Dissolved organic matter

DOM includes labile organic compounds that are available to microorganisms for biological uptake as well as refractory material that is less easily assimilated and may be exported great distances (Section 7.3). Groundwater, rainfall and throughfall, and leaching of leaves are primary sources of dissolved organic C. Concentrations vary with terrestrial vegetation, soil flow pathways, and the presence of wetlands, and may be enriched by domestic sewage or agricultural runoff. DOC enters streams primarily through groundwater inputs at low flows, while more DOC enters via lateral flows during storms (Mulholland 1997b). DOC inputs increase during storms due to flushing of locations of DOC accumulation (particularly in organicrich riparian zones and surface soils on hillslopes), canopy throughfall, and possibly due to leaching of newly entrained material (Meyer and Tate 1983, McDowell and Likens 1988).

The amount of DOM in river water varies on daily, seasonal, and yearly timescales; and spatially, in accord with local geology, vegetation, and rainfall (Table 12.1). DOM concentrations ranged between 0.5 and 36.6 mg L 1 in a survey of 33 streams and rivers in the Caribbean, North America, Europe, and Antarctica (Mulholland 1997b). On a worldwide basis, DOC exceeds POC by approximately 2:1, but this depends on type of stream and discharge regime. Based on annual means, reported DOC to POC ratios for North American streams range from 0.09:1 to 70:1 (Moeller et al. 1979). The total export of organic matter from 86 Finnish catchments was dominated by DOC, which made up 94% of total organic C (Mattsson et al. 2005). In headwater streams of British Columbia, DOM also represented a high percentage (84%) of the total organic matter export, and the remainder was mostly FPOM (Karlsson et al. 2005). In larger rivers, however, POC and DOC concentrations are similar, and at high discharge POC can exceed DOC (Thurman 1985).

DOC inputs are influenced by catchment characteristics such as geology, soils, and

TABLE 12.1 Typical concentrations of dissolved organic carbon (DOC) in running waters and groundwater. (From Thurman 1985.)

Range Average

TABLE 12.1 Typical concentrations of dissolved organic carbon (DOC) in running waters and groundwater. (From Thurman 1985.)

Range Average

Groundwater

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