Determining suitable submerged macrophyte biomass in terms of dissolved oxygen concentration and biodiversity in the South Basin of Lake Biwa, Japan

After a regime shift in eutrophic lakes from a turbid water state with phytoplankton blooms to a clear water state, troublesome issues caused by excessive growths of aquatic plants occurred. This study investigated the relationships between submerged macrophyte biomass, and associated fauna and flora, and environmental factors at 52 stations in the South Basin of Lake Biwa. Polynomial regression analysis gave 5 significant regression equations ( P < 0.05, n = 52) of submerged macrophyte biomass versus depth, periphyton biomass, and periphyton, epifauna and epi-shellfish taxa richness. We added one equation ( P < 0.1, n = 52), which was not significant, but was important for complying with the environmental quality standard for submerged macrophyte biomass versus bottom dissolved oxygen (DO) in Japan. Eleven significant regression equations of percentage volume submerged macrophyte infestation (PVI) versus depth, bottom DO, east–west and north–south averaged from surface to bottom current velocities absolute value, total chlorophyll a (chl. a ) and chl. a of green and blue-green algae, epifauna biomass, submerged macrophyte species richness, periphyton and epifauna taxa richness ( P < 0.05, n = 52) were also generated. When submerged macrophyte biomass was > 6000 g m −2 or PVI was > 60%, DO concentrations became too low for the survival of fish and invertebrates (< 4.3 mg l −1 ). In addition, the cubic regressions of periphyton and epifauna taxa richness showed inflection points were at 3000 g m −2 of submerged macrophyte biomass. Epifaunal biomass, submerged macrophyte species richness, periphyton and epifaunal taxa richness showed inflection points at 30% PVI. Previous studies suggested that regime shifts occur at particular thresholds, and community structure and biodiversity change significantly. Therefore, we assumed regression inflection points were threshold indicators. We recommend an appropriate submerged macrophyte biomass of 3000–6000 g m −2 or a PVI of 30–60% to maintain the ecosystem in a clear water state with fewer troublesome issues — starting at these levels macrophyte growth should be controlled in Lake Biwa.