Ecotonal and other properties of the Hyporheic Zone

Distinct boundaries, where biological and physicochemical environments change abruptly, occur in nature and, at least superficially, riverbanks appear to separate terrestrial from aquatic systems. However, at depth, catchment groundwater is linked to channel water via the hyporheic zone. Complex hydrological interactions between these two water masses in the hyporheic zone, combined with biogeochemical processes, produce a set of physicochemical conditions that support the hyporheos - a community of invertebrates and microorganisms. There is evidence that the hyporheos contains unique elements, but also some that indicates transition to benthic communities and processes above, and to those of true groundwater below. While some aspects of these communities are becoming known, for example their composition is influenced by depth-related variables and interstitial water temperature, many others, such as the response of bacteria to different sources and concentrations of DOC and nitrogen, and whether protists provide a trophic link between microorganism and invertebrates, are still poorly understood. Drawing on historical and new knowledge revealed by a variety of contemporary techniques for characterizing interstitial water and its inhabitants, this paper will attempt to address some of these uncertainties and also, formally, examine the candidacy of the hyporheic zone as an ecotone.