mercury compounds are toxic even at concentrations of only 100 ng/l = 0.1 p.g/I.
They inhibit the photosynthetic efficiency of the diatom Nitzschia delicatissima
(Harris et aI. 1970) or impair the growth of plankton algae (Fig. 60). This toxic concentration is merely about 10 times higher than the natural one in seawater. Because
especially effective organic complexes of mercury have been used in the experiments,
a direct comparison to the toxicity of mercury in seawater, however, is not permitted.
But there are interesting observations from nature, too. In upwelling regions, seawater from the deep sea comes in contact with the surface layer of the sea, and a
good supply with nutrients and energy from the sun stimulate blooms of planktic
algae. However, sensitive dinoflagellate algae cannot develop before more resistant
diatom algae have conditioned the seawater by excreting organic compounds. It is
a hypothesis that such organic compounds have a chelating effect on ions of heavy
metals, and decrease their toxicity. If this hypothesis is correct, the natural concentrations of some heavy metals in clean, unpolluted seawater are toxic to marine life,
and certainly double concentrations would be more toxic. However, up to date it is
not clear which of the different highly toxic elements in seawater could be responsible for the toxic effect of fresh upwelling water; besides mercury, it could be copper, cadmium, lead, or another heavy metal.
A doubling of actual natural mercury concentrations in seawater would certainly
result in about double mercury concentrations in all marine organisms, including fish,
seals, and marine birds. It seems that some of the long-lived large fish-eating species
like seals and tuna at present have mercury concentrations in some tissues which are
close to the threshold of toxic effects (see Chap. 8.2). There is at least a possibility
that these species could not cope with double concentrations. Certainly, too, under
present health standards, tuna and some other large fish would be rendered unfit for
human consumption if mercury concentrations should be double the present value
(see Cap. 8.1).
Therefore it is good to know that according to the present state of knowledge man
does not alter the mercury concentrations in the biosphere on a world-wide scale.
However, the concentration of mercury in the oceans has surely changed in the
course of the Earth's history and was higher at periods of strong erosion and volcanic
activity. Whether or not that had consequences for the organisms of earlier periods
is not known. Maybe they could adapt to live with higher mercury concentrations in
their tissues, and the detoxification effect of chelating organic compounds in seawater prevented toxic effects of elevated seawater concentrations. But maybe, too,
certain species did not survive periods of high mercury concentrations, which at the
same time were periods with high concentrations of other heavy metals in general.
8.6 Contamination of the Oceans with Cadmium
A commission of experts of the World Food and Agriculture Organization (F AO) and
the World Health Organization (WHO) came to the temporary conclusion in 1972 that
the tolerable amount of cadmium for humans is not more than 0.0075 mg/kg body
weight or 0.5 mg per person per week. No more than 10 p.g/l of cadmium can be permitted in drinking water. These maximum amounts are thus just slightly over those
158
They inhibit the photosynthetic efficiency of the diatom Nitzschia delicatissima
(Harris et aI. 1970) or impair the growth of plankton algae (Fig. 60). This toxic concentration is merely about 10 times higher than the natural one in seawater. Because
especially effective organic complexes of mercury have been used in the experiments,
a direct comparison to the toxicity of mercury in seawater, however, is not permitted.
But there are interesting observations from nature, too. In upwelling regions, seawater from the deep sea comes in contact with the surface layer of the sea, and a
good supply with nutrients and energy from the sun stimulate blooms of planktic
algae. However, sensitive dinoflagellate algae cannot develop before more resistant
diatom algae have conditioned the seawater by excreting organic compounds. It is
a hypothesis that such organic compounds have a chelating effect on ions of heavy
metals, and decrease their toxicity. If this hypothesis is correct, the natural concentrations of some heavy metals in clean, unpolluted seawater are toxic to marine life,
and certainly double concentrations would be more toxic. However, up to date it is
not clear which of the different highly toxic elements in seawater could be responsible for the toxic effect of fresh upwelling water; besides mercury, it could be copper, cadmium, lead, or another heavy metal.
A doubling of actual natural mercury concentrations in seawater would certainly
result in about double mercury concentrations in all marine organisms, including fish,
seals, and marine birds. It seems that some of the long-lived large fish-eating species
like seals and tuna at present have mercury concentrations in some tissues which are
close to the threshold of toxic effects (see Chap. 8.2). There is at least a possibility
that these species could not cope with double concentrations. Certainly, too, under
present health standards, tuna and some other large fish would be rendered unfit for
human consumption if mercury concentrations should be double the present value
(see Cap. 8.1).
Therefore it is good to know that according to the present state of knowledge man
does not alter the mercury concentrations in the biosphere on a world-wide scale.
However, the concentration of mercury in the oceans has surely changed in the
course of the Earth's history and was higher at periods of strong erosion and volcanic
activity. Whether or not that had consequences for the organisms of earlier periods
is not known. Maybe they could adapt to live with higher mercury concentrations in
their tissues, and the detoxification effect of chelating organic compounds in seawater prevented toxic effects of elevated seawater concentrations. But maybe, too,
certain species did not survive periods of high mercury concentrations, which at the
same time were periods with high concentrations of other heavy metals in general.
8.6 Contamination of the Oceans with Cadmium
A commission of experts of the World Food and Agriculture Organization (F AO) and
the World Health Organization (WHO) came to the temporary conclusion in 1972 that
the tolerable amount of cadmium for humans is not more than 0.0075 mg/kg body
weight or 0.5 mg per person per week. No more than 10 p.g/l of cadmium can be permitted in drinking water. These maximum amounts are thus just slightly over those
158