Biomagnification

Biomagnification is a process where the concentration of a substance increases at higher levels of a food chain. It is also known as bioamplification or biological magnification. This process occurs when substances move through different trophic levels, which are the different steps in a food web. As a substance moves from prey to predator, it becomes more concentrated in the tissues of the organisms. This can lead to very high levels of toxins in top consumers. Understanding this process is vital for protecting ecosystems and human health.

To understand how this works, we must look at the specific properties of certain chemicals. Many substances that biomagnify are lipophilic, meaning they are highly soluble in fats but insoluble in water. They are also often hydrophobic, which means they do not dissolve well in water. Because these substances stay in the fat or adipose tissue of an organism, they are not easily excreted. When a predator eats many prey animals, it consumes all the accumulated toxins stored in the prey's fat. The predator then stores those toxins in its own body. This creates a chain reaction where each level has a higher concentration than the one below it.

It is important to distinguish biomagnification from two related processes: bioaccumulation and bioconcentration. Bioaccumulation occurs within a single trophic level. It is the increase in a substance's concentration in an organism's tissues due to absorption from food and the environment. Bioconcentration happens when an organism takes up a substance from water at a rate faster than it can excrete it. While bioaccumulation and bioconcentration happen within an individual organism, biomagnification describes the increase in concentration across the entire food chain.

Scientists use specific mathematical tools to quantify how much a substance is magnifying. One method is the Biomagnification Factor, or BMF. The BMF is a dimensionless ratio that compares the contaminant concentration in a predator to the concentration in its prey once a steady state is reached. If the BMF is greater than 1, the substance is accumulating more strongly in the predator than in its food. Another method is the Trophic Magnification Factor, or TMF. The TMF looks at the entire food web by plotting the logarithm of contaminant concentrations against trophic levels. A TMF greater than 1 indicates biomagnification, while a TMF less than 1 indicates trophic dilution.

History shows us the real-world impact of this process, particularly with the pesticide DDT. In the 1950s, researchers noticed that DDT was causing significant declines in North American predatory bird populations. Specifically, DDT caused eggshell thinning in species like the bald eagle and the peregrine falcon. Because DDT is a persistent organic pollutant, it does not degrade easily and accumulates in adipose tissue. Following the ban of DDT in many parts of the world, these bird populations have successfully recovered. This recovery serves as a major testament to the importance of managing substances that biomagnify.

We can see clear examples of this in the ocean using heavy metals like mercury. Mercury is often absorbed by algae as methylmercury, which is one of its most harmful forms. Methylmercury is efficiently absorbed by organisms but is only very slowly excreted. This leads to massive differences in concentration between species. For instance, herring may contain only about 0.01 parts per million (ppm) of mercury. In contrast, a shark can have a concentration of greater than 1 ppm. This demonstrates how predators like sharks and swordfish accumulate much higher levels than could be explained by direct exposure alone.

Substances that biomagnify generally fall into two categories: organochlorines and inorganic compounds. Organochlorines, such as DDT, PCBs, and toxaphene, are known as persistent organic pollutants (POPs). These are difficult to degrade because organisms have not evolved specific detoxification mechanisms to handle them. Inorganic compounds include heavy metals like mercury, arsenic, cadmium, and lead. Unlike organic pollutants, metals are chemical elements and cannot be degraded. While many organisms have ways to sequester or excrete metals, problems arise when exposure levels are too high for their natural defenses to manage.