<![CDATA[Plankton are important bioindicators for assessing the ecological condition of water bodies. However, an abundance based approach does not always reflect energy distribution within an ecosystem because small organisms can dominate numerically without contributing comparable biomass. his situation can cause ecological decoupling, a discrepancy in ecological interpretation between abundance-based and biomass-based indicators. This study aims to analyze and compare plankton community structure based on abundance and biomass and to evaluate the presence of ecological decoupling in the Koto Panjang Reservoir as a representative of a tropical reservoir influenced by hydrological dynamics and anthropogenic pressures. The study was conducted from May 2025 to April 2026 at six stations representing the riverine, transitional, and lacustrine zones. Plankton samples were analyzed using the Sedgwick–Rafter Counting Cell method, while biomass was estimated using the biovolume approach. The analysis included Bray–Curtis cluster analysis, ecological indices, Pearson correlation, linear regression, and Bland–Altman analysis. The results revealed three distinct spatial zones (riverine, transitional, and lacustrine), with plankton communities dominated by Staurastrum spp., members of the Desmidiaceae family. Phytoplankton abundance ranged from 531,000 to 1,297,000 cells L⁻¹, while phytoplankton biomass ranged from 0.19 to 0.85 mg L⁻¹. Abundance-based indices showed higher Shannon diversity values (2.74–2.97) compared to biomass-based indices (0.54–0.66). The relationship between the two approaches was relatively weak (r = 0.41–0.48; R² = 0.17–0.23 indicating differences in how community structure is represented. Pearson correlation analysis, linear regression, and Bland–Altman plots revealed a weak relationship and low agreement between the two approaches, confirming ecological decoupling. These findings suggest that integrating abundance- and biomass-based indicators is necessary to make assessments of the ecological condition of tropical reservoirs more comprehensive and representative of community structure and energy distribution within the ecosystem.]]>
