The algae of Gaborone wastewater stabilization ponds: Implications for effluent quality

The types of algae found in the wastewater stabilization ponds in Gaborone were studied. Being the base of the food chain in any aquatic habitat, algae contribute significantly to the functioning and value of the ponds. The (liversit)' and abundance of the algae in the two pond systems at Broadhurst and Phakalane were determined. The nutrient composition i.e nitrates ami phosphates of the ponds were also determined. The ponds were found to have lower nutriellfs than the effluents from wastewater  treatment plant. Phakalane ponds had higher amounts of nutrients  compared to Broadhurst ponds. Blue-green algae (c)'anobacteria) and green algae were found to be the dominant algal groups. The Broadhurst ponds were dominated by green algae (58%) of all algae recorded  followed by blue-green algae (23.8%), diatoms (9.2%) and euglenoids  (8.5%). However, the blue-green algae were the most abundant group ill the Phakalane ponds (57.5%) followed by green algae (27%), diatoms (10.9%) and euglenoids (4.7%). Chlorella sp, Coelastrum combricum, C. microporium, Pediastrum boryanum and Scenedesmlls quadracauda were the most abandunt green algal species in the ponds. The blue-green algae Microcystis aeruginosa and M. flos-aquae were the most dominant species in both ponds, often forming heavy blooms. The blooms were the most striking feature of the ponds especially during the warm months. In this paper, the implications of the occurrence of algal blooms in the ponds and the use of the effluent for irrigation of edible plants and other uses are discussed. Key words : algal bloom, algal diversity, wastewater treatmellt, alga toxins, Microcysti