Hydrologic linkage and hyporheic metabolism in a Sonoran Desert stream

Ecosystems are composed of a mosaic of patches that differ in species composition, and material and energy distributions. Patches are linked through material, energy, and organism exchanges and may act as sources or sinks for abiotic and biotic elements. This research centered on spatial heterogeneity of organic matter production and decomposition in Sycamore Creek, a Sonoran Desert stream, southwestern USA. Specifically, metabolism in the hyporheic zone (sediments underlying the wetted stream channel) was studied, addressing questions of the sources of organic detritus supporting respiration, factors limiting respiration rate, and consequences of spatial heterogeneity of microbial processes for ecosystem functioning. Organic matter decomposition in the hyporheic zone accounted for the major proportion (90%) of total ecosystem respiration and occurred through a number of metabolic pathways. As water flowed through sediments, terminal electron acceptors were selectively consumed, resulting in a decline of redox potential and gradation of metabolic pathways along paths of subsurface flow. At the upstream end of subsurface flowpaths, where surface water and dissolved oxygen downwelled into sediments, aerobic respiration rate was high and supported by import of organic matter derived from benthic algal production. Chemoautotrophic production was also high in downwelling zones, presumably linked to heterotrophic decomposition. Much of this chemoautotrophy was nitrification, supported by mineralization of organic nitrogen during microbial respiration. Downstream along subsurface flow where anoxia prevailed, methanogenesis was an important metabolic pathway. The rate of organic matter decomposition by methanogens in anoxic bank sediments were nearly equal to the aerobic respiration rate in oxic hyporheic sediments. Length of subsurface flowpaths determined the kinds and amounts of materials exported from the hyporheic zone to the stream surface and atmosphere. Short flowpaths exported carbon dioxide, whereas long flowpaths exported carbon dioxide and methane. Distribution and extent of metabolic processes along flowpaths were affected by a number of abiotic variables including flash floods, benthic algal biomass, organic matter storage in sediments, and temperature. The hyporheic zone is an integral component of streams and must be considered in integrated research programs. If hyporheic respiration in Sycamore Creek is any indication of the potential metabolic activity in stream sediments elsewhere, our current understanding of metabolism in lotic ecosystems is deficient.