Hyporheic invertebrates as bioindicators of ecological health in temporary rivers: A meta-analysis

Worldwide, many rivers cease flow and dry either naturally or owing to human activities such as water extraction. However, even when surface water is absent, diverse assemblages of aquatic invertebrates inhabit the saturated sediments below the river bed (hyporheic zone). In the absence of surface water or flow, biota of this zone may be sampled as an alternative to surface water-based ecological assessments. The potential of hyporheic invertebrates as ecological indicators of river health, however, is largely unexplored. We analysed hyporheic taxa lists from the international literature on temporary rivers to assess compositional similarity among broad-scale regions and sampling conditions, including the presence or absence of surface waters and flow, and the regional effect of hydrological phase (dry channel, non-flowing waters, surface flow) on richness. We hypothesised that if consistent patterns were found, then effects of human disturbances in temporary rivers may be assessable using hyporheic bioindicators. Assemblages differed geographically and by climate, but hydrological phase did not have a strong effect at the global scale. However, hyporheic assemblage composition within regions varied along a gradient of higher richness during wetter phases. This indicates that within geographic regions, hyporheic responses to surface drying are predictable and, by extension, hyporheic invertebrates are potentially useful ecological indicators of temporary river health. With many rivers now experiencing, or predicted to experience, lower flows and longer dry phases owing to climate change, the development of ecological assessment methods specific to flow intermittency is a priority. We advocate expanded monitoring of hyporheic zones in temporary rivers and recommend hyporheic invertebrates as potential bioindicators to complement surface water assessments.