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.
