Stream periphyton and benthic insect responses to additions of treated acid mine drainage in a continuous-flow on-site Mesocosm

An on-site continuous-flow trough mesocosm was used to examine changes in the composition and abundance of periphytic algae and benthic invertebrates from additions of a solution of treated acid mine drainage (AMD). Five control and five treatment troughs supported an assemblage of periphyton and invertebrates that colonized from water withdrawn from Foxy Creek, a stream that receives limed AMD from the Equity Silver Mine, central British Columbia, Canada. A water intake for the mesocosm was located upstream of the AMD discharge. The treated AMD was delivered to the apparatus through a pipeline laid in a canal that carried the AMD to Foxy Creek. After three weeks of colonization in the troughs, additions of one part AMD to 10 parts Foxy Creek water was delivered to the treatment troughs and continued for three weeks. Analyses of variance of measurements of abundance and biomass indexes contained high power values and indicated that the AMD addition did not significantly change the algal and insect composition and abundance. Advantages and disadvantages of the mesocosm with regard to the relative sensitivity of the measured parameters for use in examining effects of the AMD additions are discussed. The conclusion was that quantitative on-site experimentation using the mesocosm apparatus is a powerful approach in setting guidelines for AMD discharge mainly due to its capability of integrating ecosystem processes in experiments where hypothesis testing is possible.