1. Food web properties can to be used in bioassessment as indicators of ecosystem stress, although logistical constraints restrict their widespread use. Size spectra (body mass – abundance relationships) are easier to produce, still incorporate much of the variation in feeding interactions and indicate the strength of the energy transfer efficiency. 2. Here we examined the effect of acid mine drainage on the size spectra of stream macroinvertebrate communities in 25 New-Zealand streams with a comparative survey. We predicted that the largest sized organisms would be most susceptible to acid mine drainage, leading to a reduction in their abundances and associated decrease in the range of body sizes present across the gradient, as well as a reduction in total community abundance. 3. The largest sized organisms were more sensitive to inputs of acid mine drainage, and were absent at the most affected sites. Surprisingly, the smallest body sizes were also removed by acid mine drainage. This led to a reduction of up to two orders of magnitude in the range of body sizes present in mine impacted sites. Total community abundance decreased along the impact gradient. 4. The changes in size spectra were also associated with changes in the proportion of functional feeding groups, suggesting concomitant changes in food web structure. Specifically communities became dominated by collector browsers and small bodied predators across the gradient. The simplification of the food web structure suggests that communities may be dominated by a few strong energy pathways, lowering their functionality and stability. However, the loss of large bodied predators also reduces top down pressure, likely increasing community stability. Further research is needed to elucidate the cumulative effects of these interacting processes.