Hydrologic influences on groundwater surface water ecotones: Heterogeneity in nutrient composition and retention

The groundwater-surface water (GW-SW) ecotone, or hyporheic zone, is an active component of stream ecosystems that influences whole-system metabolism and nutrient retention. Because hydrologic fluxes affect the supply of carbon nutrients, and oxygen to the GW-SW ecotone, the biogeochemical structure of the ecotone (i.e., nutrient content) and the role of the ecotone in nutrient retention are expected to vary under differing hydrologic conditions. In this paper, we employ an inter-basin comparison of headwater streams to assess the influence of ecosystem hydrology on the structure and functioning of GW-SW ecotones. Specifically, we address how differing rate and extent of GW-SW interaction influences heterogeneity in interstitial nutrient content and how variation in GW-SW interaction alters the role of the ecotone in whole-system nutrient retention. A multiple regression model derived from 6 solute-injection experiments identified the extent and rate of hydrologic exchange between the stream and its aquifer as critical variables that determine the retention of biologically important solutes. This approach emphasizes that the nature of GW-SW interaction is established by catchment geology (i.e., alluvial hydrogeologic properties), is modified by changing discharge within a catchment, and is a strong determinant of nutrient retention. At the landscape scale, identifying catchment geologic composition may be a starting point for comparative studies of GW-SW ecotones that could contribute to a more robust model of Iotic ecosystem functioning.