|dc.description.abstract||This study investigates the effect of aquaculture on Phytoplankton diversity and abundance in Lake Kariba. Water parameters that were monitored included: phosphate, nitrate and silica concentrations. Water samples were analysed from an aquaculture station at Yalelo fish cages located at Kamimbi area and a non-aquaculture station at Sampa Kaluma Island on Lake Kariba in Siavonga. A total of 44 species were identified belonging to 24 taxonomic groups. Oscillatoria sp. dominated the phytoplankton community in terms of relative abundance and contributed 29.6 per cent of the total observed phytoplankton at Kamimbi area and 24.3 per cent at Sampa Kaluma Island. The dominance was followed by Microcystis aeruginosa and Chlorella sp. Many of the algae are grazed by protozoa. Cyanobacteria such as Oscillatoria sp. and Microcystis aeruginosa are not grazed to the same extent as other algae, hence out competing other organisms (phytoplankton). They require little energy for maintenance of cell function and hence having high growth rate than other phytoplankton. They were also other species whose relative abundance was quite low.
Results showed that concentration of nitrate was the same in the two stations, at Kamimbi area where there are fish cages the assimilation capacity for nitrate was not exceeded by the input of the nutrient from aquaculture hence having a constant concentration in the two stations. The constant concentration of nitrate resulted in a condition of having no correlation (r2 = 0) between nitrate and diversity and abundance of phytoplankton in both stations. Results also showed that there was a significant difference (P < 0.05) in concentrations of phosphate and silica between the two stations. There was more phosphate at Sampa Kaluma Island than at Kamimbi area. The concentration at Kamimbi remained at 0.01 mg/cm3 at all depths because of assimilation capacity which was not exceeded by the input from aquaculture while at Sampa Kaluma Island the high concentration resulted from hydrology causing diffusion from the sediments. Kamimbi area had more concentration of silica because of its close proximity to the shore which was the source and also as a result of diffusion from the sediments that are not as deep as that of Sampa Kaluma Island sediments.
There was no correlation (r2 = 0) between phosphate and diversity and abundance of phytoplankton at Kamimbi area due to constant concentration of phosphate, however there was a very low correlation (r2 = 0.086) between phosphate and abundance of phytoplankton at Sampa Kaluma Island, because of variation in terms of concentration. There was also a correlation (r2 = 0.172) between phosphate and diversity of phytoplankton at Sampa Kaluma Island due to higher concentration of phosphate. There was a correlation between silica and abundance of phytoplankton in both stations (Kamimbi area; r2 = - 0.120 and Sampa Kaluma Island; r2 = 0.57). From the results obtained, it shows that aquaculture has had no effect on the phytoplankton diversity and abundance; aquaculture has had no effect on the concentration of nitrate and phosphate. There has been an input of phosphate and nitrate from aquaculture, but without a net change in nutrient balance. Aquaculture on Lake Kariba still remains at a small scale; no wonder the lake is still oligotrophic. However, the abundance of phytoplankton is higher at Kamimbi area compared to Sampa Kaluma Island.
Keywords: Aquaculture; Phytoplankton; Nutrients; Abundance; Diversity||en