Do diatom and nematode assemblages reflect the same ecological status in a tropical eutrophic reservoir?

It remains uncertain whether diatom and nematode assemblages can reflect the same ecological status in eutrophic reservoirs. To address this uncertainty, we collected diatom and nematode assemblages and measured environmental variables in the tropical eutrophic Tri An Reservoir (TAR) in Vietnam. We assessed the ecological status of each community using various biological metrics, including the Shannon-Weiner diversity index (H′), the Trophic Diatom Index (TDI), and the index of trophic diversity (ITD) of nematodes. Multivariate direct gradient analysis was conducted to determine the environmental variables influencing the dominant diatom and nematode species. We then conducted a nonparametric permutational analysis of variance with two fixed factors to assess differences in all biological variables across communities. The analysis had a cross-sectional design that incorporated “station” and “season” as factors. Our findings indicated that the surface water in the TAR ranges from eutrophic to hypertrophic. Based on these TDI values, we classified the ecological and trophic status of the TAR into two groups: meso-eutrophic and eutrophic. However, the ITD of the nematodes suggested that the ecological status varied from poor to moderate or high. Our findings suggest inconsistency between two bioindicator groups in their ability to reflect surface water quality in the TAR. This study highlights the potential of using the TDI of diatoms and emphasizes the need to revise the H′ and ITD indices of nematodes if they are to be incorporated into lake monitoring programs for assessing ecological status.