Fungal diversity in a mountain lake is explained by weather patterns

Fungi are an important component of aquatic ecosystems because of their multifaceted roles such as organic matter decomposition, nutrient cycling and energy transfer to higher trophic levels. Despite their importance, little is known about the diversity and ecological functions of fungi in mountain lakes in general and specifically for different layers. We investigated the spatiotemporal distribution of fungi in mountain Lake Tovel (Northern Italy) over more than 3 years (September 2021 to February 2025) using metabarcoding (amplicon sequence variants—ASVs—of the ITS gene). We sampled the upper and lower euphotic layers and the hypolimnion of the deep basin and the surface and bottom of the shallow basin. The sampling period covered the year 2022, one of the hottest in Europe, and the year 2024, a very rainy year in Northern Italy. Fungi showed year differences in % read abundance at different taxonomic levels. Observed richness and Chao1 diversity of the upper and lower euphotic layer were lower in warm 2022 than rainy 2024. Basidiomycota showed lower mean % read abundance in 2022 than 2024 in the intermediate layer. In the lower euphotic layer and the hypolimnion, most fungal classes (one and six fungal classes, respectively) had a lower % read abundance in 2022 than 2024. In NMDS with ASVs (stress = 0.16), samples of the years 2022 and 2024 were generally separated. In a partial redundancy analysis conditioned on layers, 17.2% of ASV variability was explained by year (p < 0.001) and season (p < 0.001) as factors and pH (p < 0.01) and Ptot (p < 0.05) as continuous variables. In path analysis, precipitation and % DO directly increased and pH directly decreased fungal diversity. The % abundance of dominant fungal genera also varied by layer: Filobasidium and Rhodotorula had higher % read abundance in the hypolimnion than lower euphotic layer while Zygophlyctis showed the opposite pattern. In network analysis, the lower euphotic layer showed the highest topological complexity, indicative of robustness against perturbations. The interaction between Zygophlyctis (primary algal parasite) and Cystobasidium (mycoparasite) pointed to hyperparasitism in the lower euphotic layer. Year differences in fungal communities indicated that the higher diversity of fungal communities in rainy 2024 can be attributed to different aspects such as wash-in of fungi and of allochthonous material by rain. Season and layer differences were weak and related to specific analyses and genera and phyla. Our study provided new information on the diversity of fungal communities occurring in mountain lakes. Fungal communities responded to altered weather patterns, an important aspect considering that ongoing climate change alters precipitation in mountain regions