Benthic diatoms are considered one of the most suitable organisms in bioassessment of water quality because of their high sensitivity to water temperature, nutrients, organic pollution and heavy metals. Since the 1970s, a number of indicator indices have been developed and used for the assessment of river quality including the IBD (Biological Diatom Index), TDI (Trophic Diatom Index), SLA (Sládeček's Index), IPS (Specific Pollution Sensitivity Index), IDG (Generic Diatom Index), DESCY (Descy Index) and CEE (European Economic Community Index). However, it is still not clear whether the ecologic signature of diatoms differs between these indicator indices. Moreover, in China, river water quality monitoring is usually limited to physi-chemical variables, which provide a snapshot picture of chemical characteristics at the time of sampling. Assessment of diatoms has the advantage that their observed status is a reflection of water quality conditions over a period of time prior to sampling.
The Dongjiang River basin, as one of the main river systems in the Pearl River Basin, provides significant ecosystem services, such as drinking water, electricity supply, transport, irrigation water supply, fishery.
In this study, seven diatom indices (IBD, TDI, SLA, IPS, IDG, DESCY and CEE) were tested to evaluate the water quality of rivers in the Dongjiang basin by means of correlation coefficient analysis, factor analysis, cluster analysis, box plot analysis and discriminant analysis. Pearson correlation coefficient analysis showed that IPS, IBD and IDG were closely related, and IPS, IBD, IDG and CEE had significant correlation with BOD₅, COD_KMnO4, NO₂-N, NO₃-N. IPS, IBD, IDG and CEE gave better performance in comparison of the seven diatom indices characterized for different water quality groups sorted by 8 key physicochemical variables (DO, BOD₅, COD_KMnO4, TN, NO₃-N, NO₂-N, TP and SiO₂) indentified by factor analysis. Results from stepwise discriminant analysis (P to entered= 0.20, P to removed= 0.25) were represented as follows: for IPS and CEE, NO₂-N was the only selected variable, and the correction percent was low by 55.6% and 48.1%. The discriminant analysis selected NO₂-N, COD_KMnO4 and DO for IBD, and the correction percent was up to 74.1%, IDG was discriminated by two variables (BOD₅ and NO₃-N), and the correction percent was 63.0%. IBD and IDG showed clear and reasonable trend in the box plots with water quality groups clustered from the diatom communities. This study suggested that the diatom indices can be applied for monitoring rivers in the Dongjiang basin, especially IBD and IDG.