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Longitudinal and temporal distribution of freshwater ascomycetes and dematiaceous hyphomycetes on submerged wood in the Lam Tsuen River, Hong Kong

Longitudinal and temporal distribution patterns of freshwater ascomycetes and dematiaceous hyphomycetes on submerged wood were investigated at 3 sites along the Lam Tsuen River in Hong Kong. Two-hundred-and-six species were recorded. Ascomycete genera were dominated by Annulatascaceae, Halosphaeriaceae, Lasiosphaeriaceae, Lophiostomataceae, and Magnaporthaceae; the most common taxa were Aniptodera chesapeakensis, Aquaticola ellipsoidea, Helicosporium griseum, Massarina purpurascens, M. thalassioidea, Ophioceras commune, and Sporoschisma uniseptatum. There were no significant differences (ANOVA, p > 0.05) in total number of species among the sites. However, the number of species of Halosphaeriaceae (p < 0.01) and Lasiosphaeriaceae (p < 0.05) was significantly higher at Site 3, and Annulatascaceae was dominant at Sites 1 and 2 (p < 0.05). Downstream changes in species composition were strongly correlated with elevated concentrations of NO3-N (r2 = 0.66), NH3-N (r2 = 0.66), and PO4-P (r2 = 0.73) resulting from human disturbance. The variation in species composition was also weakly correlated with other physico-chemical variables such as conductivity (r2 = 0.16) and aluminum (r2 = 0.11). Species diversity fluctuated temporally but did not change (ANOVA, p > 0.05) between dry and wet seasons. Species composition shifted and the frequency of occurrence of certain dominant taxa declined after heavy rainfall associated with El Niño, possibly because increased discharge washed away well-colonized woody substrata.