Variation in benthic community structure: the role of the macroenvironment along a downstream gradient.

McKittrick Creek, a discontinuous mountain-desert stream in Guadalupe Mountains National Park, Texas, was sampled at discrete 1 ocations(pools ) to test for differences among biomass, density, species diversity, equitability, and species richness in the benthic macroinvertebrate community. Significant differences occurred among pools for the biotic characteristics of the benthic community along the downstream gradient. Connected pools were more similar to each other than pools separated by dry reaches. The greatest biomass (700-1200 mg dry weight /m ) and species diversity (2.5-3*2) values occurred at the two furthest downstream pools and two furthest upstream pools, while the least biomass (150-600 mg dry weight/m ) and species diversity (2.2-2.14) values occurred at two midstream pools. Equitability (0.65-0.80) and species richness (11.0-7.6) values varied considerably among pools, but were generally greatest in downstream pools. Density values also varied considerably among pools (2500-14000 organisms/m ) with the greatest density at the furthest downstream pool. Variation in macroenvironmental characterist i cs associated with food availability was more important than substrate size and fish predation in determining the benthic community structure of McKittrick Creek. Variance in biomass and density of the benthic community was explained best by differences in quantities of coarse detritus. Pools with greater quantities of coarse detritus had less invertebrate biomass and density. Variance in species diversity and species richness values was explained best by changes in periphyton biomass. Variance in equitability values was not associated with any macroenvironmental characteristics. Although the type of food available (e.g., coarse detritus or periphyton) explained most of the variance among pools for benthic community characteristics, interaction among available food type, substrate size, and predation by fish may actually determine the benthic community structure of some pools