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Environmental scientists sometimes forget that the dose determines the response. A well recognized toxic material can be desparately needed by living creatures at very low concentration. It may, in fact, be an essential nutrient.
For instance: A remarkably small amount of selenium determines
	the survival of daphnids.
0.1 ppb permits reproduction amidst gross deformity. 1.0 ppb allows apparent health, but all die of premature old age. 2.0 ppb allows normal life IF sufficient Zn and Cu are available. 5 ppb assures their normal life. 60 ppb begins to limit fecundity.
Adding organic anti-oxidants spares the damages due to selenium deprivation		Nickel improves life in old age
	Zn is far more available via the water than via solid foods.