92
Lesson N°15
Metals :
Plants and animals vary widely in their ability to regulate their metal
content ; most can do so only over a limited range , and metals that
can not be excreted remain in the body and are conntinually added to
over the life of the organism. This is known as bioaccumulation.
Animals feeding of bioaccumulators
have a diet enriched in these
conservative materials and if , as is commonly the case , they too are
unable to excrete them , they in turn acquire an even greater body
burden of the substance . this is biomagnification.
Measures of contamination :
Concentrations of contaminants in organisms are expressed as ug g
-1 (
parts per million , ppm) and ug kg
-1 ( parts per billion ppb).
The concentrations may be calculated on the basis of the wet weight ,
fresh weight , or dry weight of tissue.
Dry weight concentrations provide the best basis for comparing the
concentrations
of a subsatance
in different tissues
or
different
organisms , but many determinations are given only on a wet weight
basis.
Concentrations of contaminants in sediments are always based on dry
weight.
Input routes :
Metals in the marine environment are either the result of human
activities or natural processes such as the erosion of ore-bearing rocks ,
wind-blown dust , volcanic activity , and forest fires.
The length of time a contaminant remains in the atmosphere , and so
the distance it travels in the air mass , depends on its reactivity if it is
a gas or its density if it is a particle.