<![CDATA[This study aims to evaluate the water quality of the Dumai River based on plankton community structure and physicochemical parameters. This research was conducted to understand the ecological response to long-standing environmental pressures in the Dumai River through community and industrial activities. This study was conducted by collecting water and plankton samples from the Dumai River using a plankton net in the downstream, middle, and upstream sections. The analysis results showed a significant decline in water quality in the downstream section, characterised by high Total Dissolved Solids (TDS) levels of 3270 mg/L and Dissolved Oxygen (DO) approaching the minimum limit. This condition reflects increasing anthropogenic pressures that are affecting ecosystem stability. The impact is visible in the plankton community structure, where Species Richness (R) decreases from upstream to downstream, allowing only species with high tolerance to these pressures to survive. Despite the decline in species abundance, Diversity (H') values remained relatively high with Evenness (E) increasing, indicating that the downstream section functions as an ecological filter producing a balanced but simpler community. Regression analysis identified a strong positive correlation (R² = 0.75–0.84) between H' and water quality parameters such as BOD, nitrate, DO, and TSS, confirming that habitat conditions in the moderate range are the main determinants of plankton diversity. These findings emphasise the need for continuous water quality monitoring to support more effective management of the Dumai River ecosystem.]]>
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