Microbial utilization of dissolved organic carbon leached from riparian litterfall

Dissolved organic carbon (DOC) in aquatic systems is abundant and used within stream food webs, but DOC quality is rarely studied. DOC in the leachates from the litter of five tree species (red alder, Alnus rubra; vine maple, Acer circinatum; western red cedar, Thuja plicata; western hemlock, Tsuga hetrophylla; and Douglas-fir, Pseudotsuga menziesii) were assessed for their chemistry and relative ability to support growth of heterotrophic, stream bacteria. Bacterial growth was measured using [H-3]leucine incorporated into protein over 24 h of exposure to nutrient-amended leachates. Bacterial growth was greatest in deciduous and western red cedar leachates, controlling for DOC concentration. Bacterial growth rates on most leachates were greatest after 1 h and then declined in a negative exponential pattern. The DOC less than 10 kDa supported lower bacterial growth rates than DOC from whole leachates on a per milligram DOC basis. The DOC C:N atomic ratio was the best predictor of bacterial growth (r(2) = 0.84). DOC release from western hemlock needles increased linearly during 7 days of leaching, whereas most red alder and western red cedar DOC was released after 1 and 2 days, respectively. Successional changes in composition of riparian forest trees may influence the stream microbial productivity based on the changes in dissolved organic carbon.