Do functional traits of chironomid assemblages respond more readily to eutrophication than taxonomic composition in Chinese floodplain lakes?

Functional trait-based measures can reflect local environmental conditions and are claimed as promising approaches for environmental monitoring and assessment programs. The chironomid larvae are a diverse and ecologically important group of benthic macroinvertebrates and have a set of functional traits that can be used to assess environmental quality of freshwater ecosystems. However, little is known regarding the relationships between chironomid functional structure and human disturbances, especially at a fine taxonomic resolution. Here, we compared the responses of functional traits and traditional taxonomic measures of chironomid assemblages to eutrophication in the Yangtze floodplain lakes. We found that functional composition showed higher sensitivity and robustness in discriminating the extents of eutrophication than taxonomic composition. Specifically, about half of functional categories examined significantly differed among the lake groups, while only two taxonomical measures (the abundance of Tanytarsini and Polypedilum) are suitable for assessing environmental quality of floodplain lakes. The macrophytic lakes showed higher abundance of scrapers, the small body size and oligotrphic individuals. Whereas the algal lakes were associated with high abundance of eutrophic/hypertrophic chironomids and those with haemoglobin. Chironomid assemblages showed significant differences among four lake groups for both trait-based and taxonomy-based variations, but with larger differences for trait-based groups. The distance-based redundancy analysis (dbRDA) indicated that taxonomic and functional assemblage structures were explained primarily by similar water quality variables (chlorophyll a, transparency and TN), but more functional variation (24.6%) was explained by environmental factors than taxonomic variation (10.9%) in chironomid assemblages. Our results indicated that chironomid trait-based approaches were more sensitive and robust to water quality gradients than taxonomy-based approaches. Therefore, we argued that the chironomid functional traits deserve considerations for biomonitoring programs in the lakes we studied and other freshwater lakes and aquatic ecosystems.