Abstract

Phytoplanktons constitute the first circle of the food chain in aquatic systems, producing high-energy organic compounds from carbon dioxide and inorganic substances by photosynthesis with light. As is well known, the food chain starts with phytoplankton and continues with zooplankton, small fish, big fish, and ends with a human. This cannot be achieved without good water quality management. This research aims to reveal the effect of seasonal variation on phytoplankton composition in the Kitoro reservoir. The phytoplankton samples were collected using a silk bolting phytoplankton net with a mesh size of 20 μm. Samples were preserved in 10 % formalin in the field and were taken to the Laboratory for analysis using the inverted binocular microscope (Olympus). The samples for physicochemical parameters were collected monthly using a 250 ml glass sampling bottle. The samples were examined at the Limnology Laboratory using standard methods. Conductivity was determined using a conductivity meter, pH using a pH meter, depth, and turbidity using a calibrated Secchi disc, DO and BOD using Winkler Azide Modification Method, Alkalinity using a titration method, and temperature using Mercury in glass thermometer. The result was subjected to statistical analysis using a T-test. The result shows a significant difference (P<𝟎.𝟎𝟓) in pH, water temperature, dissolved oxygen, turbidity, alkalinity, and depth, while there was no significant difference (P>𝟎.𝟎𝟓) in biological oxygen demand and water conductivity. Chlorophyceae dominated the group of phytoplankton in the wet season (79%), followed by Cyanophyceae (12%) and Bacillariophyceae (9%). In the dry season, chlorophyceae (58%) dominated the group, Cyanophyceae (25%) was next, then Bacillariophyceae (17%). The significant difference in turbidity of the water could be the reason for higher phytoplankton in wet season. This could be because light penetration increases with higher turbidity and thus increases photosynthesis. Finally, seasonal variation affects water quality parameters and phytoplankton abundance in the Kitoro reservoir.