Benthic Population Distributions in Red Rock Reservoir, Iowa Summer 1972

46 leaves.The problem. To determine the distribution of benthic invertebrates in Red Rock Reservoir and to relate this distribution to the physiochemical characteristics of the substrate and bottom meter of water along with water retention time after three years of impoundment. Procedure. Benthic samples were obtained at weekly intervals with a Ponar dredge from two transects of six stations each from June 9 to August 1, 1972. Bottom sediments were analyzed for organic content, pH and particle size. Bottom waters were analyzed for dissolved oxygen content, alkalinity and free carbon dioxide. Depth and temperature readings were recorded at each station. Findings. During the study three major groups comprised the benthos. Of the total numbers collected during the study Oligochaeta comprised 52.4%, Chironomidae 44.8% and Chaoborus 2.8%. Total mean numbers collected ranged from 24 to 212 organisms/m2. Thermal and chemical stratification of the bottom waters occurred temporarily. Conclusions. The benthic standing crop in Red Rock Reservoir has increased since 1970 but the amount of biomass present and the mean density of organisms was below values obtained from other reservoirs. Low numbers of Chaoborus present were due to emergence patterns. Factors which appeared to influence benthic population distributions include substrate organic content and particle size, low water retention time and presence of the old river channel. Recommendations. Future studies should determine the exact species composition of the benthos and how life cycle patterns are related to benthic biomass production and distribution. The influence of fish predation and sedimentation and the loss of production via reservoir flowthrough should be investigated