Abstract/Summary

Microbial communities play important roles in lake ecosystems and are sensitive to environmental change. However, our understanding of their responses to long-term change such as eutrophication is limited, as long-term lake monitoring is rare, and traditional paleolimnological techniques (pigments and microfossils) are restricted to a low taxonomic resolution, or organisms with well-preserved structures. Sedimentary DNA (sedDNA) is a promising technique to reconstruct past microbial communities in sediments, but taphonomic processes and the ability of sedDNA to record bac-terial pelagic history accurately are largely unknown. Here, we sequenced the 16S rRNA gene in triplicate sediment cores from Esthwaite Water (English Lake District) which has concurrent long-term monitoring and observational data. The sediment record spanned 113 years and included an episode of increased nutrient availability from the 1970s, followed by a more recent decline. Trends in bacterial community composition were broadly similar among the three sediment cores. Cyanobacterial richness in the sediment cores correlated significantly with that of cyanobacteria in a 65- year microscopy- based monitoring record, and some known pelagic bacterial taxa were detected in the sediment. sedDNA revealed distinct shifts in community com-position in response to changing lake physicochemical conditions. The relative abun-dance of cyanobacteria closely reflected nutrient enrichment, and Proteobacteria, Bacteroidetes, and Verrucomicrobia were relatively more abundant in recent sedi-ments, while Chloroflexi, Firmicutes, Acidobacteria, Nitrospirae, Spirochaetes, and Planctomycetes declined in more recent sediments. Following lake restoration efforts to reduce nutrient enrichment, the relative abundance of cyanobacteria returned to pre- 1970 levels, but the bacterial community did not fully recover from the period of intense eutrophication within the time scale of our study. These results suggest that sedDNA is a valuable approach to reconstruct lake microbial community com-position over the 100-year time scale studied, but an improved understanding of DNA deposition and degradation is required to further the application of sedDNA in paleolimnology.