Are nutrients or salinity the drivers of macroinvertebrate community composition in wetlands of the desert Southwest?

Abstract The relatively rare freshwater ecosystems in the arid southwestern United States serve as biodiversity hotspots, yet they remain among the most threatened systems in the world due to human impacts and climate change. Globally, arid region wetlands remain understudied with respect to their ecology, making assessments of quality or restoration efforts challenging. To address these needs, this project aims to better understand the factors that drive water quality and macroinvertebrate community composition of wetlands of the US desert Southwest. Water quality and macroinvertebrate data were collected over three years from 14 different wetland and riparian sites spanning across West Texas, New Mexico and Arizona. Principal Component Analysis (PCA) indicated that salinity related variables such as chloride, sulfate and conductivity were the greatest drivers of environmental variance (32%) among sampled desert wetlands. Nutrients such as nitrate and phosphate described a second axis, with 22% of variation in environmental data explained, where we found a clear distinction between wastewater and non-wastewater wetlands. Nutrients were shown to have the greatest impact on macroinvertebrate communities with wetlands receiving wastewater showing more uneven distribution of functional feeding groups and lower Simpson Index scores. These sites were dominated by filter feeders and had lower relative abundances of predator and collector-gatherer taxa. There was also a significant decrease in metrics related to diversity and environmental sensitivity such as % Ephemeroptera-Odonata-Tricoptera (EOT) within high nutrient sites. Increased salinity levels were also shown to correlate with lower Simpson Index scores indicating that increased salinity resulted in a decline in macroinvertebrate diversity and evenness. To enhance the water quality and diversity in their sites, we suggest that managers of these valuable created habitats that are fed with wastewater might try to find less nutrient-rich water sources, or dilute effluent with another water source such as groundwater. Overall, the nutrients within effluent water have shown to significantly alter community composition especially in desert wetlands where macroinvertebrates may be more adapted to salinity. Though macroinvertebrate communities in wastewater sites may not fully resemble those of natural wetlands over time, creation of these sites can still benefit landscape level diversity.