The Uptake of Dissolved Organic Matter in Groundwater by Stream Sediments — A Case Study

Abstract

The investigation of energy flow in streams has caused considerable interest in the decomposition of dissolved allochthonous organic matter. Dissolved organic matter (DOM) has been shown to be the most important source of organic energy in headwater streams (Wetzel and Manny, 1972; Hobbie and Likens, 1973; Fisher and Likens, 1973) and the pool of DOM in small streams is thought to be in rapid, dynamic equilibrium (Manny and Wetzel, 1973). The source of this organic material is generally considered to be of terrestrial origin (Hynes, 1963; Hynes, 1970; Fisher and Likens, 1973; McDowell and Fisher, 1976; Cummins et al., 1972) but, until recently, it has been assumed that the leaching of dead vegetation which fell into the stream was the only important source of dissolved organic matter. It is known, however, that groundwater is the major component of stream flow throughout the year (Viessman et al. 1977; Freeze, 1974) and that it contains DOM (Kasper and Knickerbocker, 1980; Whitelaw and Edwards, 1980; Leenheer et al., 1974; Wallis et al., in press). The possibility that this DOM contributes large amounts of energy to stream production has not been widely recognized. Fisher and Likens (1973) realized that subsurface inputs of DOM to Bear Brook constituted 21% of the organic energy budget but, because they could find no concentration difference between spring water and stream water, they concluded that groundwater DOM neither diluted nor enriched the stream. This belief was supported by the knowledge that DOM in natural ground and surface waters is refractory in nature, consisting mostly of large molecular weight humic and fulvic acids (Larson, 1978; Otsuki and Wetzel, 1973; de Haan, 1972; McDowell and Fisher, 1976). These substances are not easily degradable and were considered to be chemically and biologically conservative. This paper presents evidence to the contrary and seeks to show that DOM from groundwater sources can contribute significant amounts of energy to the detrital pathways in streams. It is important to note that the “dissolved” organic matter referred to throughout this study actually consists, of molecules in a spectrum of sizes and states of hydration ranging from truly dissolved through to colloidal organic species (Lock et al., 1977).