Temporal Water Quality and Phytoplankton Dynamics: A Basis for Environmental Monitoring of Cage Aquaculture in Napoleon Gulf, Lake Victoria, Uganda

Jessy Lugya1*, Rakel Guðmundsdóttir2, Amanda G. Vang3, Mark Olokotum1 1National Agricultural Research Organisation, Uganda, 2Marine and Freshwater Research Institute, Iceland, 3University of the Faroe Islands

Cage aquaculture on Lake Victoria started in 2006, and it expanded rapidly to more than 12,000 cages by 2019. Cage fish farming affects water quality through the accumulation of organic waste from uneaten feed and feces, leading to oxygen depletion, algal blooms, and risks to fish health. Recent incidents of mass fish mortalities in cages have raised concern among farmers and environmentalists. Yet, the lake is already affected by land-based human activities, seasonal changes, and hydrodynamic shifts. This study examined changes in key water quality parameters and phytoplankton communities at a cage farming site in Napoleon Gulf. Data was collected quarterly between 2011 and 2019 from four sampling stations located upstream, within, between, and downstream of the cages, together with a reference site for phytoplankton assessment. Water temperature, dissolved oxygen, conductivity, transparency, and total depth were measured in situ. Phytoplankton samples were preserved in Lugol´s solution and analyzed microscopically to identify species and determine biovolumes.

The results showed significant differences (p < 0.05) in the sampled parameters among sites. From 2011 to 2018, temperature increased annually by 0.061±0.011°C, while conductivity increased by 0.232 µS/cm, with the highest values recorded downstream and within cage sites. Dissolved oxygen and water transparency declined annually by 0.0438 ± 0.02 mg/l and 0.033 ± 0.005 m, respectively. The fitted smooth trends revealed a significant decline in dissolved oxygen and water transparency at sites located within, between, and downstream of cages compared to upstream sites, indicating impacts from cage farming activities. Total algal biovolume declined from more than 900 µm³/L in 2011 to 13.5 µm³/L in 2019, with cyanobacteria remaining dominant throughout the study period. Species richness ranged from 11 to 106 species, with the highest richness recorded in 2011 and the lowest in 2019. Community analysis showed changes in phytoplankton structure over time, while lower water transparency was associated with less diverse communities.

Therefore, cage farming contributes to environmental changes at cage sites. Continuous monitoring, harvesting during dry seasons, fallowing practices, cage biomass limits, and harmonized environmental monitoring procedures are essential for sustainable cage aquaculture in shared and transboundary water bodies.