Contrasting patterns of long-term zoobenthic diversity dynamics in response to sedimentary phosphorus input in eutrophic and mesotrophic lakes

• Alpha diversity of chironomid communities decreased in eutrophic lakes, but increased in mesotrophic lakes since the 1900 s. • The loss of species and functional traits accelerated with increasing sedimentary total phosphorus in eutrophic lakes. • The gain of new functional traits slowed down with nutrient inputs in most mesotrophic lakes. • Chironomid richness increased with sedimentary total phosphorus up to 1.25 mg g −1 , beyond which it declined. The response of ecosystem structures and functions to eutrophication has been investigated worldwide. Yet, present knowledge on the effects of nutrient inputs on biotic diversity is unsystematic and mainly based on taxonomic composition. In this study, both taxonomic and functional trait diversity indices (alpha and beta diversity) of sedimentary chironomid (Insecta: Diptera) communities were analysed in mesotrophic and eutrophic lakes of the middle Yangtze River reaches. Functional trait-based alpha diversity displayed similar temporal trends to taxonomic diversity in most lakes. With an increase in sedimentary total phosphorus (TP), taxonomic and functional trait alpha diversity generally decreased in eutrophic lakes, but increased in mesotrophic lakes. In eutrophic lakes, beta diversity was positively correlated with sedimentary TP, indicative of the accelerating loss of species and functional traits induced by eutrophication. The functional trait-based beta diversity generally decreased with increasing sedimentary TP in most mesotrophic lakes, suggesting that the gain of new functional traits slowed down with nutrient enrichment. Sediment TP concentration of 1.25 mg g −1 served as a critical threshold for shifts in chironomid community richness in this region. Taken together, taxonomic and functional trait diversity of sedimentary chironomids can be useful indices to indicate long-term nutrient dynamics in floodplain lakes.