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In this study, we evaluated the toxic effects of heavy metals (Cd, Cu, Zn) on the populationgrowth rate (r) of the marine diatom, Skeletonema costatum. S. costatum. The population growthrate (r) of the species was determined after 96 hrs. of exposure to Cd (0, 0.63, 1.25, 2.50, 5.00, 10.00ppm), Cu (0, 0.25, 0.50, 0.75, 1.00, 1.25, 1.50 ppm) and Zn (0, 0.31, 0.63, 1.25, 2.00, 2.50, 5.00 ppm).
It was observed that ‘r’ in the control (absence of Cd, Cu and Zn) were greater than 0.05, howeversuddenly decreased with increased heavy metal concentrations. Cd, Cu and Zn reduced ‘r’ in adose-dependent manner and a significant reduction were occurred at concentration of greaterthan 1.25, 1.25 and 2.50 ppm, respectively. Based on the toxicity, the heavy metal were ranked asCu¤Zn¤Cd, with EC50 values of 1.11, 2.13 and 6.84 ppm, respectively. The lowest-observedeffective-concentration (LOEC) of ‘r’ in exposed to Cd, Cu and Zn were 1.25, 1.00, 2.00 ppm,respectively. Precisely, a concentration of greater than 1.25 ppm of Cd, 1.00 ppm of Cu and 2.00ppm of Zn in marine ecosystems induced toxic effect on the ‘r’ of S. costatum. Based on our results,we suggested that the ‘r’ of S. costatum might be a useful bio indicator for the toxicity assessmentof heavy metals in marine ecosystems.