Cetaceans: Guardians of Marine Ecosystems

Anthropogenic toxic substances accumulate in marine ecosystems and their inhabitants. Cetaceans, which are particularly affected, can therefore be used to monitor anthropogenic contaminants in the oceans. A better understanding of the distribution of these particles within cetacean populations allows for a more accurate assessment of their presence in ecosystems.   Many human-made toxic substances are released into marine ecosystems. Originating from various sources, these contaminants often biodegrade poorly and slowly. Many persist for decades, or even centuries. And unsurprisingly, their concentration in the environment has increased exponentially in recent years. Without singling out any one contaminant over another, plastic is a very telling example. As a particularly harmful form of marine debris, plastics degrade very slowly and accumulate rapidly in the environment… and in animals. And beyond simply accumulating in the ocean—and through ingestion—in all kinds of organisms, plastics and other debris contain and attract harmful contaminants that can accumulate in organisms and pose a threat to their health, including their reproduction, development, and immunity. This is especially true since these toxic chemicals tend to bioaccumulate in the marine food chain and thus accumulate in the body tissues of higher-trophic-level organisms. This occurs to such an extent that certain elements—which have biological functions and are essential for life—become lethal at excessively high concentrations. Examples include iron, manganese, and zinc. As predators at the top of the food chain with relatively long lifespans, certain cetaceans are particularly affected. Concentrations of inorganic elements in odontocetes, or toothed cetaceans, have been documented in numerous tissues, including blubber, liver, kidneys, skin, and blood, causing adverse effects such as kidney damage, immunosuppression, and neurological, developmental, and reproductive disorders. Odontocetes are therefore sentinel species for ecosystem health and anthropogenic chemical threats. They can thus be used to monitor anthropogenic contaminants in the oceans. In fact, in addition to the many pollutants already present in the biosphere, hundreds of new compounds—for which toxicity testing is incomplete—enter the consumer market each year and thus end up in the oceans. This results in a considerable time lag between when these chemicals are released onto the market and when their effects on the health of individuals, populations, and ecosystems are understood. In an effort to keep pace with chemical industry production and provide more information on existing anthropogenic contaminants in wild odontocetes, American scientists sought to determine, using mass spectrometry, the baseline concentrations of several widely used and dispersed anthropogenic contaminants in the liver and blubber of 83 stranded odontocetes from the southeastern United States between 2012 and 2018, based on demographic parameters (species, sex, age class, and stranding location), while also describing histopathological lesions. The researchers note that concentrations of substances with toxicity vary significantly by species, sex, age, and location. Compared to pygmy sperm whales, bottlenose dolphins had higher concentrations of lead, manganese, mercury, selenium, thallium, and zinc, and lower concentrations of NPE, arsenic, cadmium, cobalt, and iron. Adult females tend to have significantly higher concentrations of arsenic and markedly lower concentrations of iron compared to adult males. Furthermore, adults had higher concentrations of lead, mercury, and selenium, and lower concentrations of manganese compared to juveniles. And dolphins stranded in Florida had higher concentrations of lead, mercury, and selenium, and lower concentrations of iron than dolphins stranded in North Carolina. These data significantly enhance the existing knowledge base regarding the concentrations of substances with toxicity in stranded odontocetes. In particular, this is the first published report to examine concentrations of atrazine, DEP, NPE, and triclosan in the blubber of stranded cetaceans. The study is also the first to report concentrations of substances with toxicity in a white-beaked dolphin and in Gervais’s beaked whales, species for which the scientific literature remains sparse. Documenting substances of toxicity in cetaceans is a critical step in tracking chemical contaminants through the marine food web and understanding their effects on biological systems. Linking these concentrations to animal demographic and histopathological data provides a better understanding of secondary factors (biotic or abiotic) that may influence the exposure to and biological effects of certain substances with toxicity, which can lead to deteriorating health and, in combination with other factors, may contribute to strandings.