Do bio-physical attributes of steps and pools differ in high-gradient mountain streams?

Abstract

The hydrologic and physical structure of streams strongly influences the biological composition of benthic macroinvertebrate communities. Research on step–pool systems in high-gradient streams has focused primarily on physical processes rather than on ecological characteristics. This study examined both the biological and physical attributes of 27 step–pool sequences in three steep mountain streams of the Smith River Basin in northern California, USA. Multivariate analysis using non-metric multidimensional scaling (NMS) found a biological separation of step and pool sites (based on benthic macroinvertebrate data) in all three study watersheds. Step habitats had greater taxa richness, diversity, %Plecoptera, %Heptageniidae, %Nemouridae, and %clingers compared to pools. Steps also differed from pools in physical characteristics such as grain size distribution, whereby steps were dominated by boulders compared to pools characterized by gravel and cobbles. Moreover, steps had higher dissolved oxygen, greater water velocity, and shallower water depths compared to pools. NMS ordinations showed a correlation between physical factors and biological communities. These results suggest the ecological importance of step–pools streams, in that the development of step sequences creates and maintains a repetitive pattern of high-quality ecological environments.