Distribution, production, and utilization of the benthic macroinvertebrate fauna of Imperial Reservoir on the lower Colorado River, Yuma County, Arizona

Abstract

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.