Temporal response of stream benthic macroinvertebrate communities to the synergistic effects of anthropogenic acidification and natural drought events

1. We evaluated whether the surprisingly weak biological recovery associated with declines in acid deposition were related to drought-induced re-acidification of streams. We used test site analysis (TSA) to characterise temporal changes (1995-2003) in the degree and nature of impairments to stream benthic macroinvertebrate (BMI) communities influenced by acid deposition and drought. 2. The BMI communities in four historically impacted test streams were compared with communities in 30 minimally impacted reference streams. Six multivariate (e.g. ordination axes scores) and four traditional (e.g. % Diptera) summary metrics were used to describe BMI communities. Using all metrics simultaneously (i.e. Mahalanobis or generalised distance), the TSA provided a single probability that a test community was impaired. If a test community was significantly impaired, a further analysis was done to identify the metric(s) important in distinguishing the test community from reference condition. 3. Results of the TSAs indicated that the generalised distances between test communities and the reference condition were inversely related to stream water pH (n = 36). The TSAs also indicated ordination metrics based on BMI abundance were important in distinguishing significantly impaired communities from reference conditions. Temporal trends indicated that there has been short-term recovery of these BMI communities, but that overall improvements have been hampered by acid or metal toxicity associated with drought-induced re-acidification of the streams. 4. Our use of a variety of summary metrics to obtain a single statistical test of significance within the context of the reference-condition approach provided a simple and unambiguous framework for evaluating the biological condition of test sites.