Biodilution, sometimes referred to as bloom dilution, is the decrease in concentration of an element or

pollutant A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like o ...

with an increase in

trophic level The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it ...

. This effect is primarily observed during algal blooms whereby an increase in algal biomass reduces the concentration of pollutants in organisms higher up in the food chain, like

zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...

daphnia ''Daphnia'' is a genus of small planktonic crustaceans, in length. ''Daphnia'' are members of the order Anomopoda, and are one of the several small aquatic crustaceans commonly called water fleas because their saltatory swimming style resembl ...

. The primary elements and pollutants of concern are heavy metals such as mercury,

cadmium Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...

, and

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

. These toxins have been shown to

bioaccumulate Bioaccumulation is the gradual accumulation of substances, such as pesticides or other chemicals, in an organism. Bioaccumulation occurs when an organism absorbs a substance at a rate faster than that at which the substance is lost or eliminated ...

up a

food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Another name for food web is consumer-resource system. Ecologists can broadly lump all life forms into one o ...

. In some cases, metals, such as mercury, can

biomagnify Biomagnification, also known as bioamplification or biological magnification, is any concentration of a toxin, such as pesticides, in the tissues of tolerant organisms at successively higher levels in a food chain. This increase can occur as a ...

. This is a major concern since methylmercury, the most toxic mercury species, can be found in high concentrations in human-consumed fish and other aquatic organisms. Numerous studies have linked lower mercury concentrations in zooplankton found in

eutrophic Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytoplan ...

(nutrient-rich and highly productive) as compared to oligotrophic (low nutrient) aquatic environments. Nutrient enrichment (mainly phosphorus and nitrogen) reduce the input of mercury, and other heavy metals, into aquatic food webs through this biodilution effect. Primary producers, such as phytoplankton, uptake these heavy metals and accumulate them into their cells. The higher the population of phytoplankton, the less concentrated these pollutants will be in their cells. Once consumed by primary consumers, such as zooplankton, these phytoplankton-bound pollutants are incorporated into the consumer’s cells. Higher phytoplankton biomass means a lower concentration of pollutants accumulated by the zooplankton, and so on up the food web. This effect causes an overall dilution of the original concentration up the food web. That is, the concentration of a pollutant will be lower in the zooplankton than the phytoplankton in a high bloom condition. Although most biodilution studies have been on freshwater environments, biodilution has been shown to occur in the marine environment as well. The Northwater Polynya, located in Baffin Bay, was found to have a negative correlation of cadmium, lead, and nickel with an increase in trophic level Cadmium and lead are both non-essential metals that will compete for calcium within an organism, which is detrimental for organism growth. Most studies measure bioaccumulation and biodilution using the δ15N isotope of nitrogen. The δ15N isotopic signature is enriched up the food web. A predator will have a higher δ15N as compared to its prey. This trend allows the trophic position of an organism to be derived. Coupled to the concentration of a specific pollutant, such as mercury, the concentration verses trophic position can be accessed. While most heavy metals bioaccumulate, under certain conditions, heavy metals and organic pollutants have the potential to biodilute, making a higher organism less exposed to the toxin.

References

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