A systemic study on the natural radionuclides such as 210Po and 210Pb in the environmental matrices and biota of Ponnusamuthiram Lake (Kumarapalayam) has been undertaken to establish a baseline data on the radiation profile of Ponnusamuthiram Lake ecosystem. The environmental samples, such as water, sediment, and biota (Catla catla, Channa punctata, Clarias batrachus, and Oreochromis mossambicus), crustaceans (Macrobrachium rosenbergii and Macrobrachium malcolmsonii), and bivalves (Lamellidens marginalis), have been subjected to analyses. It has been observed that the concentration of 210Po and 210Pb in the water samples of Ponnusamuthiram Lake was 1.95 and 3.70 mBq L−1, respectively. The activity concentration of 210Po and 210Pb in the sediment sample was 65.31 and 21.12 Bq kg−1, respectively. The bivalve mollusc Lamellidens marginalis have been identified to accumulate higher concentrations of 210Po and 210Pb suggesting that they could serve as bioindicators of radionuclides in the Ponnusamuthiram Lake ecosystem. The concentration factor of 210Po for the biotic components ranged from 1.0 × 103 to 3.0 × 105, while for 210Pb, it ranged from 1.0 × 103 to 4.7 × 104. The committed effective dose for human beings was found at 21.74–248.20 and 16.84–118.67 μSv y−1 for 210Po and 210Pb, respectively.