Long Term Monitoring of Water Quality and Phytoplankton Changes in Lake Maninjau, West Sumatra, Indonesia

Lake Maninjau is one of the eutrophic lakes in Indonesia as indicated by the occurrence of Microcystis aeruginosa blooms in 2000 and 2011. This lake has been utilized to generate electric power since 1983 and to develop fish culture in cages since 1990. The fish culture has been suspected of increasing the organic matter and stimulating blue-green algae growth. This study evaluates the water quality and phytoplankton composition changes in Lake Maninjau. The Secchi depth has tended to decrease and showed an annual variability ranges from 5.1 to 0.8 m. Monitoring of the DO concentration showed that the anoxic zone continued to increase to the upper column of waters, from 40 m in May 2006 to 15 m in June and December 2011. pH and conductivity in the surface water tended to increase. The value of pH > 9 was found from August 2009 until April 2014. The trophic state index (TSI) tended to increase, it was mesotrophic (41.9 to 50.9) between 2001 and 2007 and eutrophic (56.2 to 64.4) between 2008 and 2014. Phosphorus tended to increase indicating a phosphorus surplus and nitrogen limitation (0.013 to 0.066 mg.L-1). Phosphorous in the epilimnion from 2001 until 2007 ranged from 0.013 to 0.066 mg.L-1, while from 2008 to 2011 it ranged from 0.032 to 0.075 mg.L-1. There were changes in the phytoplankton composition. In 2001 and 2005 it was dominated by Chrysophyta and Chlorophyta respectively. In 2009 and 2014, phytoplankton was dominated by Cyanophyta. There were changes in water quality and the composition of phytoplankton indicating that Lake Maninjau is continuously becoming more degraded. Oseanologi dan Limnologi di Indonesia, Vol. 41, No. 3. Hal. 339-353 ISSN 0125-9830