Environmental and overgrazing effects on diatom communities in high mountain mires.

High mountain mires are affected by anthropogenic activities such as livestock, which have changed land use, fragmented habitats, and affected the structure and functioning of aquatic communities such as diatoms. Diatoms in mires exhibit high diversity, with a significant proportion of rare and threatened species, making them vulnerable to livestock pressure. Despite their common use as indicators of ecological status, little is known about how overgrazing and trampling affect diatom communities. To assess the impact of livestock pressure, we conducted experiments in six high-mountain mires in the Pyrenees, where livestock exclusion fences were installed. We identified a total of 370 species belonging to 60 genera, with a maximum diversity per site of 77 species. The variables most influencing diatom composition were pH, elevation, Bryophyta cover, water cover, livestock pressure, and mire area. Our results indicated an impact on diatom composition at the two locations with higher livestock pressure; however, we observed no effects on species richness or the Shannon index, suggesting that diatom communities are resistant to livestock pressure. The pH was the most important environmental variable for diatom community composition, demonstrating a positive relationship with diatom richness. Livestock pressure in high mountain mires causes habitat fragmentation and increased dry density of the soil, making diatom communities more vulnerable to environmental changes. However, these communities exhibit notable resistance to significant pressure, with pH emerging as the most critical factor influencing their structure. Our study showed that high livestock pressure affects diatom communities in mires. Considering that a high percentage of diatom species are red-listed as vulnerable or endangered, it is important to protect mires to avoid loss of diversity.