The coastal waters of Chennai face increasing threats from pollution caused by untreated sewage, industrial effluents, and urban runoff, particularly through creeks such as Ennore, Napier, and Adyar. This study assesses the extent of coastal pollution using phytoplankton and zooplankton communities as bioindicators. Environmental parameters including temperature, pH, salinity, total dissolved solids (TDS), dissolved oxygen (DO), nitrate, inorganic phosphate, and total chlorophyll were measured at each site to evaluate water quality. Results revealed significant spatial variability in physicochemical parameters, with Napier Creek exhibiting the highest pollution load, characterized by low DO levels (<2 mg/L) and reduced plankton densities. In contrast, Adyar Creek showed elevated plankton densities (>500 individuals/L), indicative of eutrophic conditions driven by nutrient enrichment. Regression analysis demonstrated strong correlations between plankton densities and environmental parameters, underscoring the sensitivity of plankton communities to changes in water quality. Phytoplankton diversity included genera such as Chlorella spp., Nitzschia spp., and Skeletonema spp., while zooplankton communities were dominated by copepods such as Paracalanus crassirostris, Tortanus discaudatus, and Oithona spp. The diversity and density of these bioindicators provided valuable insights into the ecological status of the creeks. Moreover, climate change is expected to intensify pollution dynamics and biodiversity shifts by altering temperature, salinity, and nutrient availability factors that directly affect planktonic community structures and overall ecosystem health. This study demonstrates the utility of plankton-based biomonitoring in assessing coastal pollution and underscores the urgent need for improved wastewater treatment and ongoing environmental surveillance to safeguard Chennai’s coastal ecosystems.