Phytoplankton Community Structure in Selected Fishponds of Aquaculture Farms in Ogun State

Phytoplankton community structure in aquaculture ponds determines primary productivity, water quality, and fish production efficiency, yet this relationship has not been documented for fishponds of commercial aquaculture farms in Ogun State, South West Nigeria. This study characterised phytoplankton community structure in five commercial catfish ponds at farms in Abeokuta North and Odeda Local Government Areas. Monthly sampling was conducted over four months using a plankton net (mesh size 55µm), and water quality parameters were simultaneously measured. Phytoplankton taxa were identified and counted using a Sedgwick-Rafter counting cell. Diversity indices, cell density, and biomass were calculated. A total of 47 phytoplankton taxa from 5 divisions were identified. Chlorophyta was the most diverse (21 taxa), followed by Bacillariophyta (14 taxa) and Cyanobacteria (9 taxa). Mean cell density ranged from 3,200 to 8,600 cells/mL across ponds, with the highest densities in ponds receiving organic supplementation. Microcystis aeruginosa was the dominant Cyanobacterium in two ponds, raising concern about cyanotoxin risk to fish. Shannon-Wiener diversity was inversely related to nutrient loading (r = -0.68, p < 0.05). Chlorophyll-a concentration correlated positively with total phosphorus (r = 0.81). The study concludes that phytoplankton community composition in Ogun State aquaculture ponds reflects nutrient management practices and recommends monthly phytoplankton monitoring as an early warning tool for bloom prevention in commercial fish production systems. Keywords: phytoplankton, aquaculture ponds, community structure, Cyanobacteria, Ogun State