Estimation of phytoplankton biomass and net primary production in relation to some physicochemical parameters in Omo Gibe III Reservoir, Southern Ethiopia

Spatio-temporal variation of phytoplankton biomass and net primary production in relation to physicochemical parameters in Omo Gibe III Reservoir, southern Ethiopia were investigated in 2024. Phytoplankton samples were collected from three sites, representing varying anthropogenic impacts across dry and wet seasons. Chlorophyll-a (Chl-a) and Net Primary production (NPP) were estimated using spectrophotometry and the light–dark bottle technique, respectively. Fifteen phytoplankton genera were identified, dominated by Bacillariophyceae (40.3%), followed by Cyanophyceae (30.2%) and Chlorophyceae (29.5%). Significant variations in biomass were observed both spatially and temporally. Biomass peaked at the high-impact site during the dry season, reaching 38.21 mg Chl-a m⁻3, but dropped to its lowest point of 4.82 mg Chl-a m⁻3 in the wet season. In contrast, the low-impact site exhibited an opposite trend, suggesting the presence of localized nutrient inputs. Net primary productivity increased across the reservoir during the wet season, particularly in offshore areas. Correlation analysis revealed that nitrate was a significant driver of seasonal productivity, showing a strong positive relationship with NPP (r = 0.6825, p < 0.05). Turbidity showed a moderate positive correlation with NPP (r = 0.312), whereas pH, temperature, and dissolved oxygen displayed weak negative correlations with one another and with NPP. Overall, human activities, nutrient availability and seasonal changes played a crucial role in influencing phytoplankton biomass and NPP.