Evidence that hyporheic zones increase heterotrophic metabolism and phosphorus uptake in forest streams

Abstract

We used nondisruptive, whole-stream methods to measure hydraulic characteristics, ecosystem metabolism, and phosphorus cycling in the west fork of Walker Branch (WB), Tennessee and in Hugh White Creek (HWC), North Carolina. Although similar in many of their hydrological and chemical characteristics, transient storage zone volume in HWC was relatively large (~ 1.5 times that of the flowing water zone), whereas transient storage zone volume in WB was small (~ 0.1 times that of the flowing water zone). Both streams were highly heterotrophic (gross primary production: total respiration ratios <0.1), although respiration rate was ~ 2.4 times greater in HWC than in WB. Phosphorus uptake rate was ~ 2.6 times greater in HWC than in WB, and P uptake length was 5 times longer in WB than in HWC. Analysis of 33P: 3H ratio profiles from 33PO4 and 3H2O injection experiments indicated that P uptake within the transient storage zone accounted for ~ 43% of the total P uptake in HWC but was negligible in WB, explaining, in part, the difference in total P uptake rate between these streams. The higher rate of P uptake in the surface zone, shorter P uptake length, and larger ratio of P uptake: respiration in HWC compared with WB suggest that the microbial community in HWC exerts a greater demand for and has a greater affinity for stream-water P, presumably as a result of a greater supply of labile organic matter and/or lower nutrient levels in HWC.