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Biological Implications General The primary impetus for physical, chemical and biological research in the upper Colorado River basin is concern about the recovery of native fish species. Ultimately, it must be determined what the critical food, water and habitat requirements are for individual species of concern. Algae and invertebrates in the streambed, or benthos, provide the food base for native fish. The structure of the benthic community of streams and rivers is determined by the interaction of a number of critical factors, both biotic and abiotic. Biotic factors include predation and competition. Abiotic factors include substrate characteristics and water quality characteristics, e.g., temperature, dissolved oxygen, nutrients, light penetration, etc. In some systems, an additional abiotic factor, disturbance, can strongly influence the benthic community by preventing it from maturing to the point where the structure is largely determined by the biotic factors of competition and predation. In systems where disturbance is frequently repeated, the community may be in a constant state of dynamic equilibrium, where change is the norm, and a climax community is rarely reached. Such is likely the case in gravel-bed rivers, where disturbance occurs on a near-annual basis when flood flows mobilize the benthos. The following two sections discuss the biological implications of substrate particle size and embeddedness and the potential importance of disturbance. Embeddedness and Substrate Particle i From a biological standpoint, relative DTE, measured in number of `free rocks', may be more relevant than total DTE, given that many small rocks should provide more surface area for invertebrates to exploit than a few large rocks. Bjornn et al. (1977) experimentally introduced fine sediment into laboratory streams and found that the degree of embeddedness had an important influence on insect abundance. When starting with a zero to one-third embeddedness (analogous to a 0.67-1.0 relative DTE), insect abundance declined by about 50% when embeddedness exceeded one-third (< 0.67 relative DTE). When one streambed plot was later cleaned of fine sediment, mayflies and stoneflies increased by up to eightfold. In rapidly flowing water all but the coarse substrate particles are washed away, resulting in an invertebrate fauna adapted for attachment, clinging or avoidance of the current. Cummins and Lauff (1969) concluded that although current, temperature or concentration of a chemical factor may limit the general ranges of habitat tolerance (macrodistribution) in aquatic insects, it was substrate particle size or food supply that probably exerted the primary microdistributional influences. Rabeni and N inshall (1977) studied the microdistribution of stream insects in relation to current velocity, substratum particle size, silt, and detritus utilizing substratum-filled trays in field experiments. They found 31% more organisms occurred in riffles than in pools and that 26