The effects of ecosystem configuration on nutrient dynamics in a Sonoran Desert stream ecosystem

Stream ecosystems are landscapes composed of interacting terrestrial and aquatic patches. As water moves downstream through a series of patches, the materials it carries are transformed and transported in different ways. This study addressed the question: how does ecosystem configuration affect nutrient dynamics? Research was conducted at Sycamore Creek, Arizona, a Sonoran Desert stream where prior studies of biogeochemical processes laid the foundations for the work.; Spatial patterns of nutrients in stream surface water were described using spatial statistics, at multiple scales of observation and at different times following flooding. Nutrients varied spatially at all scales sampled, and variation increased over successional time. The limiting nutrient (nitrogen) was most variable. Changes in nutrient concentration downstream were used to infer intervening biogeochemical processes. Processes in subsurface patches were particularly influential, affecting nutrients in surface water at scales from meters to kilometers. A model of the feedback between surface nutrients, surface periphyton, and subsurface microbial processes showed that exchange between surface and subsurface patches influenced periphyton biomass, and that the rate of post-flood recovery depended on processing rates in subsurface sediments.; Connections between patches in a stream ecosystem are primarily hydrological. An investigation of then connections using a conservative tracer addition revealed that the dominant direction of water movement was from the surface stream out to the lateral subsystem (the opposite direction from mesic streams). Calculations based on this information showed that local variation in hydrological parameters and connections between subsystems could influence nutrient retention. A simulation model was then developed to investigate how different configurations of surface and subsurface patches affected both spatial patterns of nutrients and nutrient retention. Results showed that increasing the numbers of connections between surface and subsurface patches produced more variation in nutrient concentrations and higher storage (retention) of nutrients.; This study demonstrates that knowledge of spatial patterns and configuration is critical to a complete understanding of material processing. Although the dominance of hydrologic transport is a distinguishing feature of streams, movement of materials between patches with different processing characteristics occurs in all ecosystems, making spatial configuration of potential importance.