Spatial Distribution of Planktonic Copepoda and Cladocera in Red Rock Reservoir, Iowa, Summer, 1972

Abstract

The problem. The measurement of spatial distribution of Copepoda and Cladocera in Red Rock Reservoir, Iowa, summer, 1972, was undertaken. Water temperature, Secchi disk, and total water depth readings were also measured. Procedure. Volumetric samples of free-swimming zooplankton were collected at weekly intervals from June 6 to September 8, 1972. Collections were made at three stations on each of four transects representing different areas of the reservoir. Samples were obtained from one meter intervals from the surface to three meters. Findings. Sater temperature varied from a low of 18.5 C to a high of 30.0 C. Secchi disk readings ranged from 6 to 70 cm. Maximum readings were obtained near the dam and decreased towards the headwaters. Retention time varied from 3.9 to 55.6 days with a study mean of 10.7 days. A gradual increase in zooplankton numbers was noted from June 16 until a high was reached on July 5 (356 organisms per liter). The numbers then rapidly decreased until a low was reached on August 15 (0 organisms per liter). The dam transect always had the highest numbers of organisms with headwater transects having lower numbers of organisms. Eight species of zooplankton were identified during the study. Conclusions. Highest zooplankton populations occurred near the dam with smaller numbers present up reservoir. Numbers of zooplankton per liter and retention times were lower than during a 1970 study. Retention time was the main controlling factor on zooplankton development. No correlations were found between zooplankton numbers and temperature, light penetration, and depth. Recommendations. More station sites and more frequent sampling could enable the study of horizontal distribution of zooplankton. Monitoring reservoir inflow and outflow would enable the determination of population development within the reservoir. The interaction of zooplankton with phytoplankton and macrophytes should be investigated.