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Phytoplankton and the correlation to primary productivity, chlorophyll-a, and nutrients in Lake Maninjau, West Sumatra, Indonesia

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, 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, 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 and 5.74 to 7.06 mg L, pH ranged from 8.0 to 9.8 and 8.1 to 9.7, BOD ranged from 0.97 to 2.36 mg L and 0.84 to 2.44 mg L, 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 and 0.12 to 0.42 mg L, total N ranged from 1.13 to 1.82 mg L and 0.52 to 3.07 mg L 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.