EVIDENCE THAT HYPORHEIC ZONES INCREASE HETEROTROPHIC METABOLISM AND PHOSPHORUS UPTAKE IN FOREST STREAMS

We used nondisruptive, whole-stream methods to measure hydraulic chara cteristics, ecosystem metabolism, and phosphorus cycling in the west f ork of Walker Branch (WE), Tennessee and in Hugh White Creek (HWC), No rth Carolina. Although similar in many of their hydrological and chemi cal characteristics, transient storage zone volume in HWC was relative ly large (similar to 1.5 times that of the flowing water zone), wherea s transient storage zone volume in WE was small (similar to 0.1 times that of the flowing water zone). Both streams were highly heterotrophi c (gross primary production:total respiration ratios <0.1), although r espiration rate was similar to 2.4 times greater in HWC than in WE. Ph osphorus uptake rate was similar to 2.6 times, greater in HWC than in WE, and P uptake length was 5 times longer in WE than in HWC. Analysis of P-33:H-3 ratio profiles from (PO4)-P-33 and (H2O)-H-3 injection ex periments indicated that P uptake within the transient storage zone ac counted for similar to 43% of the total P uptake in HWC but was neglig ible in WE, explaining, in part, the difference in total P uptake rate between these streams. The higher rate of P uptake in the surface zon e, shorter P uptake length, and larger ratio of P uptake:respiration i n HWC compared with WE suggest that the microbial community in HWC exe rts a greater demand for and has a greater affinity for streamwater P presumably as a result of a greater supply of labile organic matter an d/or lower nutrient levels in HWC.