Water quality before and after the break out of submerged plants at Lake Shinji

• Published in: Rikusuigaku zasshi Vol. 75; no. 2; pp. 99 - 105
• Main Authors: Yamamuro, Masumi, Kamiya, Hiroshi, Ishitobi, Yu
• Format: Journal Article
• Language: Japanese
• Published: 19.05.2015
• Subjects: INW, Japan, Honshu, Shimane Prefect., Matsue, Shinji L
• ISSN: 0021-5104; 1882-4897

Recently, submerged plants in Lake Shinji have spread widely. As heavy attachment of aquatic plants to fish nets had occurred in the autumn of 2008, we supposed that the spread of submerged plants started in 2007. We examined the water quality (temperature, electric conductivity, suspended solids, phytopigments, transparency) at the lake center from January 2001 to December 2012. In addition, we also examined the annual sales records of herbicides for use in rice cultivation from 2002 to 2012. We also compared the water quality during May at seven monitoring sites, prior to spread (2001-2006) and after the spread (2007-2012). Temperature and electric conductivity were higher at all monitoring sites after the spread. Although trends in suspended solids and transparency were different among monitoring sites, phytopigment was higher after spread at all sites. The average values prior and after the spread of submerged plants were not significant in terms of water quality except phytopigments. Furthermore, the amount of the effective ingredients in the herbicides used for rice cultivation was 4291 kg in 2006 and decreased to 3305 kg in 2007. At present, this has continuously decreased to the level of 2000 kg. We therefore concluded that the most likely cause for the spread of submerged plants in Lake Shinji can be attributed to the decrease in the amount of herbicides used for rice cultivation from 2007.