POPs Dirtiest Secret: Persistent Organic Pollutants in the Marine Environment
In recent years, the world has finally awoken to the detrimental effects that pollution in the marine environment is having on not only life in our oceans, but to humans too. The term ‘pollutant’ refers to any chemical or substance that is harmful in some way to the environment; often a waste product from an industrial process. Pollutants can range from huge oil spills; an example of which is the BP oil spill of 2010 in the Gulf of Mexico that was estimated to have spilled 699588120 litres of oil in to the marine environment; all the way down to microscopic pieces of plastic. Some of these pollutants come under the term ‘POPs’.
So, what are POPs?
POPs – Persistent organic pollutants – are a series of chemicals and substances (e.g. pesticides, and insecticides) that persist for a long time in the environment, creating long term damage to the organisms that come in contact with them. These chemicals take an age to degrade and have the potential to travel hundreds and even thousands of kilometres away from the site from which they were released, e.g. via the atmosphere and the marine environment (see figure 1). The majority of POPs are actually produced and released from industry and agriculture in the Northern Hemisphere. They are extremely persistent in nature which means that they have the ability to bioaccumulate (see figure 2) in the food chain, so any animal that sits at the top of the food chain is at the most risk. A lot of POPs do not dissolve in water so can accumulate together quite easily.
A reality of these particular pollutants is their ability to join forces with plastic, another man-made pollutant, to create a ‘super pollutant’. Microplastics slowly absorb POPs and, in theory, ‘clean’ the oceans, removing POPs from the water. However, POPs can become highly concentrated in microplastics and due to the size of these plastics, can very easily enter the food chain. An example of this can be seen in the lug worm Arenicola marina, a prey of many wading bird species, where a laboratory study has shown the worm has the ability to ingest microplastics thus showing the potential for POPs to enter the food chain.
POPs in action
A recent study looked at the accumulation of POPs in Antarctic fish; these fish live in an area of the world where there is very little direct input of pollutants in the water due to relatively little anthropogenic activity in the area. This, however, seems not to be a limiting factor at all as POPs are capable of travelling great distances, spreading themselves all over the globe. The results of this study found greater amounts of POPs in the flesh of these fish than studies from previous years have reported, suggesting an increase in the amount of POPs that the fish are accumulating. Bioaccumulation from the food chain was also evident as piscivorous fish had higher concentration of POPs in their flesh than the fish that ate only plankton.
Another victim of the presence of POPs are marine mammals. Marine mammals are often described as top predators which means that bioaccumulation of POPs is much more of a problem for these animals; marine mammals are actually exposed to more contaminants than any other animal in the world. This increased level of exposure has actually been seen to suppress the immune systems of these organisms. This, sadly, can lead to other physiological problems such as reproductive capabilities; reproductive success is tied with population growth and maintenance, so reducing reproductive capabilities can lead to devastation in the numbers of a population. Population changes will inevitably have a top down effect on the trophic levels below. This is just an example of the long range of damage POPs can have on entire marine ecosystems.
What does all of this mean for us?
Research has shown us that POPs are now entering the human food chain from our diet that comprises of seafood and other marine organisms. There have been studies on what sort of problems this increased exposure will create for us; some of which have suggested that increased exposure to POPs in our diet can lead to health problems such as diabetes. However, it’s not all doom on the horizon; in 2004 an international treaty (the Stockholm Convention) to reduce and eliminate the production and use of POPs became effective. The original listed ‘dirty dozen’ has since expanded with more still in review, hopefully one day eliminating POPs once and for all.