SEASONAL BOUNDARY DYNAMICS OF A GROUNDWATER SURFACE-WATER ECOTONE/

Interstitial water, faunal samples, and hydrogeological data were coll ected beneath a riffle on the Speed River, southern Ontario, Canada. T he purpose was to identify the location and seasonal fluctuation of th e hyporheic/groundwater interface and to examine several aspects of wa ter mass chemical signatures and the dynamics of the interstitial faun a. Concentration discontinuities in several water chemistry parameters delineated the chemical boundary between the true groundwater and hyp orheic habitats. The groundwater mass was characterized as having high er levels of ammonium, alkalinity, and conductivity, and lower nitrate levels. Differences in water chemistry between the hyporheic and grou ndwater zones persisted throughout the year, though no single variable differed quantitatively between these two zones on all occasions. The location of the chemical discontinuity varied seasonally. Whereas hyp orheic and groundwater faunal subunits of the interstitial community w ere identified and the location of the subunits coincided with the che mical breaklines, response to shifts in the position of the hyporheic/ groundwater interface was taxon rather than subunit based. Fauna there fore provided poor spatial resolution in terms of pinpointing the loca tion of the interface. Boundary fluctuation coincided with extremes in seasonal discharge patterns and was regulated by the relative strengt h of the upward force of baseflow and the downward force of advecting surface water. Identifying patterns of fluctuation of the hyporheic/gr oundwater interface, and consequently hyporheic habitat volume, may ha ve important consequences for the storage, retention, and cycling of n utrients in lotic ecosystems.