Influence of hydrologic exchange patterns on water chemistry and hyporheic invertebrate communities in three gravel‐bed rivers

Abstract

The influence of hydraulic exchange patterns on hyporheic water chemistry and invertebrate community composition was measured in three gravel‐bed rivers of southern Hawke's Bay, North Island, New Zealand. Based on patterns reported overseas we predict that down‐welling waters will be similar to surface waters in chemical characteristics and epigean animals will be more common in down‐welling waters than in up‐wellings, which would have a different chemical signature to the surface waters and a fauna dominated by hypogean invertebrates. Conductivity, pH, and temperature of hyporheic water were least like those of surface waters at up‐wellings in the groundwater‐fed Makaretu River. Only 6% of the invertebrate fauna of these up‐wellings comprised epigean animals. In contrast, in the groundwater‐recharging Tukituki and Waipawa Rivers, hyporheic water chemistry was similar to that of surface water in down‐wellings and 40% of the community comprised epigean animals by abundance. Significant increases (P < 0.05) in species diversity and taxonomic richness was also identified in down‐welling zones compared with up‐wellings. Species diversity and taxonomic richness were lowest in up‐wellings in the Makaretu where hypogean animals dominated the hyporheic fauna. Species diversity and taxonomic richness were higher in down‐wellings than up‐wellings because more epigean taxa were present.