Bioaccumulationoccurs within a trophic level, and is the increase in concentration of asubstance in an individual's tissues
due to uptake from food and sediments Inan aquatic milieu. Bioconcentration is defined as occurring when uptake fromthe water is greater than excretion. (Landrum and Fisher, 1999). Thusbioconcentration and bioaccumulation occur within an organism, andbiomagnification occurs across trophic (food chain) levels.Fatsoluble (lipophilic) substances cannot be excreted in urine, a water-basedmedium, and so
accumulate in fatty tissues of an organism if the organism lacksenzymes to degrade them. When eaten by another organism, fats are absorbed inthe gut, carrying the
substance, which then accumulates in the fats of thepredator. Since at each level of the food chain there is a lot of energy loss,a predator must consume many prey, including all of their lipophilicsubstances.Forexample, though mercury is only present in small amounts inseawater , it isabsorbed by algae (generally asmethimelcury). It is efficiently absorbed, butonly very slowly excreted by organisms (Croteau et al, 2005). Bioaccumulationand biomagnification build it up in the fat tissues of successive trophiclevels: consumer plankton, small fish, larger fish. Anything which eats thesefish also consumes the higher level of mercury the fish have accumulated. Thisprocess explains why predatory fish such as swordfish and sharks or birds likeospray and eagles have higherconcentrations of mercury in their tissue than could be accounted for by directexposure alone. For example, herring contains mercury at approximately 0.01 ppmand shark contains mercury at greater than 1 ppm (EPA 1997)Bioconcentration considers uptake from thenon-living environment while biomagnification describes uptake through thefoodchain. For many fat-soluble and persistent chemicals (POPS), biomagnificationis the dominant factor.Everythingin a biological system has abiological hilfile, that is, a measure of how longit will stay in that system until it is lost, is excreted, degrades, reactsinto something different, or ends its presence in some other way. Mostsubstances have a short half-life, as they aremetabolized, or excreted aswaste.However,some compounds may stay in a system for a much longer period of time. Forexample, calcium in the human body is laid down in bones and teeth, and evenwhen bone cells die, their calcium is used again in the building of bones. Thisis a sensible and efficient re-use of scarce resources.Theproblem arises when toxic substances stay in the body for a long period oftime. They are not acutelypoisonous, otherwise they would kill straight away,but are associated with chronic poisoning.If theinput of a toxic substance to an organism is greater than the rate at which thesubstance is lost, the organism is said to be bioaccumulating that substance. Thus,the longer the biological half-life of the substance the greater the risk ofchronic poisoning, even if environmental levels of the toxin are very low.This isone reason why chronic poisoning is a common aspect of environmental health inthe workplace. As people spend so much time, for so many years in these environments,very low levels of toxins can be lethal over time.Anexample of poisoning in the workplace can be seen from the phrase "as madas a hatter". The process for stiffening the felt used in making hatsinvolvedmercury, which forms organic species such asmethilmercury, which islipid soluble, and tends to accumulate in the brain resulting inmercurypoisonning.Other lipid (fat) soluble poisons include compounds (the tetra etylead inleadedpetrol), and DDT. These compounds are stored in the body's fat, and intimes of famine, when the fatty tissues are used for energy, the compounds arereleased and cause acute poisoning.Stontium90, part of the fall out from atomic bombs, is mistaken by the human body forcalcium, and is laid down in the bone, where its radiation can cause damage fora long time.Naturallyproduced tons can also bioaccumulate. The marine algal blooms known as "redtides" can result in local filter feeding organisms such as mussels and ovstersbecoming toxic; coral fish can be responsible for the poisoning known as ciguaterawhen they accumulate a toxin called ciguatoxin from reef algae.Othercompounds that are not normally considered toxic can be accumulated to toxiclevels in organisms. The classic example is ofVitamine A, which becomesconcentrated in carnivore livers of e.g.polar bears: as a pure carnivore thatfeeds on other carnivores (seals), they accumulate extremely large amounts ofVitamin A in their livers. It was known by the native peoples of the arcticthat the livers should not be eaten, but arctic explorers have suffered HypervitaminosA from eating the bear livers (and there has been at least one example ofsimilar poisoning of Antarctic explorers eating huskydog livers).