Daily variation in short-term static toxicity of unionized ammonia in Litopenaeus vannamei (Boone) postlarvae

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 degrees 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: alpha (pH 8, 26 degrees C) and beta (pH 9, 30 degrees C). In environmental scenario alpha, ammonium concentrations up to 18 mg L-1 pose no short-term mortality risks for ages 1-30 days. In scenario beta, 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.