Zooplankton diversity and seasonal variability in Kalmadu reservoir, northern Sri Lanka

Abstract

Zooplankton plays a vital role in freshwater ecosystems by contributing to energy transfer and nutrient cycling. This study examined how does seasonal variation influences zooplankton diversity and abundance 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. At each location, five replicate samples were taken using horizontal bucket tows, to ensure representative sampling of both surface and subsurface layers. Species identification was conducted out using standard taxonomic keys, and population diversity was estimated using Lackey’s drop count method. A total of ten zooplankton species were recorded, including Paramecium aurelia, Vorticella citrina, Moina brachiata, and Mesocyclops leukarti. Species abundance varied seasonally, with higher densities observed during periods of elevated water levels. The Simpson Diversity Index ranged from 0.9664 to 0.9993, indicating high species dominance, while the Shannon index (0.0031–0.1108) suggested low species evenness. The dominance of protozoan species, particularly Vorticella citrina, indicates eutrophic conditions in the reservoir. These findings underscore the ecological importance of zooplankton diversity in maintaining ecosystem function and supporting fisheries productivity. The observed dominance of protozoan species, low evenness, and seasonal shifts suggest early signs of ecological stress. These conditions imply reduced energy transfer efficiency to fish populations and a risk of algal blooms due to nutrient buildup. Regular monitoring and controlled nutrient input are essential for sustaining the reservoir’s biodiversity and fisheries. Such studies provide valuable insights for conservation and effective water resource management.