Diversity and functions of leaf-decaying fungi in human-altered streams
Résumé
1. Stream conditions have been evaluated using leaf breakdown, and aquatic hyphomy- cetes are a diverse group of fungal decomposers which contribute to this process. 2. In field surveys of three pairs of impact-control stream sites we assessed the effect of eutrophication, mine pollution and modification of riparian vegetation on alder leaf breakdown rate in coarse and fine mesh bags and on mycelial biomass, spore production and species diversity of leaf-colonizing fungi. 3. In addition, we gathered published information on the response of leaf-colonizing fungi to these three types of perturbations. We conducted a meta-analysis of 23 published papers to look for consistent patterns across studies and to determine the relevance of four fungal- based metrics (microbial breakdown rate, maximum spore production, maximum mycelial biomass and total species richness) to detect stream impairment. 4. In our field surveys, leaf breakdown rates in coarse mesh bags were lower at impact than at paired control sites regardless of perturbation type. A similar trend was observed for leaf breakdown rates in fine mesh bags. Mycelial biomass and spore production were higher in the eutrophied stream than in the control stream. Spore production was depressed in the mine polluted stream, while it was slightly enhanced in the stream affected by forestry. Fungal diversity tended to be lower at impact than at paired control sites, though the mean and cumulative species richness values were often inconsistent. 5. Results of the meta-analysis confirmed that mine pollution reduces fungal diversity and performance. Eutrophication was not found to affect microbial breakdown rate, maximum spore production and maximum mycelial biomass in a predictable manner because both positive and negative effects were reported in the literature. However, fungal species richness was consistently reduced in eutrophied streams. Modification of riparian vegetation had at most a small stimulating effect on maximum spore production. Among the four fungal-based metrics included in the meta-analysis, maximum spore production emerged as the most sensitive indicator of human impact on streams. 6. Taken together, our findings indicate that human activities can affect the diversity and functions of aquatic hyphomycetes in streams. We also show that leaf breakdown rate and simple fungal-based metrics, such as spore production, are relevant to assess stream condition.
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