Changes in lake macrophyte communities over three decades: Tracing the impacts of global change in shallow Alpine lakes of the Pyrenees.

Submerged vegetation constitutes a key component of lake biodiversity, generating structure, regulating biogeochemistry at lake interphases, and providing habitat, refuge, and food for a multitude of organisms. However, macrophytes are undergoing a global decline. High mountain lakes, due to their extreme sensitivity to environmental changes, constitute especially interestingsystems for studying the response of macrophyte communities to global change. This study investigates the changes in macrophyte communities in thirty Pyrenean lakes over the past three decades, based on semiquantitative inventories on repeated transects across two different periods. Changes in the occurrence, coverage, and depth distribution of eighteen macrophyte species were studied. Changes in species richness and spatial beta diversity between periods were examined, and multivariate analysis were used to study both community level changes and water chemistry shifts, as well as the relationship between the two. The potential effects of minnow ( Phoxinus sp.) introductions on water chemistry and macrophyte communities were also evaluated. The results show a relatively stable flora at the regional scale, with no significant changes in richness or spatial beta diversity, suggesting no homogenization. However, several notable trends emerged. Few natopotamids, typical from more eutrophic waters, have expanded, while mosses, indicators of extreme oligotrophy, and some floating-leaved species have regressed. Increases in conductivity and alkalinity, alongside a general reduction in nitrogen forms, were recorded. No significant relationship was found between the presence of minnows and water chemistry shifts. Nevertheless, greater community changes tend to occur when conductivity has exceeded a threshold of 20 µS/cm.