PHYTOPLANKTON AND TROPHIC STATUS IN LAKE TOBA, NORTH SUMATRA
The relationship between phytoplankton community and water quality can be used as indicator of trophic states in Lake Toba. The aim of this study was to describe the structure of phytoplankton communities and trophic states in Lake Toba. This study was conducted in 20-24 October 2014 at 23 stations. The results showed that the average concentration of ammonia, nitrate, nitrite, and phosphate in Lake Toba in a row are as follows 0,11 mg/L; 1,66 mg/L; 0,01 mg/L; and 0,74 mg/L. The abundance of phytoplankton ranged 216-68 319 716 cells/m3, consists of 35 genera from four classes, i.e. Chlorophyceae, Cyanophyceae, Bacillariophyceae, and Dinophyceae. Chorophyceae was a group that dominated by the composition of species (44.56%) and abundance (50.48%). Structure of phytoplankton communities in Lake Toba was dominated by Anabaena, Gloeotilla, and Spaerocystis. Based on Nygaard index, Lake Toba classified as eutrophic lakes. The highest thropic status are at stations close to the floating cage.