Bioaccumulation & Biomagnification

02/20/08

Based on Mader, Sylvia S. 1996. Biology - 5th Ed. WCB and Cox, G.W. 1997. Conservation Biology - 2nd ed. WCB

In this section, we will examine how pollutants move through the various trophic levels in an ecosystem.  To understand this material, you have to understand trophic levels, food chains, and food webs, as well as pyramids of biomass.  If you haven't reviewed basic ecology yet; perhaps you better do so now:  Review Basic Ecology

There are two basic terms we are discussing here.  Bioaccumulation refers to how pollutants enter a food chain; biomagnification refers to the tendency of pollutants to concentrate as they move from one trophic level to the next.  Here are some definitions of these terms:

Bioaccumulation:
increase in concentration of a pollutant from the environment to the first organism in a food chain
Biomagnification:
increase in concentration of a pollutant from one link in a food chain to another

We are concerned about these phenomena because together they mean that even small concentrations of chemicals in the environment can find their way into organisms in high enough dosages to cause problems.  In order for biomagnification to occur, the pollutant must be:

  1. long-lived
  2. mobile
  3. soluble in fats
  4. biologically active

If a pollutant is short-lived, it will be broken down before it can become dangerous.  If it is not mobile, it will stay in one place and is unlikely to be taken up by organisms.  If the pollutant is soluble in water it will be excreted by the organism. Pollutants that dissolve in fats, however, may be retained for a long time.  It is traditional to measure the amount of pollutants in fatty tissues of organisms such as fish.  In mammals, we often test the milk produced by females, since the milk has a lot of fat in it and because the very young are often more susceptible to damage from toxins (poisons).  If a pollutant is not active biologically, it may biomagnify, but we really don't worry about it much, since it probably won't cause any problems.

Classic example: DDT

DDT stands for dichloro, diphenyl trichloroethane.  It is a chlorinated hydrocarbon, a class of chemicals which often fit the characteristics necessary for biomagnification.  DDT has a half-life of 15 years, which means if you use 100 kg of DDT, it will break down as follows:

Year

Amount Remaining

0

100 kg

15

50 kg

30

25 kg

45

12.5 kg

60

6.25 kg

75

3.13 kg

90

1.56 kg

105

0.78 kg

120

0.39 kg

This means that after 100 years, there will still be over a pound of DDT in the environment.  If it does bioaccumulate and biomagnify, much of the DDT will be in the bodies of organisms.  DDT actually has rather low toxicity to humans (but high toxicity to insects, hence its use as an insecticide).  Because it could be safely handled by humans, it was extensively used shortly after its discovery just before WW II.  During the war, it was used to reduce mosquito populations and thus control malaria in areas where US troops were fighting (particularly in the tropics).  It was also used on civilian populations in Europe, to prevent the spread of lice and the diseases they carried.  Refugee populations and those living in destroyed cities would have otherwise faced epidemics of louse-born diseases.  After the war, DDT became popular not only to protect humans from insect-borne diseases, but to protect crops as well.  As the first of the modern pesticides, it was overused, and soon led to the discovery of the phenomena of insect resistance to pesticides, bioaccumulation, and biomagnification.

By the 1960's, global problems with DDT and other pesticides were becoming so pervasive that they began to attract much attention.  Credit for sounding the warning about DDT and biomagnification usually goes to the scientist Rachel Carson, who wrote the influential book Silent Spring (1962).  The silent spring alluded to in the title describes a world in which all the songbirds have been poisoned.  Her book of course was attacked by many with vested interests.  

I recently came across an essay from Jonathan Tolman at the Competitive Enterprises Institute which completely misses the main point we get from Silent Spring.  I guess the fact that people are still scared of the book 35 years later says something about its message.  Sure, scientific discoveries have shown weaknesses in some of Carson's positions, but the basic message that indiscriminate use of pesticides will have lasting and detrimental effects remains strong.  The author's point in the CEI essay seems to be that if nature makes dangerous chemicals, why should we be concerned when humans make more dangerous chemicals and in huge quantities, then spread them around out of airplanes so everyone gets a dose? He also focuses on cancer, which is only one of the potential problems. It's amazing what people will say to make a buck.  I note that his on-line resume lists a bachelor's degree in Political Science, presumably that degree qualified him for work as an environmental and chemical analyst.   Anyway, back to science: 

Animations

Flash        PowerPoint

Case study: Long Island Estuary (Figure 22.1 in Cox)

In his textbook, Cox reports on a study done in 1967 on Long Island Sound.  The levels of DDT in tissues of various animals in the sound showed bioaccumulation factors of 800x, and  biomagnification factors up to 31 times.  When we look at the whole food chain, the overall magnification is over 200,000x!

water to zooplankton:

800x  
zooplankton  to fish #1: 31x  
fish #1  to fish #2: 1.7x  
fish #2 to  gull: 4.8x  
overall:   202,368x

While DDT isn't particularly lethal (except to insects, and we need many of them around), it has a number of sub-lethal effects.  Most prominent is the phenomenon of shell-thinning in birds, particularly carnivorous birds (raptors) - birds that eat other birds, birds that eat carrion (dead animals), and birds that eat fish. Ospreys are one of the raptors that have been adversely affected, as have bald eagles. Other fish-eating water birds have been affected as well.  Because of the DDT, the shells are too thin to brood.   Many populations have recovered following the banning of DDT in the US, but migratory birds may be exposed to pesticides in other countries.  Recently, some studies have shown effects on sex ratios in some species of birds, with the males becoming "feminized", presumably the result of compounds in the environment mimicing the female hormone estrogen.

Heavy metals and other substances

DDT is not the only toxin to biomagnify.  All of the following have the potential to biomagnify:

Substance

Use & Problems

Links

PCB's
polychlorinated biphenyls
  • insulators in transformers
  • plasticizer
  • fire retardant
  • biomagnifies
  • impairs reproduction
  • widespread in aquatic systems
PAH's
polynuclear aromatic hydrocarbons
  • component of petroleum products
  • carcinogenic
 
Heavy metals:
  • mercury
  • copper
  • cadmium
  • chromium
  • lead
  • nickel
  • zinc
  • tin (TBT or tributyltin)
  • mercury from gold mining
  • many from metal processing
  • may affect nervous system
  • may affect reproduction
cyanide
  • used in leaching gold
  • used in fishing
  • toxic
selenium
  • concentrated by farming desert soils
  • reproductive failures
  • toxic
     

Modern pesticides, such as carbamates and organophosphates, are "safer" in that they are not persistent, one of the requirements for biomagnification.  They are, however, more toxic, and insects are developing resistance to them.  It must be remembered that we use pesticides for more that making pretty produce.  Pesticides are sometimes necessary to protect a basic food supply and to protect human health.  The concept of integrated pest management, or IPM, has been developed to improve control of pests while decreasing the need for pesticides.  IPM uses a variety of methods to control pests.  These include biological controls, and cultural practices such as timing planting and harvest to avoid periods of peak activity by pest species, and scouting to determine how big a problem the pests are actually causing (rather than just spraying to prevent a problem that may never arise).  Economics are watched closely; pesticides are never used if the cost of the pesticide would exceed the cost of the crops being saved (you'd be amazed at how often people have spent more on pesticides than the crop was worth). IPM relies heavily on information, and the internet is being used extensively.

Other pollutants:

Other pollutants of importance are plastics, radioisotopes (which may be both toxic and radioactive!) and oil.  Plastics are eaten by many organisms and can cause mechanical injury, strangulation, or starvation.  Radioisotopes can damage biological molecules, particularly DNA, leading to cancer, other illnesses, or death.  Oil smothers aquatic organisms, cutting them off from oxygen.  It can also infiltrate the insulating feathers of seabirds (or fur of sea-going mammals) and cause them to die from hypothermia (or cause them to sink). Oil spills are a serious problem in marine environments.

Regulations:

Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA)

London Dumping Convention

International Convention for the Prevention of Pollution from Ships (MARPOL)

More Federal Acts