Biomagnification and Bioaccumulation of Organochlorine Compounds in Marine Mammals

Photo credit: NOAA

Duke University
Biology 217: Ecology and Global Change
Elizabeth Griffin and Jessica Maher
April 2002

Marine mammals currently face a large number of threats, including many forms of pollution (noise, chemical contamination etc...), entanglement in fishing gear, and harassment from humans (for definition of harassment see NOAA). The potential harm caused to marine mammals from exposure to many chemicals is a concern that has begun receiving more attention as our understanding of the various ways anthropogenic chemicals can impact the environment has expanded. Because marine mammals are protected in US waters by the Marine Mammal Protection Act, it is important to understand any potential harms these animals face as a result of exposure to chemical contaminants. This site provides information on the possible effects of four types of organochlorines, a class of chemical compounds, on marine mammals. Specifically, this page considers PCBs, DDT and its metabolites, cyclodienes and toxaphene because they are 4 of 12 persistent organic pollutants recently put forth by the United Nations as needing urgent action towards reduction and eventual elimination in use (Seaweb).

Introduction to organochlorine compounds
All organochlorine compounds belong to a broader class of chemicals called persistent organic pollutants (POPs). POPs are chemical substances that persist in the environment, bioaccumulate though food webs because of their lipophilic and hydrophobic nature, and pose a risk of causing adverse effects to human health and the environment (European Environment Agency).

Organochlorines are a diverse group of compounds that have numerous applications in both industry and agriculture. Examples of organochlorine compounds include polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT), cyclodienes - including aldrin, dieldrin, and endrin, toxaphene, and hexachlorocyclohexane (HCH). All organochlorine compounds are very stable and remain in the environment for long periods of time. With respect to marine mammals, certain organochlorine compunds are thought to cause both lethal and sub lethal problems. Specifically, organochlorines have been implicated in many health problems, for example infertility and reproductive failure, birth defects, cancer, behavioral change, immune and nervous system complications, and alteration of hormone levels (Seaweb). To date, organochlorines have been found in many different marine mammal species: at least 23 Pinniped species, 44 odontocete (toothed whales) species, 11 mysticetes (baleen whales), as well as sea otters, sirenians (manatees and dugongs), and polar bears (O'Shea, 1999).

As with most anthropogenic compounds, these organochlorine compounds end up in the world's oceans (PBS; O'Shea, 1999). Atmospheric deposition is the primary way that organochlorines are transported to marine environments, but inputs from rivers and agricultural run-off also bring these compounds to the world's oceans (Global Programme of Action for the Protection of the Marine Environment from Land-based Activities- GPA). Once in the water, these compounds are readily taken up through the food web. Many are resistant to metabolic degradation and are highly lipophilic with low water solubilities, all of which allow for rapid uptake and concentration in marine food webs. Organochlorines bioaccumulate, meaning their concentrations in organisms increase over time relative to their concentration in the environment, and then biomagnify, meaning their concentrations increase as they move from a lower level of the food web to a higher one, in water ecosystems. Basically, bioaccumulation is how compounds get in the food web and biomagnification is the tendency to become more concentrated as they move up the food web. It is these properties that make organochlorines potentially problematic for marine mammals- they not only ingest them while feeding on prey that have high organochlorine levels, but they also store the compounds in their blubber. In fact, the main repository of organochlorine compounds in marine mammals is blubber.

Potential impacts of organochlorines on marine mammal populations
According to O'Shea (1999), there are 3 potential ways in which organochlorines can impact marine mammals; they are 1) through direct mortality, 2) through reproductive impairment, and 3) by increasing susceptibility to disease. To date, there are only a few studies providing evidence for direct mortality and reproductive impairment, and even these studies have confounding factors making it difficult to establish any type of causal relationship (O'Shea, 1999). Additionally, there is wide variation in the impact of organochlorine compounds on marine mammals, even among species in the same genus, which makes generalizations quite difficult (O'Shea, 1999).

General patterns of organochlorine concentration in marine mammals
Age, sex, and reproductive status play a large role in shaping the concentration of organochlorine residues in the blubber of marine mammals (O'Shea, 1999). Sub-adult males and females do not have significantly different levels of organochlorines in their blubber, but over time, there is an increase in such concentrations in the blubber of males and a decrease the blubber of females. This is attributed to a transfer of the chemicals from females to their young during gestation and lactation (O'Shea, 1999). In addition to age, sex, and reproductive status, regional, ecological, and temporal variation shape the concentration of organochlorines in marine mammals. Those species inhabiting inshore and coastal waters near industrial and agricultural areas tend to have increased levels of organochlorines in their tissues, whereas pelagic species, specifically many baleen species, tend to have lower organochlorine residues (O'Shea, 1999).

DDT
PCBs
Toxaphene
Cyclodienes
Future Policy
Bibliography