Our objectives were to describe the spatial distribution of invertebrates within the South Platte River basin (62,900 km²) and determine factors affecting this distribution. This study is a part of the U.S. Geological Survey's National Water Quality Assessment program: South Platte River Study Unit. Invertebrates were collected between July 27-August 7, 1992 at 23 sites ranging in altitude from 850-3132 m using a surber sampler (210-µm mesh), ekman dredge, or by cutting snags. Invertebrate densities ranged from 105 to 18,435/m² and varied throughout the basin; greater numbers were found on stable substrates. In contrast, the # of taxa/site decreased along the altitudinal gradient from the mountains (mean=27) to the plains (mean=14). Shannon-Weaver Diversity Index ranged from 2.19 to 3.80 in the mountains and 0.84 to 3.19 in the plains. Dominant taxa were Plecoptera, Ephemeroptera, Trichoptera, and Diptera in the mountains and Diptera, Ephemeroptera, and Oligochaeta in the plains. Differences in # of taxa, diversity and dominant taxa between the mountains and plains may be due to a combination of factors, such as cool water temperatures, low nutrient concentrations, predictable flow, and stable substrates (cobble/boulder) in the mountains compared to warm temperatures, higher nutrient concentrations, unpredictable flow, and shifting sand/gravel substrate in the plains. Nutrient chemistry appeared to have little direct effect on the invertebrate distribution.
1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225