Diatom-inferred limnological changes in Lake Inawashiro-ko, Japan, over the past 1700 years

We inferred paleoenvironmental conditions in Lake Inawashiro-ko, Japan, over the past 1700 years, using diatom assemblages in the uppermost 2 m of sediment core INW2012. The core was characterized by three diatom zones: Zone 3 (302–1556 CE) was dominated by the planktonic taxon Aulacoseira nivalis; Zone 2 (1556–1991 CE) was predominantly planktonic Cyclotella radiosa and Aulacoseira subarctica; Zone 1 (1991–2012 CE) had highest relative abundance of Fragilaria neoproducta, accompanied by Cyclotella radiosa and Aulacoseira subarctica. Correlations between recorded diatom assemblages and water quality in previous studies enabled us to infer past environmental conditions, which indicate the lake became more eutrophic over the past ~ 30 years. Support for this inference comes from the predominance of benthic Fragilaria neoproducta in the limnetic assemblage of Zone 1. The habitat of F. neoproducta expanded from the littoral zone to offshore areas as a result of progressive eutrophication. The diatom record also suggests that alkalization of Lake Inawashiro-ko occurred over the past 1700 years. If, however, most production took place in the littoral zone, past pH values throughout much of the lake may have been lower than those indicated by the diatom assemblages. This points to the need for more diatom-based paleoenvironmental studies in lakes that display a broad range of environmental conditions across space, i.e. from the littoral to the limnetic zone.