Toxicity of Two Pulsed Metal Exposures to Daphnia magna: Relative Effects of Pulsed Duration-Concentration and Influence of Interpulse Period

Abstract

Aquatic organisms living in surface waters experience fluctuating contaminant exposures that vary in concentration, duration, and frequency. This study characterized the role of pulsed concentration, pulsed duration, and the interval between pulses on the toxicity of four metals (Cu, Zn, Se, and As) to Daphnia magna. During 21-d toxicity tests, neonatal D. magna were exposed to single or double pulses. Pulsed concentrations and durations ranged from 32 to 6000 microg/L and 8 to 96 h, respectively. Intervals between two pulses ranged from 24 to 288 h. Mortality, growth, and reproduction were characterized for exposures. For single-pulse exposures of Cu and As, metal concentration had a stronger effect on survival of D. magna than did pulsed duration: pulses with 2X concentration and 1Y duration resulted in more mortality than did pulses with 1X concentration and 2Y duration. In contrast, effects of pulsed duration were stronger than metal concentration for Zn. However, the effects of duration and concentration were similar for Se. The relative effects of pulsed concentration and duration found in the present study revealed that the common method using area under the curve (AUC = concentration x duration) may not always accurately estimate environmental risk from metals (e.g., for Cu, Zn, As). In addition, the occurrence of delayed mortality in the present study revealed that using continuous exposure bioassays might underestimate metal toxicity to aquatic biota. For double-pulse exposures, the toxicity of the second pulse was influenced by the first pulse for all four metals. This influence was dependent on the pulsed concentration and duration and the interval between pulses. Further, toxicity caused by the second pulse decreased as the time between the exposures increased. For all four metals, there existed an interval great enough that the toxicity of the two pulses was independent. This would result in less toxicity for multiple exposures than continuous exposures with the same total exposure duration. The interval time at which the effects of the two pulses were independent increased with increasing concentration. Growth and cumulative reproduction of D. magna over 21 d were not significantly affected by pulsed exposures examined in the present study, indicating recovery of the organisms.

Toxicity to Melita plumulosa from intermittent and continuous exposures to dissolved copper

“…This further supported the finding that different exposures with similar TACs resulted in similar toxicity. This trend is supported by studies in which similar toxicity was the result of different exposure scenarios with equivalent metal TACs for D. magna exposure to Se [20] and P. promelas exposure to Cd [25]. The results of the current study indicated that continuous exposure bioassays with M. plumulosa are likely to accurately predict the acute-lethality effects of pulsed exposures with similar TACs, and supports the use of TAC-effect data to predict the risk of acute toxicity [2,6].…”

“…These results indicated that the pulse concentration had a greater influence on toxicity than the pulse duration. This may occur because pulses with higher concentrations facilitate metal transport to the target sites within organisms, as reported for the green mussel Perna viridis exposed to Cd, S. gairdnerii exposed to Cu, and D. magna exposed to As and Cu [14,20,31].…”

“…The delayed deaths may be attributable to slow Cu depuration rates after the pulsed exposures ended [20]. The rate of depuration reported for adult M. plumulosa (40 d old) after 24-h exposure to radiolabeled dissolved Cu was much lower than the rate of uptake [22].…”

Time-averaged copper concentrations from continuous exposures predicts pulsed exposure toxicity to the marine diatom, Phaeodactylum tricornutum: Importance of uptake and elimination

“…However, many studies have reported the opposite, with shorter duration pulsed exposures of higher concentrations causing greater toxicity than longer exposures of lower concentrations because the higher concentrations that achieve an equivalent dose overwhelm the test organisms for even short exposures. For exposures scenarios with equivalent doses, shorter exposures of higher concentrations were more toxic to survival of D. magna exposed to arsenic and copper (Hoang et al, 2007), survival of Daphnia pulex exposed to copper (Ingersoll and Winner, 1982), survival of Melanotaenia fluviatilis exposed to fenvalerate (Holdway et al, 1994), and growth (body mass) of Limnephilus lunatus exposed to fenvalerate (Schulz and Liess, 2000). Several studies have also reported no difference in toxicity to organisms caused by continuous and pulsed exposures when expressing effects based on time-averaged dose, such as survival of Melita plumulosa and D. magna exposed to dissolved copper (Angel et al, 2010) and selenium (Hoang et al, 2007), respectively.…”

“…The effects of pulsed exposure to organic contaminants such as insecticides and pesticides have been the most studied class of contaminants historically, with fewer and more recent studies of metal contaminants (Diamond et al, 2005;Hoang et al, 2007;Holdway et al, 1994;Naddy et al, 2000;Schulz and Liess, 2000). The observed responses to and relationships between pulse concentration, duration and recovery time in toxicant-free media have been mixed, with the toxic responses varying for different types of contaminants and exposure scenarios, biological species and stage of their life cycle.…”