Impact of headwater stream acidification on the trophic structure of macroinvertebrate communities

In order to investigate the impact of freshwater acidification on the troph ic structure of macroinvertebrate communities, we performed a study on 22 f orested headwater streams characterised by different degrees of acidificati on (mean pH = 4.49 to 6.98). Results showed that in acidic streams all func tional feeding groups were affected in terms of taxonomic richness. As far as the population density was concerned, only a few acid-tolerant taxa of s hredders and predators showed an increasing abundance under acidic conditio ns. Trophic structure of acidified streams appeared to be deeply impacted w ith a large contribution of shredders and a complete disappearance of scrap ers. In contrast, in circumneutral streams, we found that each functional f eeding group had an almost equal share of the trophic web.