Spatial pattern of benthic diatoms and water quality assessment using diatom indices in a subtropical river, China

Background/Question/Methods

Benthic diatoms are one of the most vital primary producers in riverine ecosystems, and their spatial distribution and assemblage structure respond sensitively to a range of environmental variables. Thus, they have been widely used as indicators of water quality in riverine systems. Here, we present a study on benthic diatoms and water quality assessment using diatom indices in the upper Han River, a subtropical river with a length of 925 km in China. Epilithic diatoms were collected from the riverine network, and species were identified. Water samples were collected simultaneously at the same sites, and analyzed for environmental variables (i.e., temperature, pH, turbidity, EC, DO, NH⁴⁺-N, NO₃-N, TP, TN, SRP, DOC, Na⁺, K⁺, Ca₂⁺, Mg₂⁺, Si, Cl^- and SO₄^2-). We calculated the Shannon-Wiener diversity index, Pielou evenness, and species richness of the diatom assemblages, and five diatom indices (i.e., CEE, EPI-D, IBD, IPS, and WAT). Two-way indicator species analysis (TWINSPAN) was performed to define species assemblages and the fidelity of species to an assemblage. Principal component analysis (PCA) was conducted to derive gradients among physical and chemical variables, and canonical correspondence analysis (CCA) was employed to explore relationships between diatoms and environmental factors.

Results/Conclusions

There were great spatial variabilities in water physical and chemical variables, and mainstream had higher nutrients concentrations than tributaries. PCA of the physical and chemical variables extracted four components with eigenvalues >1.0, accounting for 69.85% of the total variance. The first factor, accounting for 36.48% of the total variance, separated SO₄^2-, NO₃-N, and TN with factor loadings > 0.80. The second component, explaining 14.65% of the total variance, was primarily composed of TP and SRP. CCA indicated that spatial patterns of diatom assemblages were primarily explained by DOC, pH, major ions, and flow velocity. TWINSPAN on the diatom assemblages grouped the sampling sites into those from mainstream and tributaries. There were also significant differences in the Shannon-Wiener diversity index, evenness index, species richness and diatom indices between the mainstream and tributary sites, and diatom indices categorized the mainstream as mesotrophic and the tributaries as oligo-mesotrophic in the upper Han River. Lower water quality in the mainstream led to the dominance of pollution tolerant species such as Eolimna subminuscula, while tributaries were characterized by Achnanthidium pyrenaicum and A. subatomus. The research demonstrates the capacity of diatom indices and indicator analysis in monitoring water quality in subtropical rivers.