Towards a biological and chemical definition of the hyporheic zone in two Canadian rivers

Abstract

SUMMARY. 1. A series of samples of interstitial water and fauna was taken along transects from the channel into the bank in two small rivers in southern Ontario, Canada. These were examined for any discontinuities which might indicate the position of the hyporheic/groundwater interface.2. There were several chemical discontinuities in Duffin Greek, with “break lines” occurring from about the river margin obliquely downwards under the bank for dissolved oxygen and carbon dioxide, B.O.D., alkalinity, suspended solids and amount of organic matter. Break lines in nitrate and sulphide concentration ran from near the margin obliquely down under the river bed. In the Rouge River, a discontinuity extending from a point approximately 1.5 m landwards from the margin obliquely down under the river was indicated by dissolved carbon dioxide, B.O.D., conductivity, suspended solids, organic matter, nitrate and alkalinity.3. Ordination (DECORANA) and community classification (TWIN‐SPAN) revealed that, in both rivers, linear distance from mid‐river was the major factor associated with community structure. In both rivers the community under the bank was distinct from the river community and these two communities were separated by another community characteristic of the river margin. In Duffin Creek the classification procedure additionally discriminated surface and interstitial sub‐sets within the river community.4. Most taxa showed no significant correlations with the chemistry of the interstitial water but. in Duffin Creek, the densities of the copepod Diacyclops crasscaudis brachycercus (Kiefer) and Oligochaeta were positively correlated with nitrate: worms were also negatively correlated with sulphide. In the Rouge River, density of hyporheic animals was negatively correlated with both conductivity and alkalinity of interstitial water; oligochactes were positively correlated with both nitrate and organic matter; and density of nematodes was positively correlated with sulphide concentration.5. The hyporheic faunas of these two rivers were dominated by insects, particularly chironomids. Compared with the hyporheos of rivers in Europe and Colorado, the two Ontario rivers lack significant numbers of harpacticoid copepods as well as bathynellacid, amphipod and isopod crustaceans.