Aravaipa Creek is one of the few, small, permanent desert streams in Arizona. It has diverse fish and invertebrate faunas but the invertebrates are not well-known. I collected benthic and 24 hr. drift samples there during 1980-1981 and found 31 taxa. The most abundant species were Baetis insignificans and Choroterpes inornata (Ephemeroptera), which exhibited the nocturnal drift periodicity characteristic of drifting invertebrates studied elsewhere. In the laboratory, observations of the drifting behaviors of five species of mayflies were made in 5 and 10 cm/s current speeds. These provided evidence that drifting insects are not necessarily swept away by the current (accidental drift), but are probably exhibiting a specific behavior triggered by local conditions such as lack of cover or food (active drift). I also tested the relationship between drift rates and the presence and absence of food (periphyton) using B. insignificans and C. inornata. With both species, the number of insects drifting increased significantly when periphyton was not present; therefore lack of food may be an important cause of drift. The sizes of the nymphs used in these experiments were determined; I found that drifters were either similar in size to non-drifters (C. inornata), or that drifters were larger than nymphs that did not drift (B. insignificans). These results suggest that direct competition by size does not play a role in determining which individuals drift. Drifting appears to be a dispersal mechanism for benthic invertebrates; it maintains the numbers and distribution of the benthos according to the rapidly changing distribution of their food resources which occurs in streams. This process fits some recent models of population regulation by dispersal.

Data for a study of the benthic macroinvertebrates in Imperial Reservoir in Yuma County, Arizona was collected 1970-1973. Imperial Reservoir is an old, heavily sedimented reservoir on the lower Colorado River. An extensive dredging program revealed that the benthic habitat supported a low number of invertebrate species (four insects and two oligochaets) in comparison to other bodies of water. Probable reasons for the low number of species included high salinity, low organic detrital input into the reservoir, low habitat and substrate diversity, temporary low dissolved oxygen concentration, possible contamination with agricultural chemicals, and the remoteness of Imperial Reservoir from other aquatic environments. Within Imperial Reservoir the benthic macroinvertebrate community was restricted to more isolated, calm, side lakes which altogether made up only 23% of the entire surface area of the reservoir. Within these side lakes benthic macroinvertebrates were found primarily on mud substrate. Invertebrates were not found beneath dense stands of Najas marina, a rooted submersed macrophyte. Only two species of invertebrates appear to inhabit rocky substrates. Community analysis suggested that there were no large differences among most habitats where benthic invertebrates were found. A study of the microdistribution of each species of invertebrate indicated that there were two patterns of spatial dispersion: (1) taxa which bred continuously appeared to have no life history related change in dispersion; and (2) taxa which formed recognizable cohorts appeared to spread out from initial egg mass and dispersion changed with time from clumped to random. Secondary net production measures were made directly on predominant benthic invertebrates at two sites in Imperial Reservoir. Both sites had similar production values which were low when compared to production values from other bodies of water. Low benthic production in Imperial Reservoir was related to several environmental factors including high water temperature and temporary low oxygen concentration near the bottom during the summer, low input of organic detritus into the reservoir, and high predation on benthic invertebrates by fish. Collection and analysis of the stomach contents of the bass, bluegill and redear sunfish revealed that the smaller fish of a species were more dependent on benthic invertebrates for food. Large bass did not use benthic invertebrates as heavily as either bluegill or redear.