Artificial Physical Disturbances on Streambed Sites of a Kenyan Mountain Stream: Effects of Inter-disturbance Intervals on Mayfly Densities

Abstract

Field experiments to determine the effects of inter-disturbance intervals on the densities of mayflies recolonizing disturbed sites in the Naro Moru River, Kenya, were undertaken from June 1993 to January 1994. Stirring, shifting and relocation of streambed substrates by hand was used to define artificial physical disturbance within the randomly selected sites. The sites were exposed to differing interdisturbance intervals, i.e. short, intermediate and long inter-disturbance intervals. Mayfly species abundances were more similar within the disturbed subsites than between disturbed versus control sites. Individual experiments showed that maximal recolonization of the different sites was relatively short (< 8 h) but highly variable. When the three experiments were systematically combined, densities of mayflies recolonizing the disturbed sites were highest on day 19, followed thereafter by a sharp density decline. Mayfly densities on the control sites were highest on days 20 and 40. The effects of short, intermediate and long inter-disturbance intervals on densities of mayflies recolonizing the disturbed sites was bifaceted, which included slight increases and strong declines in the densities.