Marine PollutionThe silent killer
Pollution in the ocean takes many forms; plastic (micro & macro), other anthropogenic debris, chemical and organic.
Plastics have been reported to enter the ocean through littering and barge spills, as well as from polluted rivers, storm drains and even washing machines! (McIlwraith et al., 2019) A lot of plastic in the ocean is micro (<5mm) and some is invisible to the human eye. Microfibres from synthetic clothing and pre-production pellets are presenting a huge issue as they begin to make their way up the food chain from ingestion by prey fish, right up to humans. The plastic itself presents a mechanical barrier within animals that ingest it, and often chemicals called PCBs (polychlorinated biphenyls) clinging to the surface of debris can bioaccumulate in long-lived individuals to cause health problems in later life, like infertility (Ross et al., 2000).
Entanglement in marine debris also presents a formidable threat at every level of marine ecosystems, with turtles, cetaceans and sharks among groups most affected, often attracted by the smell of prey on the fishing debris they become entangled within (Duncan et al., 2017, Parton et al., 2020). The ocean is complex and large, with a myriad of currents, upwellings and gyres, which facilitates the vast and widespread distribution of pollution and debris. This often means it is hard to pinpoint the origin of pollution and try and prevent it from entering the ocean at the source.
In Koh Tao, very much like most of the world, micro and macro plastics have become a large problem. A lot of the plastic we encounter here has commercial sources, but are rarely from Koh Tao itself. Fishing gear that has been discarded also floats onto dive sites and beaches from areas not protected by law from fishing activity. All of this breaks down into microplastics over the course of many years.
Chemical pollutants are harder to track unless it’s immediately following wreckage where oil or fuel is spilt. These occurrences are incredibly rare here on Koh Tao – thankfully!
Here are some of the types of pollution in the oceans that are currently affecting ecosystems:
Microplastics is a term used to describe a truly diverse collection of plastic particles ranging in size from only a few microns, that remain unseen to the naked human eye, up to particles 5mm in diameter. These particles can vary greatly in appearance and texture from fibrous plastics to small pellets and sheet fragments. A range of colours have also been reported, however, those with brighter non-natural colours such as red and blue are more commonly spotted in the marine environment as they are less likely to be mistaken for organic matter.
The word microplastics has been widely used since 2004 when the term was used by Thompson et al to describe an accumulation of microscopic plastic pieces in the water column and sediments of European waters. Contamination by microplastics of every ocean has now been reported, and these tiny plastic pieces have since been discovered in marine animals at every trophic level from the smallest zooplankton to the largest whales and sharks. Through the food chain, microplastics have now made their way into the human diet, and there is no knowing how serious the consequences may be in years to come.
Microplastics can come from many sources and are hard to track. Being so tiny, they travel incredibly easily on marine currents and can even be consumed and transported by marine animals that then excrete them along migratory routes. Through years of research, the scientific community is now beginning to understand and explore several key sources of microplastics: commercial, industrial and domestic.
Commercial microplastics usually start their journey in the ocean as larger pieces of plastic that are broken down over time by wave action and sunlight, every piece of plastic that enters the ocean will eventually become a microplastic over many decades, releasing toxins such as bisphenol A (BPA) into the environment as they do so.
Industrial microplastics usually manifest themselves in the form of pellets called nurdles or bio beads. Nurdles are pre-production pellets, i.e., how all plastics start out. These are usually only a few millimetres in diameter, have a smooth surface and come in a huge array of colours. These often enter the ocean in their millions due to shipping container spills during transportation. The other type of industrial plastic we encounter all over the world are bio beads, unlike nurdles, these are rough and usually clear, brown or black (meaning they blend into the sand and sediment incredibly well!). These are used in the treatment of sewage water and often are saturated with highly toxic chemicals. These enter the ocean when water treatment plants have a ‘spill’ usually due to a malfunction in their facilities.
Domestic microplastics are microplastics that come from our home and everyday life, this is one source of plastic into the oceans we can highly minimize with behaviour change and conscious choices, The microplastics that leave our homes and enter the ocean are particles such as microbeads in cosmetics and plastic fibres from synthetic clothing. These are too small to be filtered at water treatment plants, however, we can stem the flow by choosing products free of microbeads and clothing made of natural fibres.
Marine debris is listed among one of the major threats to marine biodiversity sheerly because of its abundance, distribution, durability and persistence. Debris is anything discarded in the ocean that doesn’t belong there and encompasses everything from macroplastics, microplastics, fishing line, rope, shoes, tires – the lot. Marine debris doesn’t only present a threat via ingestion like microplastics, but also via entanglement and chemical accumulation.
Ingestion of marine debris affects marine animals of every taxa however it is noted to be a top threat, especially to marine turtles due to their feeding behaviours and strategies. A recent study by Duncan et al found marine plastic ingestion to be ubiquitous in every marine turtle sampled across all 7 species in three ocean basins (Atlantic, Mediterranean and Pacific). Why is this? Turtles have been found to select and eat marine debris that looks similar in shape, size and colour to their prey item of choice, and therefore have been concluded not to accidentally be consuming debris, but rather confusing the debris for their preferred food.
Entanglement in marine debris is recognized to be the greatest threat to large marine vertebrate species such as whales, sharks, dolphins and turtles, as these large marine vertebrates migrate great distances and are more likely to come into contact with floating abandoned fishing rope and other debris they’re vulnerable to become entangled in. A recent review by Parton et al revealed entanglement in debris to be a very real risk to sharks and rays worldwide and found that 74% of animals reported to be affected were entangled in ghost fishing gear. It is incredibly important that as conservationists, we collaborate with the fishing community to better facilitate the recovery of dangerous gear such as this to prevent further mortality.
Pollutants in the ocean are sometimes not as visible as plastic debris or ghost fishing gear and come in the form of organic and chemical contaminants which remain invisible to the human eye. However, these two types of pollutants are intrinsically linked. Plastics act as vectors for hydrophobic organic contaminants (HOCs) present in the environment, transporting them all over the world and facilitating their ingestion by marine life,
These contaminants accumulate in the tissues of marine animals all the way through the food chain, becoming more and more concentrated at each trophic level. Starting from zooplankton consuming microplastics saturated with chemicals, which are then eaten by small baitfish, and then larger predators, and then finally long-lived apex predators like cetaceans and sharks.
The following chlorinated industrial compounds and pesticides are of concern for marine life: polychlorinated biphenyls (PCBs), 2,3,7,8- tetrachlorodibenzo-p-dioxin (dioxin) as a member of the polychlorinated dibenzodioxins (PCDD), polychlorinated dibenzofurans (PCDFs), 1,1,1- trichloro-2,2-bis( p-chlorophenyl) ethane and metabolites (DDTs), chlorinated cyclodienes (chlordane, dieldrin, mirex/kepone), chlorinated bornanes (toxaphenes), chlorinated benzenes (including hexachlorobenzene or HCB), chlorinated cyclohexanes (HCHs) and, in very specific cases, benzo[a]pyrene (a PAH)… That’s a long list of very complex words right? What does this really mean?
These are all chemicals that come from human sources, whether that be plastics leaching them into seawater, pesticide runoff from agricultural land or from gases given off by certain industrial processes. More than 30 types of persistent organic pollutant were created specifically for use as pesticides alone, and each time it rains, freshwater saturated with these chemicals enters coastal seas.
What does this mean for marine animals specifically? Animals affected by high concentrations of these contaminants are now exhibiting symptoms such as low immune function, reduced reproductive rates and in extreme cases, infertility. Cetaceans are particularly affected due to their position in food webs as top predators with long life spans. They accumulate high concentrations of organic contaminants in their lipid tissue, as these lipophilic compounds bind to fatty acids in the blubber. This will no doubt have huge knock-on effects on cetacean populations in the future, distorting ecosystem balance all over the world.
At Big Blue Conservation, we work hard to collaborate with all of the awesome dive centres and community projects on the island to keep Koh Tao clean through beach clean-ups and underwater clean up dives! We analyze the collected rubbish from these cleans to try and detect the main sources of pollution in the ocean and to combat it at the source.