Some Factors Affecting Drift Rates of Baetis and Simuliidae in a Large River

Abstract

Effects of eight factors on the drift rates of Baetis nymphs and Simuliidae larvae were tested with multiple regression analyses. Illumination, population density of all other organisms and temperature had significant influences on drift rates of both organisms. Turbidity and water—level fluctuations were related to changes in drift rates indirectly through influence upon light penetration and population density, respectively. Dissolved—oxygen concentration, calendar date, and depth of water at the sample site did not clearly affect drift rates of either organism. In areas where population densities were high, the eight factors tested accounted for 65% to 81% of the variability observed in drift rates. Maximum drift rates were 170 x 106 Baetis sp. nymphs (63.2 kg) and 10.9 x 106 Simuliidae larvae (5.4 kg) per day.