Hydra viridissima rapidly recovers from multiple magnesium pulse exposures

The time taken for organisms to recover from a pulsed toxicant exposure is an important consideration when applying water quality guidelines to intermittent events in the environment. Organisms may appear to have recovered using standard toxicity testing methods but could carry residual toxicant or damage that may make them more sensitive to subsequent pulses. Such cumulative effects may render guidelines under-protective. The present study evaluated recovery of the freshwater cnidarian, Hydra viridissima, following multiple pulse exposure to magnesium. H. viridissima were exposed to 4-h pulses of 790 and 1100 mg/L separated by 2, 10, 18, 24, 48 and 72-h recovery periods. Twenty four-hour pulses of 570, 910 and 940 mg/L were separated by 24, 48 and 168-h recovery periods. All treatments showed similar or reduced sensitivity to the second pulse when compared with the single pulse, indicating full recovery occurred prior to a second pulse-exposure. Five variations of equivalent time-weighted average concentrations were used to compare sensitivity of hydra to various pulse scenarios. The sensitivity of the organisms to the multiple pulses was significantly lower than the time-weighted average continuous exposure response in three of the four scenarios tested, indicating that the hydra benefited from inter-pulse recovery periods. The findings will be utilised alongside that of other species to inform the use of a site-specific, duration-based WQG for Mg and provides an example of the use of empirical data in the regulation of toxicant pulses in the environment.