Journal des effets et du contrôle de la pollution
Libre accès
ISSN: 2375-4397
ISSN: 2375-4397
Ali AS* and Tsega DK
Legedadi reservoir, which is the most important single purpose reservoir, provides about eighty percent of the household water supply to Addis Ababa City. However, its water qualities are declining from time and time. For several years, the reservoir has grieved from irregular massive algal blooms, increased turbidity and undesirable odor and taste. The objective of this study was to investigate the spatial and temporal phytoplankton and nutrient dynamics in Legedadi reservoir. For this study, biweekly samples were collected from three sampling stations for ten months and pooled to a monthly data. Composite samples, prepared from discrete depths, were used for the analysis of nitrate, ammonium, orthophosphate and silica. Plankton net with 10 μm mesh size was used to collect sample for phytoplankton species identification and estimation of their relative contribution to the community. All the limiting nutrients significantly varied both temporally and spatially (P<0.05) with most peak values coincided with the short and long rains. Although the reservoir didn’t show green patches or algal scums on the surface, the phytoplankton biomass was found to be very high and significantly varied seasonally (P<0.05). Blue green algae, particularly Microcystis and Anabaena dominated the community (63-95%) throughout the study period particularly during the dry and short rainy months. This study showed that the algal bloom was initiated by the significant temporal variabilities of transparency and high levels of nutrients. The year-round persistent turbidity, relatively high temperature and low under-water irradiance favored blue green algae; particularly Microcystis and Anabaena spp. seem to be associated with the prevailing algal bloom.