RESISTANCE OF LOTIC ECOSYSTEMS TO A LIGHT ELIMINATION DISTURBANCE - A LABORATORY STREAM STUDY

Black plastic was placed over laboratory stream channels for 92 d to examine the resistance of lotic ecosystems to light elimination. Prior to the disturbance, 4 treatments (high grazing-recirculated flow, high grazing-once-through flow, low grazing-recirculated flow, low grazing-once-through flow) were imposed on the streams, resulting in systems with different biomass levels and recycling indices. Light elimination resulted in significant declines of all functional and most structural properties associated with the streams. Declines in species diversity and number were greater in high grazed streams, but high grazed streams appeared more resistant with respect to autotrophic biomass and carbon fixation. The relatively small effect of the treatments on system resistance compared with the large influence of light elimination suggests that resistance may be more dependent on the qualities of the disturbance (eg magnitude and duration) than those of the system. -from Authors