A stream “mesocosm” was used to support an experiment designed to examine the
effect of additions of treated acid mine drainage (AMD) on the composition and
abundance of periphytic algae and aquatic invertebrates downstream of AMD discharge
from the Equity mine site. An apparatus consisting of 10 flow-through troughs, suitable
for invertebrate and periphytic algal colonization and growth was installed and tested at
the site. After three weeks of running stream water without AMD through 10 troughs to
allow colonization by stream invertebrates, additions of treated AMD at an operational
dilution rate of 10% to 5 randomly allocated troughs was started and continued for three
additional weeks. Water, algal, and insect (drift, adult emergence, and benthos) samples
were collected from the 5 treated and 5 untreated troughs during and at the end of the
6-week experiment for examination of the effects of the AMD addition on indices of
invertebrate abundance and algal growth. Results showed that the experimental approach
using a mesocosm apparatus was highly sensitive and yielded analyses of variante with high
power. Results showed conclusively that at the operational 10% AMD dilution rate, the
addition of treated AMD to Foxy Creek does not impact on aquatic insect composition
and abundance. The success of applying the mesocosm approach within a rigid and well
structured experimental design in this study is discussed with respect to its application to
many water quality problems. It can be a powerful tool to accurately determine ecosystem
response curves for effluent discharges during mine operations or at closure. It would also
be ideal for exploring alternative AMD treatment strategies at specific sites. An important
advantage of the method over more common single-species bioassays, is the power of
predicting changes in ecosystem functioning which is fundamental for establishing water
quality criteria.