Heterogeneity in diatom diversity response to decadal scale eutrophication in floodplain lakes of the middle Yangtze reaches

Eutrophication substantially alters biotic communities and has become a major threat to biodiversity conservation in lake ecosystems. However, little is known about how long-term diversity dynamics respond to nutrient enrichment among lakes in different ecological states. Based on a dataset of diatom records in 210Pb-dated cores collected from eight shallow lakes in the middle reaches of the Yangtze River (central China), this study evaluates temporal changes in species richness and ss-diversity over recent decades, highlighting three distinct trends in diversity dynamics. In heavily-polluted lakes (e.g., Shahu and Sanliqi), severe eutrophication caused the replacement of many resident species by few pollution-tolerant species. However, in transitional lakes (e.g., East Dongting and Luhu), slightly increased nutrients promoted a net species gain at an accelerating rate. While in macrophyte-dominated lakes (e.g., Futou), nutrient enrichment caused species gain to slow down. Slightly increased nutrients probably promoted the spread of some cosmopolitan species, but severe eutrophication caused the extinction of many resident species. Given that the expansion of cosmopolitan species would cause biotic homogenization at a regional scale, species gain in individual lakes cannot be assumed to be beneficial to ecosystem functioning. For conserving local diatom diversity, individual lakes characterized by species gain at an accelerating rate are clear management targets. For conserving regional diatom diversity, controls on both catchment external nutrient inputs and in-lake internal loads are required to promote heterogenous habitats and maintain diatom diversity. Exploring past diversity dynamics is an essential solution to inform and direct the sustainable management of ecologically diverse floodplain lakes.