Improving the interpretation and applicability of epiphytic lichen biomonitoring for air quality assessment

Public Thesis Defense of Hugo Counoy 

"Improving the interpretation and applicability of epiphytic lichen biomonitoring for air quality assessment: 

Advances in bioindication, bioaccumulation, and citizen science" 

Summary 

Air pollution remains a major threat to human health and ecosystems, yet conventional monitoring networks are costly, too sparse to capture fine-scale variation in exposure, and restricted to a few pollutants. Biomonitoring with lichens offers a complementary approach, since these organisms respond to atmospheric conditions, integrate many contaminants over time and can be surveyed across large areas. 

However, inferring air quality from the composition of lichen communities (bioindication) remains hindered by identification difficulties and incomplete knowledge of how species respond to individual pollutants, while the contaminants accumulated in lichen tissues (bioaccumulation) are hard to trace back to their sources, notably because they reflect integrated and partly legacy inputs. 

This thesis aims to improve the interpretation and applicability of lichen bioindication and bioaccumulation. 

First, a European meta-analysis identifies species with consistent responses to NH3, NOx, SO2 and climate variables. 

These responses are then translated into operational indices which, tested in Wallonia (Belgium), partially discriminate NH3 from NOx. In the same region, trace elements accumulated in herbarium and contemporary samples of Xanthoria parietina reveal a marked, element-specific decline in enrichment since the 1960s and identify industry, non-exhaust traffic, and dust resuspension as the main current sources. Analyzing nitrogen content and δ15N indicates that isotopic signatures alone cannot discriminate sources in multi-source landscapes and vary with spatial scale. 

Finally, to open biomonitoring to non-specialists, the Lichens GO citizen science protocol is evaluated and adapted: targeted changes to sampling and identification procedures improve data quality without increasing sampling effort. 

Overall, this thesis supports a more pollutant-specific, source-informed and widely deployable use of lichens to complement conventional monitoring.

Jury members 

Prof. Yannick Agnan (UCLouvain), Supervisor
Dr. Laure Turcati (Sorbonne Université), Supervisor
Prof. Emmanuel Hanert (UCLouvain), Chairperson
Prof. Guillaume Lobet (UCLouvain), Secretary
Prof. Gabrielle Martin (Toulouse Univ., Fr.)
Prof. Nadine Mattielli (ULB)
Prof. Nathalie Séjalon-Delmas (Toulouse Univ., Fr.)