THE ROLE OF WATER EXCHANGE BETWEEN A STREAM CHANNEL AND ITS HYPORHEICZONE IN NITROGEN CYCLING AT THE TERRESTRIAL AQUATIC INTERFACE

The subsurface riparian zone was examined as an ecotone with two inter faces. Inland is a terrestrial boundary, where transport of water and dissolved solutes is toward the channel and controlled by watershed hy drology. Streamside is an aquatic boundary, where exchange of surface water and dissolved solutes is bi-directional and flux is strongly inf luenced by channel hydraulics. Streamside, bi-directional exchange of water was qualitatively defined using biologically conservative tracer s in a third order stream. In several experiments, penetration of surf ace water extended 18 m inland. Travel time of water from the channel to bankside sediments was highly variable. Subsurface chemical gradien ts were indirectly related to the travel time. Sites with long travel times tended to be low in nitrate and DO (dissolved oxygen) but high i n ammonium and DOC (dissolved organic carbon). Sites with short travel times tended to be high in nitrate and DO but low in ammonium and DOC . Ammonium concentration of interstitial water also was influenced by sorption-desorption processes that involved clay minerals in hyporheic sediments. Denitrification potential in subsurface sediments increase d with distance from the channel, and was limited by nitrate at inland sites and by DO in the channel sediments. Conversely, nitrification p otential decreased with distance from the channel, and was limited by DO at inland sites and by ammonium at channel locations. Advection of water and dissolved oxygen away from the channel resulted in an oxidiz ed subsurface habitat equivalent to that previously defined as the hyp orheic zone. The hyporheic zone is viewed as stream habitat because of its high proportion of surface water and the occurrence of channel or ganisms. Beyond the channel's hydrologic exchange zone, interstitial w ater is often chemically reduced. Interstitial water that has not prev iously entered the channel, groundwater, is viewed as a terrestrial co mponent of the riparian ecotone. Thus, surface water habitats may exte nd under riparian vegetation, and terrestrial groundwater habitats may be found beneath the stream channel.