Ammonia toxicity to macrophytes (common duckweed and rice) using static and renewal methods

Ammonia is a widespread contaminant that occurs in municipal wastewaters, industrial wastewaters, and agricultural wastes. Most ammonia toxicity studies deal with species of fish and macroinvertebrates, but very little information is available on ammonia phytotoxicity. The objectives of this study were to compare ammonia toxicity by using static and renewal methods and to compare phytotoxicity tests by using the vascular plants common duckweed and rice. Un-ionized ammonia-N at the rate of 8.85 mg/L inhibited rice seedling growth by 27%, using the renewal method, whereas the same amount caused insignificant effects on rice growth, using the static method. Ammonia-N at up to 8.85 mg/L did not inhibit duckweed growth, using the static method. By using the renewal method, un-ionized ammonia-N at concentrations greater than 3.0 mg/L depressed duckweed growth noticeably (20% or more). The increased ammonia concentration inhibited duckweed growth correspondingly, producing a linear relation for the concentration-inhibition effect. An un-ionized ammonia-N concentration of 7.16 mg/L was calculated to cause 50% duckweed growth inhibition (IC50), and 95% confidence limits ranged from 6.33 to 8.47 mg/L.