Hydrologic variation influences stream fish assemblage dynamics through flow regime and drought

Area of Science:

• Ecology
• Hydrology
• Ichthyology

Background:

• Stream fish assemblages are structured by hydrologic variation.
• Flow regimes influence the relationship between stream hydrology and fish biology.
• Understanding these dynamics is crucial for aquatic biodiversity conservation.

Purpose of the Study:

• To test the hypothesis that variable flow regimes result in lower species richness, higher turnover, lower assemblage stability, and stronger abiotic-fish relationships compared to stable flow regimes.
• To investigate the influence of hydrologic variation on fish assemblage structure and dynamics in Ozark streams.

Main Methods:

• Seasonal sampling of fish assemblages in Runoff/Intermittent Flashy (variable flow) and Groundwater Flashy (stable flow) streams over two years (drought and wet).
• Backpack electrofishing and three-pass removal techniques to estimate fish species richness, abundance, and density.
• Mixed-effects models and redundancy analysis to assess relationships between hydrology, habitat, season, and fish assemblage characteristics.

Main Results:

• Fish species richness and abundance were relatively stable, but densities fluctuated with habitat volume.
• Hydrologic variation, not flow regime, influenced seasonal relationships between fish and habitat.
• Seasonal fish species turnover was higher in the drought year.
• Assemblage stability was lower in variable flow streams during the drought year.
• Fish species densities were clearly separated by flow regime, with distinct species groups (periodic, opportunistic, equilibrium) characterizing each stream type.

Conclusions:

• Spatial and temporal hydrologic variation strongly influences fish assemblage dynamics in Ozark streams.
• More hydrologically variable streams exhibit lower assemblage stability and greater density fluctuations.
• Understanding hydrologic variation's impact on fish assemblages is vital for biodiversity conservation.
• Future research should address the effects of altered hydrologic variation on biotic assemblages.