Whole-stream metabolism in two montane streams: Contribution of the hyporheic zone

We used whole-stream and benthic chamber methods to measure rates of metabo lism and determine the contribution of the hyporheic zone to ecosystem resp iration (R) in two streams with differing surface-subsurface exchange chara cteristics, Rio Calaveras and Gallina Creek, New Mexico. We used the differ ence between whole-stream and benthic R to calculate the rate of hyporheic zone R and coupled this estimate to an independent measure of hyporheic sed iment R to estimate the cross-sectional area of the hyporheic zone (A(H)) f or 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 (A(s)). The hyporheic zone contributed a substantial proporti on of whole-stream R in all four study reaches, ranging from 40 to 93%. Who le stream R, hyporheic R, and percent contribution of hyporheic R all incre ased as transient storage increased, with whole-stream and hyporheic R exhi biting significant relationships with A(s). All three measures of respirati on and values of A(H) were much greater for both reaches of the stream with greater surface-subsurface exchange. A(H) is valuable for cross-site compa risons because it accounts for differences in rates of both benthic and hyp orheic sediment R and can be used to predict the importance of the hyporhei c zone to other stream ecosystem processes.