Murniarti2013-04-302013-04-302013-04-30sumarnihttp://repository.unri.ac.id:80/handle/123456789/3097A study on the diversity of phytoplankton in the Rengas Lake was conducted from November 2012 to January 2013. This research aims to understand the abundance and diversity of phytoplankton in that area. There were three stations with 3 sampling points in each station. Samples were taken 3 times, once a week and they were analyzed in the Aquatic Productivity Laboratory of the Fisheries and Marine Science Faculty, Riau University. Result shown that the phytoplankton obtained were consist of 29 species, they were belonged to of 5 classes, namely Bacillariophyceae (3 species), Chlorophyceae (18 species), Chrysophyceae (2 species), Cyanophyceae (3 species) and Xanthophyceae (3 species). The average of phytoplankton abundance was around 64,303 – 86,852 cells/l. The most common phytoplankton was Phormidium sp. (50983 cells/l, Cyanophyceae) and the lowest was Vacuolaria sp. (9 cells/l, Xanthophyceae). Diversity index (H') was 1.58 – 2.18, dominancy index (C) was 0.28 – 1.24 and equitabity index (E) was 0.50 – 0.66. General water quality parameters are as follow: temperature: 27 – 300C, brightness: 0.34 – 0.94 m, pH: 6, DO: 1.38 – 2.17 mg/l, CO2: 9.00 – 10.98 mg/l, nitrate: 0.00 – 0.53 mg/l and phosphate 0.41 – 0.72 mg/l. Based on phytoplankton abundance, the Rengas lake can be categorized as eutrophic lake.otherPhytoplankton AbundancePhormidium spVacuolaria spRengas Lakestudent Paper Post Degree