Toxicity of pulsed metal exposures in aquatic ecosystems: Development of a predictive model based on the responses of Daphnia magna

Toxicant concentrations in the natural environment vary with time and location. Aquatic organisms living in surface waters would be exposed to fluctuating contaminant concentrations in either scenario. Hence, results of standardized laboratory bioassays may not represent responses of indigenous organisms in receiving surface waters. This research characterized the responses of Daphnia magna to pulsed metal exposures. During 21-d toxicity tests, D. magna were exposed to single or double pulses of Cu, Zn, Se, or As. Metal concentrations ranged from 32 to 64 mug/L for Cu, 125 to 3000 mug/L for Zn, 800 to 2000 mug/L for Se, and 3000 to 6000 mug/L for As. Duration of exposure ranged from 3 to 144 h for Cu, 6 to 504 h for Zn, 3 to 24 h for Se, and 3 to 120 h for As. Recovery time between the two metal pulses ranged from 0 to 288 h. Twenty-one day mortality increased with increasing exposure concentration and duration. Increase recovery time between pulsed exposures decreased mortality, except for arsenic exposures, during which mortality slightly increased with increasing recovery time. Twenty-one day growth and accumulative reproduction of D. magna were not affected by pulsed metal exposures due to post exposure recovery, except for copper. Twenty-one day pulsed exposure mortality and accumulative reproduction of D. magna exposed to all four metals exhibited biphasic age-response relationships. Peak sensitivity occurred in 4-day old organisms exposed to Cu and Zn while peak sensitivity occurred in 2-3 day old organisms for As and Se. The influence of organism age on the toxicity of Se and As was greater than that of Cu and Zn. In general, 21-d pulsed exposure mortality was a function of exposure concentration, duration, and recovery time between pulsed exposures. A model based on first order metal uptake kinetics was developed to explain the variation in 21-d mortality as a function of metal concentration, duration of exposure, and recovery time between exposures. Results of this research suggest that water quality criteria may be overly conservative unless results of fluctuating contaminant exposure bioassays are considered in ecological risk assessments and development of water quality criteria.