Immobilization of dissolved organic matter from groundwater discharging through the stream bed

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Abstract
SUMMARY. 1. In laboratory experiments, 9.7–25.7% of dissolved organic carbon (DOC) in groundwater (at concentrations of 18.7–24.8 mg 1‐−1) was immobilized after perfusion through 8‐cm‐deep (22‐cm‐diameter) cores of stony stream‐bed substratum.2. This represented immobilization rates of 7.1–23.5 mg m−2 h−1 or, extrapolated across the year, potential immobilization rates within the stream bed of 62.2–205.9g m−2 yr−1. Actual rates in the entire stream bed were probably higher because perfusion through the experimental cores did not reduce groundwater DOC concentrations to levels measured in the adjacent stream.3. Natural concentrations of dissolved free amino acids (DFAAs) in groundwater were generally unchanged following perfusion through the cores, suggesting the maintenance of a dynamic equilibrium in their concentrations.4. Selective enrichments of amino acids in groundwater (up to 20‐fold) were entirely immobilized following perfusion, indicating their rapid retention and flux in this environment. Thus, immobilization of the bulk DOC in stream‐bed cores probably did not reflect net reductions in dissolved free, low‐molecular‐weight material, with higher molecular weight, more ‘refractory’ material being immobilized instead.5. We conclude that groundwater can contribute substantial amounts of DOC, both high and low molecular weight, to a stream ecosystem. The stream bed is the site at which much of this material could be initially immobilized and made available to the stream trophic structure.