The Effects of Temporal
Heterogenety on the Structure of Stream Fish Assemblages within Isolated Pools of the
Upper Brazos River, Texas
Kenneth G. Ostrand and Gene R. Wilde Department
of Range, Wildlife, and Fisheries Management Box 42125 Texas Tech University Lubbock,
Texas 79409 E-Mail c7okg@ttacs.ttu.edu
Keywords: community structure; environmental
variability; fish assemblages; streams; Texas
The Brazos River upstream from Possum Kingdom Reservoir,
Texas, is largely intermittent. During summer dry spells, the river consists of a series
of isolated pools with varying physical and chemical characteristics. We examined fish
assemblage patterns in dry-season pools to examine two hypotheses: (1) that physical and
chemical variables within dry-season pools structure species assemblages, (2) species
composition and abundance in dry-season pools change in response to changes in
environmental factors. We found that species assemblages within dry-season pools were
structured by maximum depth, turbidity, salinity, ammonia, and pH levels. Cyprinids had
higher relative abundance within deeper pools having higher pH, and turbidity. Whereas
shallower dry-season pools having greater salinity, and ammonia levels had higher relative
abundance of cyprinodonts (P < 0.05). Within pools temperature and conductivity
increase through time whereas pH and maximum depth decrease (P < 0.05). As these
environmental factors change cypinids relative abundance decreases while cyprinodonts
increase in relative abundance (P < 0.05). Changes in species assemblages within
individual pools paralleled those observed among pools on a larger geographic range.
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