Macroinvertebrate community development and resilience to channel dredging following flow restoration using effluent in an urban desert river

Freshwater species face numerous threats across the globe, including urbanization. Within cities in regions with drier climates, dewatering and channelization of rivers commonly occur and reduce or eliminate freshwater biodiversity. The discharge of effluent (treated wastewater) has been used to restore flow in some of these rivers, but our knowledge is negligible about how ecological communities develop and change in these unique but increasingly common ecosystems. In this study, we quantified aquatic macroinvertebrate community development in the Santa Cruz River (Arizona, U.S.A.), where effluent-restored flow more than 100 years after the river dried up. We tracked community development over a 2-year period in reaches where flow had been restored and compared those findings with data from a reference reach. Our study period also encompassed a massive disturbance where effluent releases temporarily ceased and sediment was dredged from the channel, allowing us to quantify the impacts of urban channel maintenance activities on recovering communities. Macroinvertebrate colonization was rapid following the initial flow restoration and channel dredging, with density and species richness values reaching or exceeding those of the reference reach within a few months, but community composition remained quite distinct after 2 years. Flow duration and the number of dry days in the month prior to sampling were the most influential factors in macroinvertebrate metrics. Simply adding effluent to dewatered urban rivers has the potential to restore diverse aquatic fauna, but targeted reintroductions may be needed for sensitive or dispersal-limited taxa.