humic substances (estuary) Natural Bacterial cell number 0% and +41%... 

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 ... 

Another component of total DOM that has received considerable attention in recent years in estuaries, and particularly in shelf waters, is (CDOM) (see review, Blough and Del Vecchio, 2002). CDOM has been commonly referred to as Gelbstoff, gilvin, and yellow substance, because of its strong association with humic substances. In fact, CDOM is... 

Allochthonous DON sources from terrestrial runoff, plant detritus leaching, soil leaching, sediments, and atmospheric deposition may also represent important inputs to estuaries (Berman and Bronk, 2003). DON typically represents about 60 to 69% of the TDN in rivers and estuaries (Berman and Bronk, 2003). The major components of DON include urea, dissolved combined amino acids (DCAA), DFAA, proteins, nucleic acids, amino sugars, and humic substances (Berman and Bronk, 2003). However, less than 20% of DON is chemically characterized. 

Alberts, J.J., Filip, Z., Price, M.T., Hedges, J.I., and Jacobsen, T.R. (1992) CuO-oxidation products, acid hydrolysable monosaccharides and amino acids of humic substances occurring in a salt marsh estuary. Qrg. Geochem. 18, 171-180. 

Alberts, J.J., and Takacs, M. (1999) Importance of humic substances for carbon and nitrogen transport into southeastern United States estuaries. Qrg. Geochem. 30, 385-395. 

In comparing these different environments, one also becomes aware of the tremendous range in environmental conditions (particulate versus dissolved, oxic versus anoxic, etc.) and in the length of time humic substances remain in an environment (hundreds of years in soil or deep aquifers compared to only weeks or months in the surface waters of lakes, streams, and estuaries). One must be equally aware of the fact that, in reality, the lithosphere is not compartmentalized into the convenient categories represented by the various chapters in this section of the book. Rather, each environment is part of an overall system where each overlaps and interacts with neighboring environments. This interaction is dramatized in the case of estuaries where the transition from a freshwater environment to a marine environment is pronounced. 

The focus of this chapter is on the geochemistry of stream humic substances and should provide the reader with an appreciation of the dynamics, importance, and uniqueness of streams within the hydrologic system. Streams should not be envisioned only as arteries connecting lake, ground water, and soil environments (which are considered in previous chapters) with estuaries and oceans (which are presented in following chapters), nor as integrators of humic substances from upgradient, but rather they should be viewed as a different and unique aquatic environment where stream humic substances also have a dififerent and unique character. 

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. 

Nevertheless, it seems reasonable to infer that aquatic humic substance concentrations in estuaries are intermediate between those of rivers and those of the open ocean, as would be expected from a mixture of river water and seawater. The concentrations of humic acids at intermediate salinities do not correspond, in a linear fashion, to the relative proportions of river water and seawater (Fig. 2), primarily because of a removal from solution of a portion of the riverine contribution. This behavior is in contrast to that of total dissolved organic carbon (DOC) which generally does show linear mixing lines when plotted versus salinity (Sholkovitz et al., 1978 Moore et al., 1979 Laane, 1980 Fox, 1983a Mantoura and Woodward, 1983). 

The chemical character of dissolved humic substances in estuaries also reflects its mixed origin, with a number of parameters exhibiting values intermediate between the riverine and oceanic endmembers. These properties include UV-visible absorptivities, C/N ratios, total acidity, molecular weight (Preston, 1979), amino acid content (Fox, 1981), and perhaps C/ C... 

TABLE 1. Concentrations of Aquatic Humic Substances in Estuaries"... 

Estuary Isolation Method Humic Substance Concentrations (mg C/L) DOC (%) Reference... 

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