Relationships were examined between chemical variation associated with additions of acid
mine drainage (AMD) to a stream, and changes in lotit community structure. The experiment
was run in Foxy Creek, close to an AMD seepage source from the Equity Silver Mine, northcentral
B.C. The AMD was metered into a 16 channel, flow-through mesocosm in an
experimental design that included seven replicated AMD dilutions: Control, 1:25000, 1: 10000,
1:2500, 1: 1000, 1:250, 1:25. Alkalinity and pH varied hyperbolically with AMD dilutions. At
dilutions less than 1: 1000, there was clear evidence (at acid neutralizing capacity of the stream
was exceeded and pH dropped abruptly from neutrality. Metals levels increased by several
orders of magnitude with declining dilution of the AMD. Metals which changed most from the
AMD additions were those known to be most toxic to aquatic organisms. Analysis of copper
complexing capacity indicated that at the lowest dilution of AMD (1:25), dissolved organic
ligands were completely hydrolyzed. At higher dilutions, complexation capacity increased, thus
suggesting reduced potential for toxicity at those dilutions. At highest dilutions, other ligands
may have been important in complexing metal cations. Evidence of protonation of organic
ligands at the 1:25 dilution indicated “competition” between H+ and cations including Cd, CU,
and Zn and the possibility that despite the high concentrations of aquo ions, hydrolysis of
exchange sites may have reduced direct toxicity by metal binding. A separate group of
processes that potentially contributed to toxicity included the ingestion of metals bound to a
series of low and high energy binding sites on food particles. Relatively loose sediment bound
metals that included in ranked order; Cd > Zn > CU > Mn could have contributed to toxicity
via ingestion. The abundance of the diatom dominated periphyton community and
mayfly/chironomid dominated benthos declined with increasing doses of AMD according to
logarithmic decay functions. Of the taxon that dominated the benthic community, only the
abundance of the Orthocladiinae did not change as a function of the AMD additions. Insect drift
analyses indicated an immediate avoidance response to the onset of AMD additions, the response
by mayflies occurring within three hours and that by the Tanytarsini within 24 hours. Drift also
increased with higher doses of the AMD. Crude estimates of insect tumover rates indicated that
the mayfly population was transient and the chironomids settled for longer periods in the
troughs. Models of indices of benthic response to the gradient of AMD additions suggested that
the structure of the benthic community in Foxy Creek would be changed with an AMD dilution
of less than or equal to 1:25,000. Comparison of these models to CCREM guidelines indicated
that levels of Cd, Mn, Zn, Al, and Fe that were found to cause change in community structure
as part of a complex metals mixture were less than the critical levels indicated in the CCREM
guidelines. Only copper levels found to cause change as part of the mixture were above the
CCREM guideline. This comparison did not indicate that the CCREM guidelines are wrong.
Rather, this study suggests that models based on community level responses to experimental
dilutions of chemical mixtures are appropriate predictors to be used after an initial screening
using CCREM guidelines, when questions related to systems no less complex than communities
are of interest. The concentration of a metal in solution that causes change in community
structure can be lower when that metal is part of a complex acidic mixture with other metals
compard to when the same metal acts individually on single organisms or simple populations
which is the level of complexity in most tests forming the basis of CCREM guidelines.