Few substances, especially non- biodegradable ones (which cannot be metabolized by the living organism) have the ability to move through various trophic levels. The movement of these substances involves two main processes:

• Bio-accumulation: It is the process of gradual increase in the concentration of toxic elements within the body of a particular organism. In general, the concentration of such pollutants becomes higher in the body of the organism compared to the external environment. It happens when the accumulating rate is much higher than the removal rate. Therefore, if the lifetime of the substance is higher, the impact of it also gets higher. Bioaccumulation occurs at the base of a food web, usually within primary producers like phytoplankton.
• Bio-magnification: It is the process in which the concentration of the toxic elements increases at the successive trophic levels in a food chain. It occurs when slightly larger organisms, such as zooplankton feed upon the contaminated phytoplankton and in turn absorb POPs into their own tissues at a higher concentration. These pollutants bio-magnify along food chains because successive trophic levels consume relatively large quantities of biomass (food) to ob tain the resources required for metabolic functioning. If that hiomass is contaminated, the contaminant will be taken up in large quantities by the consumer.

Hence, bio-accumulation occurs within a trophic level while bio-magnification occurs between trophic levels.

Their effects include

• Impact on human health: Bio-magnification makes humans more prone to cancers, organ failure, respiratory disorders, birth defects, brain damage, heart diseases etc. For instance, diseases like hepatitis and cancer have been attributed to consuming seafood that has been poisoned by mercury (Minamata disease) and polycyclic aromatic hydrocarbons (PAHs).
• Effects on aquatic life: The various toxic chemicals and elements accumulate in the vital organs of the various aquatic creatures affecting their reproduction and development. For instance, seabird eggs are laid with thinner shells than normal, and can result in the birds crushing their eggs instead of incubating them. Increased concentration of Mercury in an organism damages the nervous, gastrointestinal and cardiovascular systems. Bio-accumulation of DDT can cause sterility etc.
• Disruption of the food chain: Many sea creatures depend on the natural food chain for survival. When chemicals and other toxins are carried into the soils, rivers, lakes or oceans and taken up by various organisms, it disrupts the interconnected relationships within the food chain.
• Destruction of the coral reefs: The coral reefs are destroyed by cyanide which is used in leaching gold and in fishing. The reefs provide for spawning, feeding, and dwelling grounds for numerous sea creatures. When destroyed, the survival of aquatic creatures is highly compromised.
• Effect on ecosystem: The destruction of keystone species due to bio-accumulation of toxic elements can disrupt the entire food chain and ultimately the entire ecosystem.

Effective solutions to these problems include elimination of the use of toxic elements like some harmful heavy metals, following proper safety measures and guidelines while using and disposing the toxic elements, increasing awareness among people about the issue ete.