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Lesson N°13 :
Measuring toxicity
Any material discharged into the sea inevitably causes some change in
the environment . The change may be great or small , long lasting or
transient , widespread or extremely localized. If the change can be
detected and is regarded as damaging , it constitutes pollution. Much
effort is devoted to measuring levels of contamination of sediments
and organisms by chemical analysis , but to determine if the observed
level of contamination causes pollution generally requires a study of its
biological effects .
Measurement of toxicity :
Toxicity states how poisonous a substance is or how large a dose is
required to kill an organisms ; the more toxic the substance the smaller
the lethal dose.
If a sample of aquatic animal is exposed to a toxin , not all die at the
same time. Instead , mortality shows a sigmoid relationship to the
period of exposure. The median lethal time is the time for the death of
50% of a sample and is written LT50.
Organisms vary widely in their sensitivity to a toxin . Different species
differ in their sensitivity , but even within a single species , sensitivity to
a toxin depends on age , sex , reproductive conditions , nutritional
state.
Organic discharge :
All human sewage contains enteric bacteria , pathogens and viruses ,
and the eggs of intestinal parasites . Contamination of bathing water
or seafood may therfore pose a health hazard to the human
population :
- Through contact with , or accidental ingestion of pathogenic
bacteria , or viruses.
- Through infection by parasites present in the sewage.
- Through the consumption of contaminated seafood.