Concentration of toxins is more in organisms occupying the highest trophic level in the food chain is because of biomagnification, i.e. it goes on increasing at each trophic level of a food chain and is therefore maximum in the organisms of highest trophic level due to their increasing accumulation at each trophic level. For example DDT was used to control mosquitoes in a lake of USA. It was noted that the DDT found in phytoplantktons was about 800 times higher than that present in the lake and zoplantktons had about 13 times more DDT than that in phytoplanktons.
