Homogenization of lake cyanobacterial communities over a century of climate change and eutrophication

Human activities have impacted terrestrial and aquatic ecosystems over the last few centuries. Climate change and eutrophication have had profound effects on lakes, which stimulated phytoplankton growth and increased the frequency and strength of harmful cyanobacterial blooms. The mitigation of phosphorus loading has contributed to the return of some lakes to a lower trophic state. However, cyanobacteria often continue to dominate the phytoplankton community, impairing ecosystem functioning and water quality. Due to the scarcity of long-term lake data (both biotic and abiotic), the state of past ecosystems and the phytoplankton response to warming and rapid changes in local conditions remain poorly understood. With the aim of bridging this gap, we investigated the dynamics of cyanobacterial communities across 10 European pre-alpine lakes over the last 150 years using sedimentary records. More specifically, we used high throughput amplicon sequencing to explore temporal changes in the regional distribution and the phylogenetic diversity of cyanobacteria. Our results show that the compositional similarity increased across lake communities over recent decades, generalising possible problems related to the presence of bloom-forming and harmful cyanobacteria. This study highlights the potential of high-resolved phylogenetic data from sedimentary records for investigating planktonic community dynamics and for disentangling the roles of local and global environmental change in shaping plankton communities.