Zooplankton Diversity and Seasonal Variability in Kalmadu Reservoir, Northern Sri Lanka

Zooplankton play a crucial role in freshwater ecosystems by contributing to energy transfer and nutrient cycling. This study assesses the diversity and of zooplankton in the Kalmadu Reservoir, Northern Sri Lanka, over a six-month period (August 2024–January 2025). Water samples were collected from ten locations using a 5L bucket and filtered through a 64-µm mesh plankton net. Standard taxonomic keys were used to identify species, and Lackey’s drop count method was applied for population density estimation. A total of 10 zooplankton species were recorded, including Paramecium aurelia, Vorticella citrina, Moina brachiata, and Mesocyclops leukarti. Species abundance varied monthly, with higher counts observed during periods of increased water levels. The Simpson index ranged from 0.9664 to 0.9993, indicating species dominance, while the Shannon index (0.0031–0.1108) suggested low species evenness. The dominance of protozoan species, particularly Vorticella citrina, suggests eutrophic conditions in the reservoir. These findings highlight the importance of zooplankton diversity in ecosystem functioning and fisheries productivity. Seasonal shifts in community structure emphasize the need for long-term monitoring and further investigation into environmental drivers such as dissolved oxygen, pH, and nutrient levels to inform conservation and water resource management strategies.