Litter – pervading the ocean
Top ten marine debris items:
- Cigarettes/ cigarette filters
- Bags (plastic)
- Food wrappers/ containers
- Beverage bottles (plastic)
- Cups, plates, forks, knives, spoons (plastic)
- Beverage bottles (glass)
- Beverage cans
- Straws, stirrers (plastic)
- Bags (paper)
Litter: Where does it come from?
Take a stroll along any beach after a storm and you will get an idea of just how much litter is floating around in the world’s oceans: the sand is strewn with plastic bottles, fish boxes, light bulbs, flip-flops, scraps of fishing net and timber. The scene is the same the world over, for the seas are full of garbage. The statistics are alarming: the National Academy of Sciences in the USA estimated in 1997 that around 6.4 million tonnes of litter enter the world’s oceans each year. However, it is difficult to arrive at an accurate estimate of the amount of garbage in the oceans because it is constantly moving, making it almost impossible to quantify.
A further complicating factor is that the litter enters the marine environment by many different pathways. By far the majority originates from land-based sources. Some of it is sewage-related debris that is washed down rivers into the sea, or wind-blown waste from refuse dumps located on the coast, but some of it comes from careless beach visitors who leave their litter lying on the sand.
Shipping also contributes to the littering of the oceans: this includes waste from commercial vessels and leisures that is deliberately dumped or accidentally lost overboard and, above all, torn fishing nets. As most of the litter is plastic, which breaks down very slowly in water and may persist for decades or even centuries, the amount of debris in the marine environment is constantly increasing.
Scientific studies have revealed regional variations in the amount of litter in the sea. In many regions, researchers have reported quantities of floating plastic debris in the range of 0 to 10 items of debris per square kilometre. Higher values were reported in the English Channel (10 to 100 items/square kilometre), but in Indonesia’s coastal waters, 4 items of debris in every square metre were reported – many orders of magnitude above the average.
The problem does not only affect the coastal areas, however. Propelled by the wind and ocean currents, the litter – which is highly persistent in the environment – travels very long distances and has become widely dispersed throughout the oceans. It can now even be found on remote beaches and uninhabited islands.
In 1997, researchers discovered that the floating debris accumulates in the middle of the oceans – in the North Pacific, for example, where massive quantities of water constantly circulate in a swirling vortex of ocean currents known as gyres, which extend for many hundreds of kilometres and are driven by light winds. The plastic debris ends its journey here. The litter circulates constantly, with new debris being added all the time. Environmental researchers call it the Great Pacific Garbage Patch. The concentration of litter is extremely high, which is particularly worrying if we consider that it is located in the open sea thousands of miles from the coast. Scientists have detected up to 1 million plastic particles per square kilometre here. Much of the debris consists of small fragments of plastic that were fished out of the water using fine-mesh nets. By contrast, studies in the English Channel, and many other surveys carried out elsewhere, are based on the visual method of quantification, which means that scientists simply count the pieces of debris that are visible as they pass by in their research vessels.
The amount of floating oceanic debris is immense. However, it is thought that around 70 per cent of the litter eventually sinks to the sea floor. The worst-affected areas are the coastal waters of densely populated regions or regions with a high level of tourism, such as Europe, the US, the Caribbean and Indonesia. In European waters, up to 100,000 pieces of litter visible to the naked eye were counted per square kilometre on the sea floor. In Indonesia, the figure was even higher – up to 690,000 pieces per square kilometre. Much of the litter is harmless, but some of it is responsible for marine mammal deaths. Seals and otters, for example, which feed on fish, crabs and sea urchins on the sea floor, are frequent casualties.
- 4.10 > The amount of litter in the oceans is constantly increasing. Much of it degrades very slowly. Plastic bottles and nylon fishing line are particularly durable. Although many plastics break down into smaller fragments, it will take decades or even centuries (estimated timescales) for them to disappear completely.
Tiny but still a threat – microplastics
For some years now, scientists have increasingly turned their attention to what remains of the plastic debris after prolonged exposure to wave action, saltwater and solar radiation. Over time, plastics break down into very tiny fragments, known as “microplastics”. Microplastics are now being detected in ocean waters, sand and sea-floor sediments all over the world. These tiny particles, just 20 to 50 microns in diameter, are thinner than a human hair. Marine organisms such as mussels filter these particles out of the water. Experimental analyses have shown that the microplastics accumulate not only in the stomachs but also in the tissue and even the body fluids of shellfish. The implications are still unclear, but as many plastics contain toxic substances such as softeners, solvents and other chemicals, there is concern that microplastics could poison marine organisms and, if they enter the food chain, possibly humans as well.
The silent killers – ghost nets
Derelict fishing gear – known as “ghost nets” – poses a particular threat to marine wildlife. These are nets which have torn away and been lost during fishing activities, or old and damaged nets that have been deliberately discarded overboard. The nets can remain adrift in the sea and continue to function for years. They pose a threat to fish, turtles, dolphins and other creatures, which can become trapped in the nets and die. The tangled mass then snags other nets, fishing lines and debris, so that over time, the ghost nets become “rafts”, which can grow to hundreds of metres in diameter. Some of these nets sink to the sea floor, where they can cause considerable environmental damage. Propelled by currents, they can tear up corals and damage other habitats such as sponge reefs. >