Immediate and habitat-specific responses of macroinvertebrates to sequential, experimental floods

Abstract

Spatial and temporal patterns of transported organic matter (seston) and macroinverte- brates (drift) and benthic macroinvertebrate densities were examined before, during, and shortly after each of a series of scheduled, experimental floods in a flow-regulated river in the Swiss National Park. Temporal patterns in the lateral transfer of seston, drift, and benthic macroinvertebrates were evaluated in the flooded riparian area during 3 to 4 separate floods of different magnitude. No clear spatial pattern was found in the lateral transfer of seston, drift, or benthic macroinvertebrates, but the concentrations of seston and the densities of macroinvertebrates in the drift usually were lower in samples collected farthest from the main channel. Seston and drift increased significantly (from ,1 g to 4-20 g ash-free dry mass/m3 and ,10 to 250-1300 ind./m3, respectively) in the initial stages of each flood, but decreased to baseflow levels after ;2 to 3 h. Macroinvertebrates responded passively to the floods, and their densities followed the hysteresis pattern of sediment and organic particles entrained during the course of each flood. The total number of macroinvertebrates drifting during each flood ranged from 33 3 106 to .300 3 106 individuals. The average density of macroinvertebrates stranded in the riparian area after each flood ranged from ;6000 to 22,000 ind./m2. Benthic mac- roinvertebrates were collected from pool, run, bedrock, and riffle habitats in the main channel the day before and the morning after 5 floods to test whether specific habitats provided flow refugia for macroinvertebrates. Floods reduced macroinvertebrate densities by 14% to 92%, averaged across hab- itat types, and the % reduction was related to flood magnitude. Fewer organisms were lost from bedrock habitats (43%) than from the other habitat types, and the most macroinvertebrates typically were lost from pools (.90%). Macroinvertebrate responses (e.g., recovery patterns) changed signifi- cantly between early floods and sequentially later floods, reflecting temporal changes in assemblage composition and abundance.