Is drift the primary process promoting the resilience of river invertebrate communities? A manipulative field experiment in an intermittent alluvial river

In river systems, aquatic invertebrate communities are surprisingly persistent over time and generally recover quickly from disturbances. Drift has long been viewed as the primary process promoting this resilience, and it plays a important role in predictive models of community composition and concepts in lotic ecology. More recently, other processes such as vertical migration from the hyporheic zone, aerial oviposition from distant refuges and the use of resistance forms (e.g. diapause) have received greater recognition and support for their importance. In this study, the view that drift is the primary process promoting invertebrate community resilience was challenged in an intermittent alluvial river using reach-scale flow manipulations. First, six treatment channels were completely dried for 1week, while three others were left flowing to be used as controls. Second, flow was re-established in channels and drift was either allowed or blocked for a 4-week period. Third, during this period the resilience of community structure, composition and function was compared between treatments, and the potential for colonisation from the drift, hyporheic zone, aerial oviposition and resistance forms was measured. Communities recovered after only 2weeks in all of the previously dried channels, and contrary to our hypotheses, invertebrate community structure, composition and functional trait composition were not altered by blocking drift, indicating it was not the primary process promoting resilience in this river. Three lines of evidence suggested colonisation from the hyporheic zone and not aerial oviposition nor resistance forms promoted resilience following rewetting including the following: (i) finding all common benthic taxa in the hyporheic zone during the drying event, (ii) a distinct decrease in invertebrate size upon rewetting in all treatment channels and (iii) a negative correlation between resilience and water table depth. This experiment highlighted the potential importance of the hyporheic zone as a key source of colonisation in alluvial rivers and emphasises the need for a three-dimensional perspective when considering community resilience in rivers. Adaptive management approaches are needed to direct attention to sources (e.g. hyporheic zone) that are essential to promoting community resilience in rivers facing increased pressures due to climate change, water abstraction and flow regime alteration.