Exchange between interstitial and surface water: Implications for stream metabolism and nutrient cycling

Metabolism of a Sonoran Desert stream was investigated by both enclosure and whole system oxygen techniques. We used recirculating chambers to estimate surface sediment metabolism and measured deep sediment respiration in isolated sediment cores. Metabolism of the stream ecosystem was determined for a 30-m reach as dark and light oxygen change with and without black plastic sheeting that darkened the stream and prevented diffusion. Average ecosystem respiration for two dates in August (440 mg O₂ m^-2 h^-1) exceeded respiration of either the surface sediment community (155 Mg O₂ m^-2 h^-1) or the hyporheic community (170 mg O₂ m^-2 h^-1) alone. Deep sediments show substantial oxygen and nitrate uptake when isolated. In the stream, this low nitrate interstitial water is exchanged with surface water. Metabolism of the isolated surface community suggests a highly productive and autotrophic system, yet gross production is balanced or exceeded by community respiration when ecosystem boundaries include the hyporheic zone. Thus, despite high rates of gross primary production (600-1200 mg O₂ m^-2 h^-1), desert streams may be heterotrophic (P_G < R) during summer.