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Comparative Toxicity of Three Organic Acids to Freshwater Organisms and Their Impact on Aquatic Ecosystems

ABSTRACT The comparative toxicity of lactic acid, acetic acid, and benzoic acid to tilapia (Oreochromis mossambicus), cladoceran crustacea (Moina micrura), and oligochaete worm (Branchiura sowerbyi) were determined using static bioassay tests. Worms were found most sensitive to all the acids whereas the cladoceran was found most resistant to lactic acid and the fish most resistant to acetic acid and benzoic acid. The 96h LC50 values of lactic acid, acetic acid, and benzoic acid, were, respectively, 257.73, 272.87, and 276.74 mg L−1 for O. mossambicus; 329.12, 163.72, and 71.65 mg L−1 for M. micrura and 50.82, 14.90, and 39.47 mg L−1 for B. sowerbyi. Tilapia lost appetite at sub-lethal concentrations as low as 2.18 mg L−1 lactic acid, 1.26 mg L−1 acetic acid, and 13.84 mg L− 1 of benzoic acid. Growth and reproduction of the fish were affected following 90-day chronic exposure to sub-lethal concentrations of the acids. Minimum effective concentration of the acids that significantly reduced food conversion efficiency (FCE), percent increase of weight, specific growth rate, yield and fecundity of the fish were 2.18, 1.47, and 3.95 mg · L−1 of lactic acid, acetic acid, and benzoic acid, respectively. Effects of acetic acid and benzoic acid on FCE, weight increase, and yield were not significantly different from each other whereas lactic acid produced different effects from acetic acid as well as benzoic acid. Mean values of dissolved oxygen, primary productivity, and plankton populations of the test medium significantly reduced from control at 16.94 mg L−1 lactic acid, 16.79 mg L−1 acetic acid, and 13.84 mg L−1 benzoic acid.