Long-term study of the drought impact on the phytoplankton concentration and assemblages in two water supply reservoirs in Namibia

Rising temperatures and increased occurrences of droughts, brought on by climate change, are expected to affect reservoir water levels. We hypothesised that the decrease in reservoir volumes in dams with desert climates will favour the growth of phytoplankton biomass, measured as chlorophyll a (Chl a), under drought conditions. Using the threshold levels method of Q50, ten drought years were recorded in the Von Bach Dam (VB) in comparison to seven in the Swakoppoort Dam (SWP). Both dams had significant reduction in percentage volume (vol %). The Chl a in SWP was 81 mu g l(-1) in drought and 48 mu g l(-1) in AAR years, compared to VB with 21 mu g l(-1) in drought and 24 mu g l(-1) in AAR years. Cyanobacterial cell counts in drought and AAR years in SWP were 129 704 and 77 838 cells ml(-1), respectively, while cell counts in drought and AAR years in VB were 5 925 cells ml(-1) and 20 070 cells ml(-1), respectively. Decreases in phytoplankton biomass and total cyanobacteria were observed in SWP but not in VB. The pattern and magnitude of the statistically significant responses (t-test, p < 0.05) between physico-chemical and biological water quality variables varied among drought and AAR years. We conclude that that there is no clear correlation between drought and phytoplankton biomass from this study and that other anthropogenic-environmental drivers, such as land use impacts, will be needed to further elucidate the response of water quality to droughts.