The Assessment of Phytoplankton Dynamics in Two Reservoirs in Southern Africa with Special Reference to Water Abstraction for Inter-Basin Transfers and Potable Water Production

Abstract

Toxic phytoplankton in the aquatic ecosystems are dynamic, affecting water quality. It remains unclear as to how possible toxic phytoplankton assemblages vary vertically and temporally in Swakoppoort and Von Bach dams, located in a dry subtropical desert region in central Namibia. The following variables were analyzed: pH, Secchi depths, turbidity, water temperature, total phosphorus, orthophosphate, chlorophyll-a, phytoplankton cells, and water depths. Cyanobacteria dominated the phytoplankton community in the autumn, winter and spring (dry) and summer (wet) seasons, at all the depth ranges in both dams. Microcystis dominated the vertical and temporal dynamics, followed by Dolichospermum. In the dry seasons, higher cyanobacteria cell numbers were observed in comparison to the rainy season in both dams. Spring blooms of cyanobacteria were evident in the Von Bach Dam while autumn and spring cyanobacteria blooms were observed in the Swakoppoort Dam. In the Swakoppoort Dam, the preferable depth ranges for toxic cyanobacteria species were at 5 to 10 m while in the Von Bach Dam at 0 to 5 m range. The findings of the current study indicate that the traditional selective withdrawal of water in the two dams should be performed with vertical and temporal dynamics of possible toxic cyanobacteria accounted for to aid the abstraction of water with the lowest possible toxic phytoplankton numbers, which could lower the public health risk.

Using Multiple Indices for the Water Resource Management of a Monomictic Man-Made Dam in Southern Africa

“…Water quality parameters considered for analysis included common major cations (NH 4 + ), anions (NO 3 − , NO 2 − ), and one system water quality parameter (pH). Although there were missing periods from the 2001-2021 dataset that was used in this study, due to challenges of not sampling caused by the expired license registration of the boat and a lack of human capacity as well as financial constraints, the study used a long-term data set analysis [17] that proved to be sufficient to provide insights into the water quality of the dam. The study employed the analysis of annual mean concentrations, which were then presented with Microsoft Excel 2016 surface graphs that showed the temporal and spatial distribution of the selected parameters.…”

“…Further unwanted effects of algal blooms include the generation of foul odor and a reduction in biodiversity [84,85]. The occurrence of potentially toxic cyanobacteria species in drinking water dams leads to the abstraction of poor raw water quality for inter-basin and potable water provision, and this could be more severe in subtropical dams located in climate regions characterized by low rainfalls and high evaporation, such as the Vaal Dam in South Africa and Swakoppoort Dam in Namibia [17].…”

“…Freshwater resources are strategic socio-economic assets in semi-arid Southern African countries and water supply systems depend predominantly on dams. The concerning fact is that the most important dams used for water supply in Southern Africa, such as Lake Chivero in Zimbabwe, the Vaal and Hartbeespoort dams in South Africa, and the Von Bach Dam in Namibia, suffer from excessive nutrient enrichment, with some already in a chronic eutrophic state [13][14][15][16][17]. The declining water resource quality threatens current and future water supply availability.…”

Advances in the integration of microalgal communities for biomonitoring of metal pollution in aquatic ecosystems of sub-Saharan Africa

“…Recent studies on South Africa’s urban Molopo River depicted anthropogenic Cu, Cr, Zn, and Pb sediment contamination with potentially deleterious ecological impacts on the benthic algal and macrofauna communities (Mohajane and Manjoro 2022 ). Additionally, the diatom-based biomonitoring tools (e.g., the “South African Diatom Index (SADI)” and the “Benthic Diatom Index (BDI)”) have been used to detect and quantify the magnitude of natural and anthropogenic impacts on stream ecosystems (Lang et al 2013 ; Harding and Taylor 2014 ; Sirunda et al 2021 ).…”

Evaluating the Application of Chitosan-Based Sorbents for the Solid-Phase Adsorption Toxin Tracking of Microcystins in Irrigation Water

“…Reports have also shown that dams such as those at Vaal and Hartbeespoort are dominated by toxic cyanobacteria such as Dolichospermum and Microcystis sp. [4]. Toxic blooms present a threat to the South African population, since water derived from the impacted reservoirs is mainly used for drinking [3] and irrigation purposes [5].…”