Acid mine drainage (AMD) is a major environmental issue worldwide. On the West Coast of the South Island, New Zealand, numerous catchments receive AMD, with significantnegative impacts on in-stream flora and fauna. Diatoms are commonly regarded as powerful biological indicators and may be found in high abundance in AMD-contaminated streams; however, relatively little work has been done on diatoms in mining environments in New Zealand. Initially, I conducted a survey of epiphytic diatom communities in 39 streams ranging from non-impacted reference streams to those severely impacted by AMD. Streams were assigned to one of four classes along an AMD gradient: circum-neutral reference, naturally acidic reference, moderately impacted, and severely impacted. There was a wide range in diatom taxonomic richness in reference and moderately impacted streams (8 – 33 taxa). Taxonomic richness was greatly reduced in severely impacted streams (1 – 5 taxa) at a threshold of pH 3.4 and was dominated by Pinnularia cf. acidophila (69 – 100% relative abundance). Community composition differed between circum-neutral reference, moderately, and severely impacted streams; however, naturally acidic and moderately impacted streams had similar diatom communities primarilycomposed of acid-tolerant Eunotia and Frustulia species. This indicated that diatoms are strongly structured by pH and able to tolerate moderate conductivity and metal concentrations without a corresponding shift in community composition. Survey data were then used to develop two diatom-based indices for streams impacted by AMD: a single Biotic Index and a Multimetric Index. While neither index was able to distinguish naturally acidic from moderately impacted streams, both indices successfully categorised streams ascircum-neutral reference, moderately or severely impacted by AMD. These indices may be useful in assessing AMD impact on circum-neutral streams or in identifying when a stream has crossed a threshold from moderately to severely impacted by AMD. Diatoms would be especially useful as bioindicators of AMD if they respond rapidly to a change in mine discharge. To test this, mature algal biofilms were reciprocally transferred between circum-neutral reference streams and streams of varying degrees of AMD over a period of 13 days. Diatom mortality increased rapidly in the reciprocal transfer between reference and severely impacted streams. Reference communities resembled the ambient diatom community of severely impacted streams 13 days post-transfer. However, in the reverse transfer, a change in community composition was slow to occur. Diatoms respond faster to an increase in pollution than to pollution amelioration. Overall, results indicated that diatom communities may be a useful tool for monitoring the presence and magnitude of AMD in New Zealand streams.
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