An effective minimum concentration of un-ionized ammonia nitrogen for controlling Prymnesium parvum

The minimum concentration of un-ionized ammonia nitrogen needed to kill the alga Prymnesium parvum in 24-48 h was determined from ammonium sulfate treatments. Five treatments [0, 3, 5, 10, and 15 mg (NH4)(2)SO4/L] were tested at pH 8 and pH 9 and temperatures of 15, 20, and 25degreesC to generate various concentrations of total and un-ionized ammonia nitrogen; their effectiveness in killing the alga was then monitored for 48 h. There was a strong relationship between treatment success and the level of ammonium sulfate or un-ionized ammonia nitrogen. The ammonium sulfate treatments that killed the alga did so within the first 24 h, and their effectiveness at 48 h was not significantly different from that at 24 h. At pH 9, ammonium sulfate concentrations above 5 mg/L eradicated the alga approximately 90% of the time, compared with 33% for the other treatments. In terms of un-ionized ammonia nitrogen, 0.055 mg/L was similar to the control in failing to kill the alga; however, 0.17 mg/L eradicated the alga 80% of the time. We suggest 0.17 mg/L of un-ionized ammonia nitrogen as the minimum concentration for therapeutic control of P. parvum.