<![CDATA[Lake Ranau is the second largest lake on Sumatra Island. This lake has a high potency for fisheries. Fisheries activities carried out in Lake Ranau are fishing and aquaculture activities with nile tilapia (Oreochromis niloticus) as the main commodity. Intensive fish farming activities will cause impacts in the form of waste caused by feces and wasted feed. This waste will increase the nutrient loads that enter the waters, triggering the nutrient enriching phenomenon known as eutrophication. Eutrophication is a process of water enrichment by nutrients, mainly nitrogen and phosphorus, which can change the community structure of phytoplankton in the waters and increase their growth. This study aims to analyze the composition of phytoplankton in the waters of Lake Ranau and to estimate the status of water fertility. Data collected was water quality data and phytoplankton data. Status of water fertility was analyzed using the saprobic index. The results show that the community structure of phytoplankton exhibits values as follows: the diversity index ranging from 1.54–3.08, the uniformity index ranging from 0.46–0.90, and dominance index ranging from 0.06–0.41. The abundance of phytoplankton is dominated by the species Nitzhia sp., Synedra, Diatoma, and Tabellaria (all are from the Bacillariophyceae class). The saprobic index obtained ranges from 0.74–1.26, thus categorized as the ?/?-mesosaprobic phase, meaning the waters of Lake Ranau are lightly polluted by a little load of organic and inorganic matters.]]>
