Numerical modeling on transition of dominant algae in Lake Kitaura, Japan

Coastal, marine, and freshwater ecosystems are vulnerable to the impact of harmful algal blooms. It is also important to understand how future climatic changes and other environmental conditions may influence the transition of dominant algae, and the frequency and severity of blooms. In this study, the transition of dominant algae in Lake Kitaura was analyzed using a three-dimensional hydrodynamic–ecosystem-coupled model. Lake Kitaura is part of Lake Kasumigaura in Japan, and is connected to Lake Nishiura and Lake Sotonasakaura. Dominant algae have shifted seasonally and annually between 2005 and 2009, between three major algal: Microcystis spp., Planktothrix spp., and Cyclotella spp. Numerical simulation was carried out for the four years of the period 2005–2009. The model reproduced well the transitions of dominant algae in the four years by calibrating ecological parameters. The biomass of Planktothrix spp. suddenly increased in the summer of 2008, and Planktothrix spp. became the dominant species. Longer periods of stratification, lower concentration of dissolved oxygen, and higher concentration of dissolved nitrogen were observed in 2008, while the sudden increase in Planktothrix spp. biomass in 2008 was likely caused by transport of algae into the lake from Lake Nishiura via Wani River.