Extreme flow events influence habitat and fish assemblage structure of groundwater dominated systems


Abstract

Background. Extreme flow events structure aquatic communities, but their effects in hydrologically stable, spring-fed rivers are less understood. These systems are often viewed as refugia for endemic fishes, yet their resilience to floods and drought remains uncertain.
Methods. We analyzed a 9-year dataset (2014–2022) from the San Marcos and Comal rivers, Texas, to evaluate the effects of a major flood and a severe drought on habitat structure and fish assemblages. Habitat variables were surveyed alongside standardized fish sampling in wadeable and non-wadeable reaches, and fishes were grouped into habitat guilds for analysis.
Results. Habitat conditions were largely resistant to flood and drought, though vegetation cover increased in wadeable reaches during drought. Fish responses were guild and habitat-specific: pelagic generalists declined during floods in wadeable reaches and during drought in non-wadeable reaches, while pelagic specialists decreased during floods in wadeable habitats. In contrast, Micropterus and the federally listed Fountain Darter (Etheostoma fonticola) increased during drought, coinciding with greater vegetation cover.
Conclusions. Groundwater-fed rivers buffered habitat from extreme flows, but fish assemblages still shifted under flood and drought, revealing vulnerabilities among pelagic species and drought-associated increases for benthic taxa. These results show that spring systems provide resistance to climatic extremes but remain susceptible to assemblage restructuring, with implications for conservation under future climate variability and groundwater extraction.
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