The 2017 relaunch of BBC’s Blue Planet this year has helped to drive a renewed interest in the oceans and the impact that humans are having upon the marine environments that make up the vast majority of our planet’s surface area. One such impact is marine plastic litter. With estimates that plastics will outnumber fish, alongside ocean warming and acidification, it could be argued that our action are pushing many species into the extremes of their natural limits.  In 2017, a big push in the UK against microplastics lead to the banning of them from cosmetics and toiletries. However, almost all plastic has the potential to break down into these pervasive microplastic particles. But how do larger plastic items interact with the marine environment and the organisms that live in it?

The award-winning image of a lone seahorse, holding onto a cottonwood in place of anchoring seaweed. Credit: Justin Hofman

New materials, new habitat

In 2014, a german research team travelled to the Sargasso Sea, a part of the Atlantic just off the Caribbean and Florida coast, characterised by high densities of the algae Sargassum found floating on the sea surface. The team compared the invertebrate communities found of the Sargassum rafts to that of those found on plastic rafts and found there to be a significant difference between the two. Could this be indicative of organisms adapting themselves to a new, plastic-filled environment? A follow up survey in the North Sea is currently being analysed. Although no results have yet been published, the rafting invertebrate communities in the North Sea are dominated by the isopod Idotea balthica, which was present in high densities in both the algal rafts and plastic. So perhaps, the example of the Sargasso Sea better highlighted the specific niche of the Sargassum algae-living community than the wider marine invertebrate species.

Carl Safina, of the Blue Ocean Institute in New York believes that as litter becomes as common as driftwood and other rafts, organisms have adapted to using it as an alternative. The attraction to plastics as rafts is not fully understood, but could be simply related to abundance. In Marine Anthropogenic Litter, the role of plastic as habitats and dispersal vectors was discussed. Barnacles seem to particularly benefit from hard, long-lasting plastic as a substrata.

Threats and Invasive Species

However, the benefit of habitat and refuge offered by plastic is far outweighed by the cost that plastics have on the wider environment, according to Charles Moore of Algalita Marine Research Foundation. A key part of this is the long life of plastics. Algal rafts break down in a matter of weeks as the invertebrates that they host feed upon the algal fronds. Driftwood has a higher longevity, but it too is digested and broken down before long. Plastic, conversely, is much more resilient and can act as a driver for invasive species as it transports organisms far further than would be possible on biotic rafts. It is estimated that marine litter can double or triple the risk of invasive species, depending on latitude. The full extent of plastics-related invasive species is not fully understood, but is thought that marine litter can now rival ship ballasts for transporting foreign organisms.

Going back to Blue Planet, one episode showed the carcasses of albatross chicks and adults found with stomachs full of plastic. It’s a sobering example of the direct impacts that marine litter can have upon species. Like many marine animals, the albatross mistakes the plastic for food, and unwittingly eats it or feeds the plastic to it’s young. Carrier bags show a remarkable likeness to jellyfish, leading to mortalities in several species of turtle across the globe. Even animals as large as whales have been found to have plastic in their digestive systems.

One of the poster images for BBC’s Blue Planet was this turtle, entangled in “ghost netting” left over from fishing vessels. Credit: BBC

A further risk of ocean plastics is that of entanglement. Images such as the cotton bud-holding seahorse above frequently make headlines as they provide an emphatic snapshot into human’s impacts in every ecosystem on the planet. Entanglement provides a huge mortality to many species. Air breathing birds, reptiles and mammals are often at most immediate risk due to drowning. However, even some fish can drown; sharks, for example, must continually swim to move enough oxygen over their gills. It is estimated that 200 species worldwide are at risk of entanglement.

Whilst a small number of species may benefit from additional refuge and habitat relating to marine litter, many more are at risk, particularly due to ingestion and entanglement. Wider environmental impacts include plastic acting as a vector for invasive species, which could lead to marked differences in community structures. In all cases, the introduction of plastics to our ocean has had a significant effect on marine life and will continue to do so whilst we still produce and throw away these seemingly immortal materials.

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