COS 117-1 - Landscape-scale hydrologic and environmental thresholds on patterns of fish and macroinvertebrate assemblage structure across stream flow regimes

Abstract: Identifying thresholds in flow ecology relationships is a goal of many environmental flows studies and decision making. We compiled landscape-scale, geo-referenced species occurrence datasets to examine and compare patterns of fish and macroinvertebrate assemblage and species-level turnover across flow regimes. A gradient forest machine learning approach was used to quantify multi-species threshold responses along hydrologic and watershed-scale disturbance gradients in groundwater (GW), runoff (RO), and intermittent streams (INT) in the Ozark and Ouachita Interior Highlands and Gulf Coastal Plains, USA. Preliminary results indicate watershed fragmentation was of high importance for assemblage turnover in RO and INT streams, while changes in dam storage were more important in GW streams. Hydrologic metrics describing seasonal and stochastic properties of daily streamflow were most important for assemblage turnover in INT streams. Timing of high flow events had significantly higher importance compared to flow magnitude, duration, and frequency metrics, especially in GW and INT streams. The frequency and timing of low flow events had high importance for assemblage turnover across all stream flow classes, while the magnitude of low flows and the magnitude and rate of change of average flows was most important for INT stream assemblages. The results of our analysis will inform multi-species conservation and management through identification of local and regional flow-ecology relationships across different flow regimes. Results of this study also provide understanding of complex nonlinear environmental and disturbance threshold relationships driving patterns in fish and macroinvertebrate assemblages and species in streams.