2020 Volume 69 Issue 12 Pages 747-752
In order to clarify the cause of hypereutrophication in Lake Miyajima-numa, one of the Ramsar Convention on Wetlands in Japan, water quality monitoring and dilution bioassay were conducted to determine the seasonal changes of the water quality and the limiting nutrients for phytoplankton growth in this lake. It was confirmed that the chlorophyll a concentration in the lake was high during the time when the white-fronted goose (Anser albifrons) flew in and decreased after it flew away. From the results of a dilution bioassay, it was shown that the limiting nutrient changed from nitrogen to phosphorus during the rapid growth of phytoplankton from late summer to autumn. This suggests that the direct load of ammonium nitrogen leaching from the excrement of geese on the lake was occurred, and then, that the growth of phytoplankton was promoted with the increase of ammonium-nitrogen concentration in water. After that, an excessive supply of ammonium nitrogen from the excrement of geese made phosphorus a limiting nutrient. These results could be useful for the conservation and regeneration of the water quality of Lake Miyajima-numa with appropriate management of migratory waterfowl to reduce nutrient loads in the future.
