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Anthropogenic disturbances and climate change have significantly altered the biotic composition across many ecosystems, leading to changes in biodiversity and even ecological collapse. An ecosystem comprises multiple trophic levels, and the issue how these disturbances affect their assembly processes remains unclear. Ecological stability of assemblages was maintained by their structure, and thus, revealing structure changes across trophic levels could improve our understanding of how ecosystems response to disturbances as a whole. In this study, we combined methods from palaeolimnology, ecology and network analysis, and observed the changes of biodiversity and network structure of two trophic levels (algae - diatoms and zoobenthos - chironomids) in Erhai lake, Southwest China over the last century. Results showed nutrient enrichment induced shifts in diatom and chironomid assemblages at ∼2001 CE, suggesting that the shift in Erhai lake may have occurred at multiple trophic levels. We found biodiversity exhibit different trends across trophic levels as it decreased in diatoms but increased in chironomids. However, network skewness declined in both trophic levels, indicating the common loss of biotic heterogeneity. The consistent decline of skewness among trophic levels long before the compositional shift is a potential parameter to warn of the shifts in lake ecosystems., Competing Interests: Declaration of competing interest None., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Aim: Over the last century, lakes all over the world have experienced significant eutrophication and become more susceptible to critical transitions. This has prompted a growing need to understand how community dynamics shift following ecological perturbations. This study aims to elucidate the present status, spatial patterns and determinants of biological diversity, and how they have changed in response to alterations driven by eutrophication. Location: Lakes of East China, mostly in the Yangtze River Basin. Taxa: Bacillariophyceae (163 species in 33 genres, see taxa names in Appendix S1). Methods: The environmental conditions of over 60 lakes were assessed based on parameters such as water quality, physical characteristics and biogeochemical constituents. Surface sediment samples and high‐resolution sediment records were collected to quantify the biodiversity of diatom communities, and principal component analysis (PCA) was used to determine the major environmental gradients. Meanwhile, linear regression, canonical correlation analysis and Mantel tests were used to quantify the correlations between variables. Results: Biodiversity metrics showed no distinct geographic patterns, although areas near large lakes showed comparatively high evenness. Smaller pairwise dissimilarities were observed among hypereutrophic lakes in comparison to lakes with a lower trophic status. The past century has witnessed a decrease in both pairwise dissimilarity and the standard deviation of evenness. A significant statistical association was found between dissimilarity and ionic concentration and composition in the surface sediment of lakes, rather than nutrient levels. Main Conclusions: Following regional eutrophication and human disturbance, a significant reduction in biodiversity differentiation was observed in lakes located in East China. Currently, the dominant factor in determining the beta diversity of diatom communities is geochemical conditions rather than nutrient concentrations. Long‐term stress caused by cultural eutrophication may also have altered the dominant determinant of regional biodiversity and diatom community dynamics. [ABSTRACT FROM AUTHOR]

[Display omitted] • Heterogeneity index(H) is a structural parameter describing the distribution of species associations. • H is proposed to unveil the species turnover in community in this paper. • Eight lakes exhibit a eutrophication trend with different degrees from the 1950s. • H , species richness, evenness and Jaccard index respond differently to eutrophication. • H can serve as a complementary biodiversity metric to quantify functional diversity. Anthropogenic eutrophication in aquatic ecosystems has had significant negative effects on biodiversity and human wellbeing. Improving our understanding of the ways in which biological communities respond to eutrophication underlies biodiversity conservation, and diversity metrics like species richness and evenness are often used to assess environmental impacts on biodiversity. However, such studies mainly consider overall gain and loss, ignoring species replacement in community succession. This paper combines paleolimnology and network science to propose a heterogeneity index (H) to measure community successions by linking long-term changes in community structure with environmental stress. Eight lakes of different types in the middle and lower reaches of the Yangtze River Basin (MLYB) were selected, and we focused on diatom communities to reconstruct their ecological trajectories after significant anthropogenic impact. The combined results suggested that human impacts have caused eutrophication in each lake since the 1950s, but led to different degrees of impact on the diatom communities. In hypereutrophic lakes, diatom assemblages shifted to high-nutrient tolerance and the proportions of epiphytic-benthic species decreased, while in mildly eutrophic lakes, a switch to mild-nutrient tolerant assemblages occurred in planktonic and epiphytic-benthic species with no change in epiphytic-benthic proportions. The H of diatom communities declined in hypereutrophic lakes, but changed little in mildly eutrophic lakes after the 1950s. Meanwhile, no regular changes in diversity indicators such as richness, evenness and the Jaccard-index occurred with lake eutrophication. The ordered species turnover in nutrient-enriched diatom communities may account for the discrepancies in H and the above indices, such as ordered epiphytic-benthic species being replaced by higher nutrient-tolerant and planktonic species with no necessarily attendant reduction in species richness. By quantifying the alteration of species associations without being influenced by species diversity, the H index effectively reveals the succession of diatom communities, and can therefore serve as a complementary indicator of biodiversity. [ABSTRACT FROM AUTHOR]