| Abstract: | Ammonium toxicity of short‐duration alkaline events and their variability, as related to 1–30 day‐old postlarvae whiteleg shrimp Litopenaeus vannamei (Boone), was assessed by determining medium lethal concentration (LC50) of total ammonium‐nitroen (TAN) and NH3‐N to 4‐h exposures. Exploratory concentrations of TAN were tested at 30°C and pH 9, until mortality from 5% to 95% occurred between 0.9 and 18 mg N L?1. To determine the daily variation of ammonium toxicity, 64 lots of 20 postlarvae were exposed to eight different ammonium concentrations (0, 0.9, 3, 6, 9, 12, 15 and 18 mg N L?1), in two different environmental scenarios: α (pH 8, 26°C) and β (pH 9, 30°C). In environmental scenario α, ammonium concentrations up to 18 mg L?1 pose no short‐term mortality risks for ages 1–30 days. In scenario β, mortality was recorded at all ages. The values of LC50 (4 h) for different postlarvae ages have daily variability, ranging from a minimum of 2.54 to a maximum of 6.02 mg L?1 of TAN (0.76 and 1.81 mg N L?1 of NH3‐N), for PL3 and PL19, respectively, with a logarithmic linear tendency to increase with age. Postlarvae mortality at 4 h and 3.0 mg N L?1 TAN exposure was lower and less variable in ages greater than 19 days old. |
