Differences in long-term impacts of un-ionized ammonia on life-history traits of three species ofDaphnia

Abstract

Daphnia spp. feeds on microalgae and can potentially be used to screen microalgae, which are grown in ammonia-containing wastewater to scavenge available nutrients. A key step for this to work is that Daphnia spp. can still feed in the presence of elevated levels of un-ionized ammonia. To test that, we studied the life-history of three species of Daphnia (Daphnia obtusa, Daphnia similoides, and Daphnia similis) exposed to ammonia-containing water for 21 days. Results showed that survival, development, and fecundity of three tested Daphnia species were negatively affected by un-ionized ammonia. Although there were some differences in details among three species, the three-parameter logistic model fitted the data of three species well and showed similar patterns of response to un-ionized ammonia toxicity. After the 21-day exposure, the increasing NH3N concentrations slightly decreased the survival time by about 1 or 2 days in D. similoides and D. similis, but by 10 days in D. obtusa. The time to maturity was delayed in all three species after being exposed to ammonia. With increasing un-ionized ammonia concentrations, the number of moults decreased from 14.50 to 8.75 in D. obtusa, 13.60 to 11.20 in D. similoides, and 14.25 to 12.25 in D. similis. The overall reproduction per female was reduced by 81.4% in D. obtusa, 82.1% in D. similoides, and 64.6% in D. similis under the NH3N concentration of 0.58 mg/L. The EC50s for total offspring per female in D. obtusa, D. similoides, and D. similis were 0.27, 0.55, and 0.54 mg/L, respectively. Based on the experimental results, D. similoides and D. similis, showing stronger ammonia-resistance than D. obtusa, were more promising for screening microalgae cultivated in ammonia-containing wastewater.