CC.18 Toxicity
ESSENTIAL TO KNOW
- A toxic chemical is a substance that can cause death or adverse sublethal effects in organisms exposed to it at a concentration above a critical threshold.
- Toxic chemicals have many possible sublethal adverse effects on organisms, such as inhibition of the ability to photosynthesize or feed. The most important sublethal effects appear to be those that interfere with reproductive success.
- All toxic chemicals have a concentration threshold below which they have no sublethal or lethal toxic effects. Hence, for every chemical in the ocean environment, there is a concentration below which it is environmentally safe.
- Many substances that are toxic at high concentrations are also essential to life, and the growth of marine organisms may be inhibited if they are not present above a certain concentration. For these substances, there is an optimum range between the minimum concentration that an organism requires to supply its needs and the concentration above which the substance is toxic.
- The range of optimum concentration can be large or small. The concentration at which toxicity occurs can be many times higher than or very close to the natural range.
- Anthropogenic inputs of toxic substances can be assimilated safely in the oceans if the amount introduced does not cause concentrations to exceed the threshold at which sublethal toxicity occurs. The quantity that can be safely assimilated is different for each substance and determined in part by its sublethal toxicity threshold and background concentrations.
- Sublethal or lethal toxicity threshold concentrations are difficult to determine because they vary among species, among substances, and with other factors, such as physical stresses and synergistic and antagonistic effects of other chemical constituents.
- Marine organisms bioaccumulate most toxic substances. Bioaccumulation occurs when the concentration in the organism is higher than the environmental concentration but the concentrations are in equilibrium.
- A few toxic substances are biomagnified in marine organisms. Biomagnification occurs when the organism retains all the toxic substance to which it is exposed in its food or environment and does not lose any of the substance, even if its environmental concentration decreases.
- Carcinogenicity, mutagenicity, and teratogenicity can be considered to be lethal or sublethal effects, but there is probably no concentration threshold below which there is no effect. Each exposed organism has a small probability of suffering an effect at any specific concentration. More individuals within a population suffer the effect as concentration increases.
- Carcinogenic, mutagenic, and teratogenic substances occur naturally. Anthropogenic inputs will increase the incidence of the effects of such substances. However, at least for some of these substances, anthropogenic inputs would need to be large before the increased incidence would be significant or measurable in comparison with the natural incidence of effects.