Nowhere to swim: interspecific responses of prairie stream fishes in isolated pools during severe drought.

Extreme drying can dramatically shift abiotic characteristics of streams and species respond differently as conditions change. We modified a conceptual model describing responses of aquatic invertebrate assemblages to stream drying to facilitate predictions of fish assemblage shifts in which thresholds of physiochemical gradients lead to punctuated losses of fish diversity as stream connectivity diminishes. We tested the applicability of this model during a record-breaking drought that reduced a perennial reach of a headwater prairie stream to a series of 12 isolated pools with variable levels of hyporheic connectivity that was associated with pool volume. We tracked fish assemblage structure in pools over seven weeks and measured abiotic conditions. Inter-specific differences in drought resistance traits were tested in the lab (i.e., thermal and hypoxia tolerance) for common species experiencing drought conditions. Species richness declined rapidly with pool volume associated with hyporheic connectivity. Abundance of the most common species prior to drought, Chrosomus erythrogaster (southern redbelly dace) declined 95% and was extirpated from five pools during drought due to its low thermal tolerance. Despite being the least tolerant to hypoxia, Campostoma anomalum (central stoneroller) populations persisted in most pools. The remaining pools became dominated by a non-native extremophile fish, Gambusia affinis (western mosquitofish) and abundances of less resistant native species declined. Our study demonstrates that richness in drying pools is driven by factors associated with pool volume, and assemblage composition shifts from dominance by native species adapted to variable hydrological conditions to non-native species able to resist increasing temperatures and hypoxia. Sudden species loss associated with reduced stream connectivity can shift abundance patterns of prairie stream fish communities with potentially lagged effects on population and community dynamics. [ABSTRACT FROM AUTHOR]