Aquatic Macroinverterbrates of the Piceance Basin, Colorado: Community Response along Spatial and Temporal Gradients of Environmental Conditions

Abstract

A long-term study of the aquatic biotopes of the Piceance Basin has enabled ana- lyses of macroinvertebrate communities along spatial (downstream) and temporal (year-to-year) gradients of environmental conditions. Cluster analysis indicates four distinct aquatic habitat types: spring sources, springbrooks, and the middle and lower reaches of Piceance Creek. Spatial differences in temperature and flow constancy influence habitat stability with respect to substrate, degree-day distribution, ice conditions, and the distribution and abundance of aquatic macro- phytes. Despite great differences in thermal patterns, total annual degree-days were ca. 3000 in all four habitat types. The principal energy source for macroinvertebrates is autochthonous detritus. Trophic structure shifts from shredders-collectors-scrapers in the headwaters to collectors- predators in lower reaches. Year-to-year variations in discharge altered the density and composi- tion of macroinvertebrates, and were reflected in the thermal regimes at mainstream locations. Data suggest that groundwater discharge from upper reaches is important in maintaining the structural and functional integrity of aquatic systems in the Piceance Basin. Because of the seasonal distribution of precipitation and runoff, headwater streams in arid regions are highly dependent on groundwater discharge for permanent flow (Davis, 1974). Stream segments receiving proportionately large inputs of groundwater exhibit relatively constant thermal, chemical and, flow regimes. With progressive distance downstream these conditions are modified by climatic fluctuations, thus forming an environmental gra- dient that influences the distribution and abundance of aquatic organisms (Deacon and Minckley, 1974). Superimposed upon this spatial gradient is a temporal gradient engendered by year-to-year variations in precipitation which, in arid regions, is most directly reflected by differences in annual dis- charge.