Autochthonous sources

Recent investigations provide new insight on the structural chemistry of dissolved organic matter (DOM) in freshwater environments and the role of these structures in contaminant binding. Molecular models of DOM derived from allochthonous and autochthonous sources show that short-chain, branched, and alicyclic structures are terminated by carboxyl or methyl groups in DOM from both sources. Allochthonous DOM, however, had aromatic structures indicative of tannin and lignin residues, whereas the autochthonous DOM was characterized by aliphatic alicyclic structures indicative of lipid hydrocarbons as the source. DOM isolated from different morphoclimatic regions had minor structural differences. 

Figure 1. Allochthonous and autochthonous sources of dissolved organic carbon in natural waters. (Reproduced with permission from reference 1. Copyright 1985 Kluwer Academic Publishers.)...

DOM is derived from autochthonous sources such as phytoplankton and photosynthetic bacteria (16) at Big Soda Lake near Fallon, Nevada. This lake is alkaline (pH 9.7) and chemically stratified. It contains DOC concentrations as high as 60 mg/L and dissolved salt concentrations as high as 88,000 mg/ L (17). The DOM in this lake is colorless. The fulvic acid fraction was isolated by adsorption chromatography (Amberlite XAD-8 resin) (18) and by zeo-trophic distillation of water from N,N-dimethylformamide (19). Average molecular model synthesis was achieved in a manner similar to that used for fulvic acid from the Suwannee River. The characterization data are presented in Table I and the structural model is presented in Structure 2. 

The clear differentiation of allochthonous from autochthonous sources of DOM in integrating freshwater environments is not presently possible. 

Supply of Dissolved Organic Matter to Aquatic Ecosystems Autochthonous Sources... 

In this chapter, we review and discuss the role of autochthonous sources of DOM in surface waters. Our focus is on the input of DOM from algae and macrophytes within aquatic ecosystems and largely confined to the major pools of DOM potentially available for use as substrates by heterotrophic bacteria. Hence, we will not consider phytoplankton/macrophyte release of specific volatile organics, vitamins, antibiotics, toxic compounds, or enzymes, unless they fall within the scope stated above. 

For lakes, relationships between DOC concentration and some measure of the relative size of the drainage area (e.g., drainage area/lake area ratio) and between DOC concentration and water residence time can be used to estimate the importance of autochthonous DOC by extrapolation. In the study of Experimental Lakes Area lakes by Curtis and Schindler (1997), the y intercept of the DOC concentration versus catchment/lake area ratio (the DOC value at minimal catchment area) is approximately l-2mgL 1. In the same study, the asymptote of the DOC concentration versus water residence time relationship at infinitely long residence times is roughly 2-3 mg L-1. This would suggest that for lakes in this region, autochthonous sources account for perhaps as much as 3 mg L 1 of the DOC present. 

Stable carbon isotope ratios have also been used to determine the sources of lake DOC. Baron et al. (1991) used 13C analysis to show that autochthonous sources dominated the DOC of an alpine lake during most periods while allochthonous sources dominated the DOC in a subalpine lake. The high DOC concentrations observed during spring snowmelt and early summer in both lakes, however, were mostly derived from allochthonous sources. As with streams and rivers, synoptic regional studies of 13C and 14C would provide important new information on broad spatial patterns and controls on the sources of DOC in lakes. 

The formula for BCD in Eq. (2) was derived under the assumption that the bacterial growth rate was mineral nutrient limited. If the supply rate of labile organic carbon from allochthonous and autochthonous sources is insufficient to meet this demand, the pool of labile dissolved organic carbon (DOC) will eventually be depleted and the bacteria will become carbon... 

Bertilsson, S., and Jones, J. B., Jr. (2003). Supply of dissolved organic matter to aquatic ecosystems Autochthonous sources. In Aquatic Ecosystems—Interactivity of Dissolved Organic Matter, Findlay, S. E. G., and Sinsabaugh, R. L., eds., Academic Press, Amsterdam, pp. 3-24. 

The dominant allochthonous inputs are from riverine, marine/estuarine plankton, and bordering terrestrial wetland sources. Autochthonous sources typically include plankton, benthic and epiphytic micro- and macroalgae, emergent and submergent (e.g., seagrasses) aquatic vegetation (EAV and SAV) within the estuary proper, and secondary production. 

The simple comparison of amino acid distributions in bacteria and phytoplankton reveals no obvious differences between these two possible autochthonous sources of dissolved proteins. Accordingly, the amino acid distribution in HMWDOM is similar to the distribution in phytoplankton, bacteria and sediment traps. The relationship between the amino acid distribution in total DOM and various particulate fractions is more variable. In general, it appears that amino acid distribution alone cannot identify the dominant source of dissolved proteins/peptides. However, assuming that the primary source of dissolved proteins in the ocean is bacteria and phytoplankton, these data support the interpretation that hydrolysable proteins in DOM and HMWDOM maintain a source-like amino acid distribution. 

Riverine fluxes. Approximately 0.2 Gt each of dissolved and particulate OC are carried from land to sea annually by rivers (Ludwig et al., 1996). Much of this riverine organic matter appears to be soil derived based on its chemical characteristics (Meybeck, 1982 Hedges et al., 1994), although autochthonous sources may be important for the dissolved fraction (Repeta et al., 2002). It is now recognized that, on a global basis, riverine... 

Phytoplankton abundance responds to nutrients, light, and grazing pressure. The primary autochthonous source of CDOM is often assumed to be photosynthetic organisms, but heterotrophic bacteria may play an important role by processing the relatively UV-transparent photosynthate and releasing modified compounds that absorb UVR more strongly [91]. Spatial and temporal linkage between CDOM and the microbial community appears complex and difficult to... 

Florida rivers of contrasting trophic status (Paul et al., 1991), while a variety of filterable organic phosphorus compounds were detected in water from a treatment wetland by capillary-electrophoresis separation and detection by mass-spectrometry (Llewelyn et al., 2002 Cooper et al.. Chapter 3, this volume). Similarly, enzymatic hydrolysis techniques have yielded information on functional organic phosphorus classes in natural waters (e.g. Herbes et a/., 1975 Shan et al., 1994). However, it is difficult to infer terrestrial origins for organic phosphorus compounds measured in aquatic environments, because they are equally likely to be derived from autochthonous sources such as algal or bacterial cells. 

Boyd, T.J. and Osbum, C.L. (2004). Changes in CDOM fluorescence from allochthonous and autochthonous sources during tidal mixing and bacterial degradation in two coastal estuaries. Mar. Chem., 89(1 ), 189-210. 

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