AACL Bioflux, 2020, Volume 13, Issue 3. http://www.bioflux.com.ro/aacl 1689 Phytoplankton and the correlation to primary productivity, chlorophyll- a , and nutrients in Lake Maninjau, West Sumatra, Indonesia 1 Jabang Nurdin, 1 Desra Irawan, 2 Hafrijal Syandri, 1 Nofrita, 1 Rizaldi 1 Department of Biology, Faculty of Mathematics and Natural Sciences, Andalas University, West Sumatra, Indonesia; 2 Faculty of Fisheries, Bung Hatta University, West Sumatra, Indonesia. Corresponding author: J. Nurdin, jabang_nurdin@yahoo.com Abstract . Intensive freshwater fish farming using floating-net-cages in Lake Maninjau has been conducted since 1992. Fish farming intensification can reduce water quality and affect aquatic biotic communities. This research aims to study the impact of floating-net-cages activities on the phytoplankton communities, and its correlation to primary productivity, chlorophyll- a, and water quality of Lake Maninjau. Five representative sampling sites from Lake Maninjau were chosen purposively. Primary productivity, chlorophyll- a, and water quality were analyzed using the oxygen method, chlorophyll extract from autotrophic organisms and the tropical index respectively, then all parameters were analyzed using principal component analysis (PCA) to present the pattern. Water quality parameters such as temperature, pH, total suspended solids (TSS), total dissolved solids (TDS), light intensity, dissolved oxygen (DO), biological oxygen demand (BOD), chemical oxygen demand (COD), total P, and total N were also measured. From the results, we found 17 species of phytoplankton from 5 classes. Phytoplankton abundance ranged from 1980 to 3540 individual L -1 , with the lowest abundance found in Sigiran Site (incubation zone) and the highest in Kubu Baru Site (surface waters). The biomass estimation value of chlorophyll- a ranged from 0.415 to 0.104 mg C m -3 , with the highest value found on the surface water in Sigiran Site and the lowest in incubation zone in Tanjung Sani Site. From the water quality assessment, the trophic index of Lake Maninjau is categorized as eutrophic. The results of the analysis indicate the surface water tends to have higher productivity than in the incubation zone. Although in Sigiran and Tanjung Sani sites, the incubation zone has higher productivity than the surface water. Physico-chemical assessment results in surface water and incubation zone for DO ranged from 5.93 to 6.82 mg L -1 and 5.74 to 7.06 mg L -1 , pH ranged from 8.0 to 9.8 and 8.1 to 9.7, BOD ranged from 0.97 to 2.36 mg L -1 and 0.84 to 2.44 mg L -1 , COD ranged from 7.10 to 25.27 and 6.72 to 27.93, total P ranged from 0.13 to 0.33 mg L -1 and 0.12 to 0.42 mg L -1 , total N ranged from 1.13 to 1.82 mg L -1 and 0.52 to 3.07 mg L -1 respectively, and light intensity ranged from 1.3 to 1.6 m. Residual feed due to floating-net-cages activity causes water quality impairment in Lake Maninjau which is known by water quality and eutrophic status. Key Words: floating-net-cages, intensive fish farming, nutrient, water quality. Introduction . Lake Maninjau is a type of volcanic lake, one of five big lakes in West Sumatra, Indonesia. Lake Maninjau supports a large amount of floating-net-cages activity. Besides that, Lake Maninjau also supports hydroelectric power generation, agriculture, tourism, and other fisheries activities. There are more than 16,210 units of floating-net-cages since it began in 1992, sedimentation occurred with values of about 9,324.98 tons per year and causing declining in water quality (Junaidi et al 2014). Intensive fish farming using floating-net-cages was enhancing the nitrogen and phosphate source from the fish feed residues in cages, which induce mass mortality of fishes (Syandri et al 2014). Also, this condition disrupts the balance of organisms and the occurrence of phytoplankton population outbursts that impacted aquatic organisms (Abuka 2012). The enrichment of nitrogen and phosphate elements in the waters can cause eutrophication. Lake Maninjau is in the eutrophic status (Junaidi et al 2014). Eutrophication under certain conditions is harmful because it can stimulate the growth of