Effect of Substrate Composition on Detritus Accumulation and Macroinvertebrate Distribution in a Southern Nevada Desert Stream

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Abstract
A 32-day colonization experiment was conducted to assess the effect of substrate particle size on the accumulation and size distribution of particulate detritus and on the distribution and abundance of benthic macroinvertebrates in a southern Nevada desert stream. Wire mesh baskets containing either gravel (mean particle diameter = 11.5 mm), pebble (mpd = 32.9 mm), cobble (mpd = 67.9 mm), or a mixture of these three sizes were used to represent substrate patches with different physical characteristics. The smallest substrate (gravel) collected the greatest quantity of fine detritus (< 1 mm) and was colonized by the greatest number of macroinvertebrates. The largest substrate (cobble) collected the smallest quantity of fine detritus and the largest quantity of coarse detritus (> 1 mm) and was colonized by the lowest number of macroinvertebrates. Although macroinvertebrate abundance was significantly higher in the gravel substrate, macroinvertebrate biomass was greatest in the cobble substrate indicating a difference in the size of organisms colonizing the different substrates. There was a significant positive correlation between macroinvertebrate abundance and quantities of fine detritus but no correlation between macroinvertebrate biomass and detritus quantities. The benthic macroinvertebrate assemblage was dominated by small-bodied collector-gatherer insects, dominated by Baetis sp. (Ephemeroptera: Baetidae) and Orthocladiinae (Diptera: Chironomidae), indicating the importance of this trophic group and their fine detrital food resource to the structure and function of desert stream ecosystems.

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