VARIABILITY IN REMOVAL OF DISSOLVED ORGANIC-CARBON IN HYPORHEIC SEDIMENTS

Dissolved organic carbon (DOC) is consumed by microbial metabolism as streamwater perfuses through a lateral gravel bar of the East Branch o f the Wappinger Creek. The rate of DOC removal was estimated from the decline in DOC and travel time through the bar. Variability in DOC rem oval together with potential regulatory factors was determined for 14 dates spanning more than a 2-y period. DOC removal was not correlated with temperature, availability of oxygen, or residence time within hyp orheic sediments. Hyporheic DOC could be predicted (r(2) = 0.68) from streamwater DOC concentrations, with a surprisingly constant 57% (+/-9 % [1 SD]) removal of DOC. This pattern suggests an initial concentrati on-dependent adsorption of DOC onto surfaces. This mechanism allows fo r efficient retention of DOC within hyporheic sediments even under con ditions (low temperature, high interstitial velocity) that might be ex pected to minimize biotic consumption of DOC.