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 a nd dematiaceous hyphomycetes on submerged wood were investigated at 3 sites along the Lam Tsuen River in Hong Kong. Two-hundred-and-six species were r ecorded. Ascomycete genera were dominated by Annulatascaceae, Halosphaeriac eae, Lasiosphaeriaceae, Lophiostomataceae, and Magnaporthaceae; the most co mmon taxa were Aniptodera chesapeakensis, Aquaticola ellipsoidea, Helicospo rium 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 Si tes 1 and 2 (p < 0.05). Downstream changes in species composition were stro ngly correlated with elevated concentrations of NO3-N (r(2) = 0.66), NH3-N (r(2) = 0.66), and PO4-P (r(2) = 0.73) resulting from human disturbance. Th e variation in species composition was also weakly correlated with other ph ysico-chemical variables such as conductivity (r(2) = 0.16) and aluminum (r (2) = 0.11). Species diversity fluctuated temporally but did not change (AN TOVA, p > 0.05) between dry and wet seasons. Species composition shifted an d the frequency of occurrence of certain dominant taxa declined after heavy rain-fall associated with El Nino, possibly because increased discharge wa shed away well-colonized woody substrata.