INFLUENCES OF HYPOXIA AND HYPERTHERMIA ON FISH SPECIES COMPOSITION INHEADWATER STREAMS

Indices of hypoxia and hyperthermia tolerance for Missouri fish assemb lages were based on laboratory measurements of lethal dissolved oxygen concentrations and temperatures, combined with field measures of the relative abundances of tolerant and sensitive species. Fish assemblage s and extreme physicochemical conditions were monitored over 3-4 years at 18 sites on headwater streams in the Prairie, Ozark Border, and Oz ark regions of Missouri. Oxygen minima ranged from 0.8 to 6.0 mg/L, an d temperature maxima ranged from 19.6 to 30.7 degrees C; oxygen minima at study sites were not correlated with temperature maxima. Hypoxia t olerances of fish assemblages were strongly correlated with minimum st ream oxygen concentrations and varied concordantly with regional, long itudinal, and temporal gradients in stream oxygen minima. Hyperthermia tolerances of fish assemblages were not correlated with maximum strea m temperatures, nor were regional, longitudinal, or temporal differenc es in hyperthermia tolerances concordant with variation in temperature maxima. Axis scores from a detrended correspondence analysis of speci es frequencies were strongly correlated with dissolved oxygen minima f or all 18 sites, but axis scores correlated with temperature maxima on ly at the four well-oxygenated sites. Low dissolved oxygen levels had a substantial effect on the composition of fish assemblages at most si tes, but maximum temperatures influenced assemblages only at the few s ites without severe levels of hypoxia.