Since 1997, the number of species known to have been affected by either entanglement or ingestion of ocean plastics has more than doubled. For marine turtles, the number of species affected in the same time period has increased to 100%, for marine mammals from 43 to 66% and for seabirds from 44 to 50%. Fish and molluscs are also increasingly impacted, where before they were of far less visible concern. Currently, 60% of the world’s major marine ecosystems exploited by humans are used unsustainably, and have been degraded and endangered beyond any tolerable measure.
Marine biodiversity is facing significant threats that are causing severe changes in the mixture of species, their distribution and, if unchecked, will result in their extinction. Based on IUCN (International Union for Conservation of Nature) lists, 25% of marine mammals, over 27% of seabirds and 27% of corals are severely threatened; and by 2100 more than half of the wold’s marine species may be threatened by extinction.
Plastic production grows, on average, by 5% per year, and thus, each year, more plastic ends up in the ocean. Trillions of plastic parts now litter almost every square mile of the world’s oceans, particularly nearer to coasts, leaving few refuges for the most vulnerable marine species. Through human waste disposal and marine animals’ ingestion, break down and excretion of smaller pieces of plastics, numbers of plastic pieces grow exponentially year upon year, with little sign of slowing.
For decades, global authorities have placed the plasticisation of the oceans as a source of major concern, and yet we are hardly any closer to halting, let alone reaching a plateau in the amount of plastics in the ocean. Viable global solutions have yet to be found, and environmental debates in climate summits often prioritise reaching net zero in greenhouse gas emissions by 2050 and limiting temperature increase from preindustrial levels to below 1.5ºC, leaving plastics often as an afterthought. This is likely because their effects on marine species are typically out of sight to most humans, yet underestimation of the problem is ignorant to the very real suffering of thousands of species, with population loss in the oceans increasing year upon year.
Marine environments are characterised by their high biodiversity, with a presence of 32 of 33 described animal phyla. As humans, we are dependant on this biodiversity for fish and seafood; fish represents a primary source of animal protein for more than one billion humans in the world. The loss of marine biodiversity also weakens the wider ocean ecosystem and its capability to withstand further disturbances, to adapt to climate change and to play its role as a global ecological and climate regulator by storing carbon. The oceans have stored up to half of all CO2 produced in the last two centuries, and phytoplankton are responsible for sequestering most of this transfer of carbon dioxide from the atmosphere to the ocean. Even small changes in the growth of phytoplankton affect atmospheric carbon dioxide concentrations, which feed back to global surface temperatures. They also play a role in reducing global temperatures, by producing a gas called Dimethyl sulphide, which, as it passes into the atmosphere, rapidly breaks down. The resulting sulphur compounds become aerosols, or microscopic particles in the atmosphere, which reduce the intensity of the sun’s light before reaching the earth’s surface. What does plastics in the oceans have to do with phytoplankton, however? Recent research from the UCC (University College Cork) have theorised that when microscopic pieces of plastic are consumed by the plankton, through a complex series of events involving algae and zooplankton, the efficiency of the transfer of CO2 to the seafloor from the atmosphere is lowered.
The most visible and obvious effect of plastic pollution on marine organisms concerns wildlife entanglement in marine debris. Entanglement occurs all over the world, from penguins in the southern oceans to whales in the Arctic, from octopuses in the pacific to crabs off the Atlantic coasts of North America. Entangled animals are hindered in their ability to move, to feed and often to breathe, making surviving in a competitive environment an unlikely occurrence. Hundreds of thousands of marine birds and mammals are known to perish in fishing gear, but, unfortunately, no estimates are possible for the actual number of animals being trapped in synthetic human debris every year.
The percentage of species that have been recorded as entangled among various groups of marine organisms is remarkably high: 100 % of marine turtles (7 of 7 species), 67 % of seals (22 of 33 species), 31 % of whales (25 of 80 species) and 25 % of seabirds (103 of 406). (Kühn et al. “Deleterious Effects of Litter on Marine Life”, Marine Anthropogenic Litter, 2015). Entangled organisms may no longer be able to acquire food and avoid predators, or become so exhausted that they starve or drown. Even if the individual does not die directly, wounds, restricted movements and reduced foraging ability will seriously affect the animal.
Many marine organisms also mistake litter for food and ingest it, leading to an accumulation of plastics in their stomachs, impacting their ability to reproduce and survive, if not direct mortality. Marine birds, turtles and mammals have received most attention, but the consequences of ingestion on other organism groups, e.g. fish and invertebrates, are becoming more evident. Ingestion of plastic debris has currently been documented for 100 % of marine turtles (7 of 7 species), 59 % of whales (47 of 80), 36 % of seals (12 of 33), and 40 % of seabirds (164 of 406). Marine turtles, for example, frequently ingest plastic bags as they may mistake them for jellyfish, a common component of their diet. Omnivorous or predatory species, such as sea birds and sharks, consume animals which themselves have ingested plastic, severely impacting their own health. They are also known to frequently inspect plastic debris and take bites out of larger plastic items, thus more directly ingesting large quantities of plastic. For example, Cadée (2002) observed that 80% of foamed plastic debris on the Dutch coast showed peckmarks of birds and suggested that the birds mistake polystyrene foam for cuttlebones or other food.
The colour of the plastic also can trick animals into mistaking it for food; Parakeet auklets (Aethia psittacula) on the Alaskan coast, feeding naturally primarily on light-brown crustaceans, consumed mainly darker plastic granules, suggesting they were mistaken for food items. When plastics are consumed, too, they are excreted as smaller particles, and become available to other trophic levels in marine and terrestrial habitats, impacting a wider portion of ocean biodiversity. Transportation of plastics across the ocean is facilitated by migratory species such as sea birds and whales, and down to the sea floor by sinking carcasses, marine snow or the faeces of zooplankton.
Ingesting plastics causes serious harm; it can cause direct mortality by blocking or severely damaging the gastrointestinal tract, or the stomach wall. If this immediate danger is passed, stomach and gut functioning can be severely slowed, causing a slow decline in health and bodily functioning. Poor nutrition and hydration may also be caused by damage to the digestive tract, and reduced space in the stomach may result in less food intake, effecting energy and ability to survive for longer periods without eating. Similarly, ingesting plastics may give animals a feeling of satiation, reducing the desire to search for food while they have actually consumed no beneficial nutrients. The chemical substances added during manufacture or adsorbed to plastics at sea are also a source of serious concern, particularly in larger species where more plastic is stored and broken down into micro plastics over longer periods, can cause long-term harm.
Microplastics, similarly, have been found in almost every marine habitat in the world, with plastic composition and environmental conditions significantly affecting their distribution. Their impacts are thought to be greater on smaller animals, especially on organisms at the cellular level; nanometre-sized plastics can pass through cell membranes. As time goes on, species at every trophic level are increasingly unable to avoid the plastic invasion. Its range of impacts are inherently tied to the pre-established relationships of co-dependant marine ecosystems, whether that be deep sea or coastal. Increasingly, these relationships are being altered by population loss, migration, and the various health effects of plastics and microplastics on marine species, so that even the few species that do not come into direct contact with plastics are impacted by it.
With new evidence that microplastics are reducing the ocean’s ability to store carbon, there has never been a better time to readdress ocean plasticisation as a true global priority. That process might be dependant on top-down initiatives to reduce plastic production, usage and disposal into waterways, which may yet see the necessarily drastic progress in years to come, though action must be taken very urgently if remaining marine biodiversity may be salvaged. In the meantime, every person can, and should, do their part to reduce plastic consumption, to recycle plastics where possible, and get involved with initiatives which aim to reduce plastic waste disposed of improperly so that they are far less likely to end up in the oceans.
