Stable successional patterns of aquatic hyphomycetes on leaves decaying in a summer cool stream.

The colonization of leaf litter (Alnus glutinosa) by aquatic hyphomycetes was studied in a summer cool stream of the French Pyrenees. In spite of the rapid decomposition of leaves, the fungal community exhibited a characteristic successional pattern with three phases. The initial colonization stage was defined by a dense sporulation of the five species Tetrachaetum elegans, Lemonniera aquatica, L. centrosphaera, L. terrestris, and in particular Flagellospora curvula. After four weeks of colonization, a mature community had established. It was characterized by high species diversity and peak fungal biomass, which was measured as ergosterol content, and coincided with about 50 % loss in leaf mass. With leaf decay progressing further, diversity diminished concomitant with a slight reduction in fungal biomass and a sharp decrease in the rate of conidial production. Typical species of this late successional stage were Clavatospora longibrachiata, Heliscella stellata and Goniopila monticola. This successional pattern proved to be stable both within the period of leaf fall in one year and between two successive years. Between-seasons differences were quite small as well, the striking lack of species replacement apparently being due to not exceeding the threshold temperature of 16-18 °C as previously defined in literature. In spite of this general stability in community structure, correspondence analysis discriminated the communities on leaf packs with equal exposure times according to season, with the cyclical arrangement of leaf packs on the principal factorial plane reflecting the seasonal cycle. The colonization of fresh (non-dried) leaf litter by aquatic hyphomycetes was delayed compared to air-dried litter; however, the lead diminished with progressing leaf decay, resulting in nearly identical communities on fresh and dried leaves after four weeks of decomposition.