INTERPOLATING FLOW REGIME AT UNGAGED SITES TO ASSESS RELATIONSHIPS WITH THE HYPORHEIC INVERTEBRATE COMMUNITY

Climate change and unsustainable groundwater extraction result in longer and more frequent periods of drying and stream intermittency in Texas rivers. Dry conditions affect and degrade the health of rivers and the associated hyporheic zone, which is the ecotone between surface water and groundwater beneath a river channel.

Little is known about how stream intermittency affects hyporheic invertebrate communities, and this study was designed to quantify these relationships. A 2-part study first classified flow regime at 34 sites across 4 rivers in central Texas using 10 indices calculated using the Indicators of Hydrological Alteration software (The Nature Conservancy, 2009). At sites that lacked long-term time-series gage data (defined as 10 or more years of continuous record), we created predictive models based on shared environmental characteristics for basins of both gaged and non-gaged sites. These models were then used to predict hydrological indices at 24 sites without time-series gage data in the same 4 rivers. Cluster analysis, based on predicted index values, resulted in distinct clusters of ‘wet’ and ‘dry’ sites. ‘Dry’ sites experienced systematic declines in discharge, or complete desiccation, because of anthropogenic effects such as river withdrawals and groundwater extraction, while ‘wet’ sites experienced higher, more stable flow.

Statistical analyses of relationships between hyporheic invertebrate community structure and flow regime classification and hydraulic conductivity revealed a significant difference in invertebrate abundance between comparatively “wet” and “dry” sites, with lower abundances at drier sites. Differences in community composition were also found between sites, driven mainly by the presence/abundance of Copepoda, Ostracoda, and Insecta taxa groups. More than 50% of variation in abundance between sites could be explained by multiple linear regression models with mean annual flow and hydraulic conductivity. This work found a strong connection between flow regime and hyporheic invertebrate communities, suggesting that decreasing flows result in lower abundance and shifts in community composition. Understanding how hyporheic communities respond to future climate scenarios will enable more effective management and mitigation of damage to river ecosystems.