Implications of Using Treated Wastewater as Habitat for Desert Fishes

Authors: Drew Eppehimer, Kelsey Hollien, Zach Nemec, Larissa Lee

Year: 2018

Abstract

Discharge of treated wastewater and upgrades to water reclamation facilities have been instrumental in returning aquatic biodiversity and riparian habitat to the Santa Cruz River in southern Arizona. The endangered Gila Topminnow has recently recolonized effluent-dependent reaches of the river, but the long-term health effects of living in effluent and the potential for these fish to persist in effluent reaches are unknown. Using non-native Western Mosquitofish (Gambusia affinis) in the lower Santa Cruz River (Tucson, AZ) as a proxy for native topminnow, we explore the challenges of living in effluent-dominated streams, including (1) fluctuating water levels, (2) the potential for microplastics to impact diet, and (3) the effects of emerging contaminants on sexual maturation. Diurnal fluctuations in discharge of effluent create daily drying events in the lower reaches of the river, which can result in the stranding and death of fish. Furthermore, in drift samples we have found microplastic concentrations as high as 5.56 particles/m3. However, in our diet analysis on a subset of 100 mosquitofish, we only documented one ingested plastic microfiber. Fish diets were otherwise comparable to natural systems and contained a variety of prey items with chironomid larvae, pupae, and adults dominating these taxa. Finally, our preliminary analysis has revealed a mosquitofish sex ratio heavily biased towards females (10.1:1), which is higher than ratios from published studies of natural systems (4.4:1). We hypothesize this pattern could be due to estrogen compounds, which can be common in effluent-dominated systems and can inhibit the sexual maturation of males. Faced with increasing human population growth and uncertainties of climate change, effluent systems will become increasingly important sources of aquatic habitat, but further studies are needed to assess their ability to support native fishes.