Phytoplankton community in Lake Kasumigaura: 2005 to 2019

Abstract

The large shallow lake complex of Lake Kasumigaura comprises of three water bodies: Lake Nishiura, Lake Kitaura, and Lake Sotonasakaura, which are connected by three rivers. The community structure and biovolume of phytoplankton are important factors for the ecology and water quality of the lake. However, few studies have been conducted recently on the phytoplankton communities of the entire Lake Kasumigaura. The aim of this study was to clarify the changes in community structure and biovolume of phytoplankton from 2005 to 2019 in Lake Kasumigaura. The results showed that the dominant phytoplankton taxa were Bacillariophyceae from 2005 to 2007, Planktothrix ( Oscillatoriales, Cyanophyceae ) from 2008 to approximately 2010, and Bacillariophyceae since 2011. The change in the phytoplankton community structure differed between lakes Nishiura and Kitaura: the ratios of the dominant genera Cyclotella and Aulacoseira ( Centrales, Bacillariophyceae ) were higher in Nishiura than in Kitaura; the ratio of the dominant genus Synedra ( Pennales, Bacillariophyceae ) was lower in Nishiura than in Kitaura. Moreover, while the abundance of Planktothrix in Nishiura was higher than that in Kitaura, the period of Planktothrix dominance in Nishiura was shorter than that in Kitaura. The phytoplankton biovolume decreased in all three water bodies during the research period because the biovolumes of the main phytoplankton genera, Planktothrix , Cyclotella , and Cryptomonas , decreased. The values of the biodiversity indices decreased drastically in 2007 at all stations. The results suggested that Planktothrix was dominant from 2007 to 2011 across Lake Kasumigaura. Furthermore, the values of biodiversity indices increased rapidly in 2011 in Nishiura and Sotonasakaura, while the increase was slower in Kitaura.