Coupling biomonitoring methods to assess trace element deposition: A new interpretative scale of lichen sensitivity to trace element pollution

Details

Supervisors

Agnan, Yannick ; Counoy, Hugo

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en sciences et technologies de l'environnement, à finalité spécialisée

Abstract

enAir pollution, particularly from trace elements (TEs), remains insufficiently characterised at regional scales, especially in urban contexts. Because conventional monitoring networks are limited in spatial coverage and mainly target other pollutants, complementary approaches are needed. This study uses lichens as biomonitors to evaluate TE pollution and build a species-level sensitivity scale. The objective was to develop an empirical sensitivity scale based on the coupling of bioindication (species inventories and frequency analysis) and bioaccumulation (chemical quantification and geochemical indices). At 41 sites across Wallonia, lichen species were surveyed to calculate relative frequencies and alpha diversity indices (LDV, Shannon). In parallel, Xanthoria parietina thalli were sampled and analysed for 13 TEs using ICP-MS. Two geochemical indices were derived: the enrichment factor (EF) and the pollution load index (PLI). Bioindication results showed that diversity decreased in urban and agricultural sites, while forested zones exhibited higher LDV and Shannon indices. NMDS ordination highlighted two dominant ecological gradients, influencing species assemblages. Although indicative of general environmental quality, community composition alone could not disentangle the specific role of TEs. Bioaccumulation revealed significant enrichment for Cd, Zn and Pb, pointing to anthropogenic sources, while Al and Fe had lower enrichment, consistent with lithogenic inputs. ΣEF and PLI captured different pollution signatures: ΣEF reflects cumulative anthropogenic enrichment; PLI relates more directly to toxic pressure and overall pollution. PCA confirmed the presence of three distinct geochemical groups (anthropogenic, lithogenic, mixed). The integration of both approaches in a partial CCA demonstrated that ΣEF had a significant independent effect on species frequency, allowing the derivation of a sensitivity scale. Species were classified as tolerant intermediate or sensitive. Nitrogen-based analyses showed only partial overlap, suggesting that TE sensitivity is a distinct ecological trait. Despite limitations (co-variation with NO₂, limited specificity for some taxa and lack of seasonal replication) this study achieves its aim: establishing a robust, field-based sensitivity scale to TEs in lichen communities.