River flow controls ecological processes and invertebrate assemblages in subsurface flowpaths of an ephemeral river reach

We present the first measurements of solutes, invertebrates, and microbial activity in the semi-perched hyporheic, parafluvial, and riparian flowpaths of an ephemeral river channel. Dissolved organic carbon (DOC). nitrogen (DON), and phosphorus (DOP) concentrations decreased as water from an adjacent river mainstem moved through the flowpaths. DOC, DON, and DOP processing rates decreased with increasing mainstem flow and increased with parafluvial zone length. These patterns suggest that the surface water zones of perched river systems are organic nutrient sources to subsurface flowpaths and that parafluvial zones of these systems are strong sinks for organic matter. No longitudinal changes were detected in NO3- concentrations, and relationships between NO3- processing and hydrological variables were not significant. NO3- concentrations were uniformly high, and microbial activity and DOC and dissolved reactive phosphorus (DRP) concentrations were low, suggesting that biological NO3- removal was carbon- or phosphorus-limited. Invertebrate assemblages also varied between subsurface zones: density and taxon richness in the hyporheic and parafluvial zones were higher than in the riparian zone, and evenness was higher in the riparian zone than in the hyporheic or parafluvial zones. Distinct invertebrate assemblages in riparian zones may reflect greater hydrologic stability compared with hyporheic and parafluvial zones.