Dissolved Organic Matter Concentration and Flux in Streams

Dissolved organic matter (DOM) in streams is important as an energy resource for food webs (Bott et al. 1984, Meyer 1990a), a regulator of nutrient uptake and cycling by heterotrophic microbes (Lock 1981, Meyer et al. 1988), a complexing agent for metals (McKnight and Bencala 1990, Mierle and Ingram 1991), and a determinant of pH and alkalinity (Oliver et al. 1983). DOM has been shown to be an important metabolic substrate in some streams, with DOM use contributing significantly to total stream metabolism (Naiman et al. 1987, Meyer and Edwards 1990). Sources of DOM are both the terrestrial watershed (McDowell and Likens 1988, Hornberger et al. 1994) and instream processes, such as leaching and decomposition of allochthonous particulate organic matter and release by stream algae (Kaplan and Bott 1982, Meyer 1990b). For small streams, the primary site of DOM use is the stream bottom, including the interstitial waters of streambed sediments (Dahm 1981, Hynes 1983, Ford and Naiman 1989, Fiebig and Lock 1991). In larger rivers, the water column may also be an important site for DOM use (Vannote et al. 1980). Stream DOM is an important indicator of watershed-scale hydrologic and biogeochemical processes. Large water-soluble pools of DOM are present in the upper soil horizons of forests and grasslands (litter, 0, and A horizons) and are sources of DOM to drainage waters; however, lower soil horizons (B and C horizons) often contain materials (e.g., iron and aluminum oxides) that effectively sorb and immobilize DOM (McDowell and Wood 1984, Cronan 1990). Hydrologic pathways involving lateral flow of water through or over surface soil layers often have relatively high concentrations of DOM, whereas water flowpaths that pass slowly through lower soil layers usually have low DOM concentrations (Moore and Jackson 1989, Kaplan and Newbold 1993). The commonly observed increase in DOM concentrations during periods of high discharge is at least partly due to shifts in dominant flowpaths from deeper routes to shallow subsurface or surface pathways (Tate and Meyer 1983, Mulholland et al. 1990, Hornberger et al 1994). Wetlands are also an important source of DOM to streams, and several analyses have shown that differences in baseflow DOM concentrations among streams are strongly related to the amount of wetland drainage contributing to streams (Mulholland and Kuenzler 1979, Eckhardt and Moore 1990). Saturated riparian soils and wetlands appear to be particularly important contributors to stream DOM, especially during storms (Fiebig et al. 1990, Hemond 1990). In this paper I focus on the concentrations and flux of DOM in streams whose organic matter budgets are presented in the preceding chapters. My objective is to define the variation in annual average DOM concentrations and flux and to identify what factors might be most responsible for this variation.