Thresholds of Floodplain Fish Assemblages in Response to River Impoundment and Flood Timing, Magnitude and Duration

Ecosystems may respond to natural and anthropogenic gradients in a relatively continuous manner or manifest profoundly when conditions approach a certain threshold.  In river-floodplain ecosystems, flooding and associated flow components (e.g. magnitude, duration, and timing) comprise a primary gradient essential to life-history strategies of species occurring in these systems.  Anthropogenic modification of the hydrologic regime by impoundment or diversion severely affects timing, magnitude and extent of seasonal flooding, and otherwise results in longitudinally and laterally disconnected systems.  Using a long-term dataset from the Upper Paraná River floodplain LTER, Brazil, we test for community threshold responses of fish assemblages to upstream river impoundment, as well as along gradients of flood magnitude, duration and timing.  Initial analyses indicate community thresholds across multiple habitat types (rivers, connected lagoons and isolated lagoons) that correspond with the closure of Porto Primavera reservoir, as well as levels of flow components that elicit strong non-linear responses in floodplain fish assemblages.  Flow regulation and depletion are primary threats to freshwater biodiversity worldwide, and thresholds identified in this study can be applied as criteria for management in this basin and help anticipate community responses to impoundment in other areas.