Wholestream metabolism in two montane streams: contribution of the hyporheic zone. Limnology and Oceanography

We used whole-stream and benthic chamber methods to measure rates of metabolism and determine the contri-bution of the hyporheic zone to ecosystem respiration (R) in two streams with differing surface–subsurface exchange characteristics, Rio Calaveras and Gallina Creek, New Mexico. We used the difference between whole-stream and benthic R to calculate the rate of hyporheic zone R and coupled this estimate to an independent measure of hyporheic sediment R to estimate the cross-sectional area of the hyporheic zone (AH) for two reaches from each stream. Conservative tracer injections and solute transport modeling were used to characterize surface–subsurface hydrologic exchange by determining values of the cross-sectional area of the transient storage zone (As). The hyporheic zone contributed a substantial proportion of whole-stream R in all four study reaches, ranging from 40 to 93%. Whole-stream R, hyporheic R, and percent contribution of hyporheic R all increased as transient storage increased, with whole-stream and hyporheic R exhibiting significant relationships with As. All three measures of respiration and values of AH were much greater for both reaches of the stream with greater surface–subsurface exchange. AH is valuable for cross-site comparisons because it accounts for differences in rates of both benthic and hyporheic sediment R and can be used to predict the importance of the hyporheic zone to other stream ecosystem processes. Aquatic ecologists are increasingly aware of the impor-tance of interactions between surface water and groundwater to the functioning of aquatic ecosystems (Boulton et al. 1998; Winter et al. 1998; Jones and Mulholland 2000). The region of mixing between groundwater and stream water (i.e., the hyporheic zone, sensu Orghidan 1959; Triska et al. 1989b) influences ecosystem functioning because its sedi-ments contain metabolically active microbial assemblage