Temporal variability of stream macroinvertebrate abundance and biomass following pesticide disturbance

We determined the extent of macroinvertebrate recovery in a former pesticid e-treated stream (FTS) relative to a reference stream (RS) by examining mac roinvertebrates colonizing red maple (Acer rubrum L.) litter bags between 5 to 10 y following pesticide treatment. Mean abundance and biomass, variabi lity in abundance and biomass (using the coefficient of variation [CV]), an d assemblage structure were compared both within and among years to assess recovery. The 5 y of study included 3 drought years followed by 2 wet years . Mean total abundance and biomass of macroinvertebrates, and that of most functional feeding groups (FFG) did not significantly differ between stream s during this study, nor did within-year variability of these means, indica ting macroinvertebrates in FTS had recovered relative to RS. Some exception s to the above patterns (shredder abundance and mean annual gatherer biomas s) resulted from the dynamics of a single taxon in each group (Leuctra and Paraleptophlebia: respectively). Macroinvertebrate assemblage structure in litter bags was similar between streams throughout the study as shown by th eir similar ordination scores; hence, assemblage structure had also recover ed. In each stream, mean annual abundance and biomass of total macroinverte brates and of each FFG, aside from shredder abundance, differed significant ly among years. However, assemblage structure was generally similar among y ears. Among-year CVs were usually lower than within-year CVs because macroi nvertebrate abundance and biomass fluctuated more during a year than it did from year to year, and different processes apparently contributed to the v ariation observed at these 2 time scales. For example, juvenile development time influenced within-year CVs, indicating that life-history characterist ics affected temporal variability of macroinvertebrate abundance and biomas s. Examination of both the means and their variances was useful for determi ning the extent of recovery and how macroinvertebrates responded to natural environmental variability. The detailed analysis of temporal dynamics at d ifferent time scales afforded by the CV supported our contention that FTS h ad recovered from the pesticide application relative to RS.