“All at Sea”- The effects of Plastic on Marine Ecosystems

Yet again I forgot. Forgot to bring my reusable shopping bags to the supermarket. A small mistake- a deadly outcome…

It seems strange that a place that is revered so highly for its mystery, aesthetic value and commercial use is where the majority of plastics are disposed of annually. 50-80% of debris found in the marine environment is plastic; found on the seabed, beaches and drifting in the sea [2]. A total of 660 species are known to encounter plastic debris, affecting the food chain from the smallest phytoplankton to top predators such as whales and birds [3].

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Fig.1 World Map showing the average concentrations of plastic at 442 sites. Grey areas signify possible areas of plastic accumulation in subsequent years, with darker areas symbolising higher densities [1].

Since their discovery in the 1900s plastics have become the most widely used material in the world. Despite only existing for just over a century, their high persistence, abundance and durability make the ‘oceanic plastic crisis’ an increasing global problem (Fig.1) [2]. In 2011, 280 million tonnes of plastic were produced, an alarming increase from the 1.5 million tonnes manufactured in the 1950s. The multi-faceted nature of plastics have led to them affecting marine ecosystems in a variety of ways depending on the size, species, social ecology and feeding strategy of marine life [5].

Entanglement

Certain groups including birds, cetaceans, crustaceans, sharks and turtles fall prey to plastic snares. Larger vertebrates become ensnared in plastic, forming ‘lethal collars’ on sharks, seals and cetaceans. These can wrap around body parts tightening as the animal grows, restricting movement, cutting off blood circulation and inhibiting predator avoidance [3].  As some plastics take 500 years to break down in natural environments, once a ‘snare’ kills its victim and the body decomposes the plastic is free to entangle more individuals resigning them to the same fate. Some mammals actively seek out plastic. Dolphins and sea-lions are typically known for their playful and inquisitive natures and are often found entangled in plastic debris where play has ended in catastrophe (Fig.3) [2].

‘Ghost fishing’ has become a term coined for marine vertebrates caught in long lines of discarded netting left to drift in the sea. Unsurprisingly, this has been found to have detrimental effects at all pelagic levels of the marine biota, affecting species diversities on the sediment and at the surface [4].

 

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Fig 2. The effects of plastics, a) Californian sea-lion (Zalophus californianus) ensnared in plastic netting, b) decomposing Hawksbill turtle (Eretmochelys imbricate) which has died from ingesting a plastic bag and c) mussels and barnacles encrusting plastic marine debris. (Source: public domain).

Ingestion

Plastic debris is often mistaken for food by a variety of fauna owing to the visual similarity between plastics and food. This can lead to stomach blockages and consequent starvation in larger vertebrates [3]. Styrofoam mistaken for cuttlefish by birds affects 100 pelagic species, blocking their digestive tracts. Plastic ingestion is a particular problem for smaller birds which store the most and seem to be unable to successfully expel it from their guts following ingestion.

One of the most charismatic animals to be affected by plastic debris are sea turtles. Spending the majority of their life at sea, turtles favourite food are jellyfish which they often mistake for plastic bags. These clog their stomachs leading to starvation and death [4].

Microplastics

The tendency for microplastic to enter the food chain is a major cause for concern. Many organisms from the lowest trophic levels capture and consume any material of the appropriate size meaning microplastic is often ingested [3]. On entering the benthos, microplastics have the ability to be ingested by a variety of an organisms and therefore can be recycled within the food chain indefinitely due to their persistence in the environment. They may be ingested by bottom-dwellers or disturbed only to float to the surface and be re-consumed. Once consumed by lower trophic levels their inability to be digested means that through predation events they can accumulate in high densities at the highest trophic levels, continually being recycled and exchanged between food webs (Fig.3) [5].

 

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Fig.3 Potential pathways that microplastics can enter and be recycled in the marine environment [5].

Drifting Plastic

The buoyancy of plastics means that if disturbed on the sea bed it can return to surface waters. This benefits very few species, however in large expanses of open sea large masses of plastic materials attract a large diversity of vertebrates (Fig.4) [4]. These ‘plastic islands’ act as nurseries for young schools of fish in the vastness of the Pacific ocean– perhaps the only good attribute that plastics have for marine organisms.

Drifting plastics have the ability to move across an entire sea, allowing organisms to attach themselves, potentially moving with plastics as ‘hitch-hikers’ [4]. People are all too clear with the term ‘invasive species’ which has been used to describe organisms such as the zebra mussel which have ‘hitch-hiked’ in the ballast water of ships across the Americas and Europe taking over and destroying habitats. This has the potential to reoccur with species traveling on plastic debris. Tunicates, bacteria, diatoms and barnacles have been found to have the ability to move up to 1,500km in this way [2]. Thus, invasive species transfer to other areas could be accelerated by plastic debris (Fig.2) [5].

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Fig.4 Plastic Island. Source:public domain.

The global plastic problem although increasing in severity is still widely ignored. Recent developments including the 5p charge for plastic bags in UK chain stores have proved an annoyance -if not very effective. In order to successfully facilitate change a more drastic message might be needed- perhaps a plastic bag bearing the picture of a leatherback turtle belching out a soggy piece of plastic would be a more successful deterrent.

References

1. Cózar, A. et al. Plastic debris in the open ocean. Proc. Natl. Acad. Sci. U. S. A. 111, 10239–10244 (2014).

2. Derraik, J. G. B. The pollution of the marine environment by plastic debris: A review. Mar. Pollut. Bull. 44, 842–852 (2002).

3. Green, D. S., Boots, B., Blockley, D. J., Rocha, C. & Thompson, R. Impacts of Discarded Plastic Bags on Marine Assemblages and Ecosystem Functioning. Environ. Sci. Technol. 49, 5380–5389 (2015).

4. Gregory, M. R. Environmental implications of plastic debris in marine settings-entanglement, ingestion, smothering, hangers-on, hitch-hiking and alien invasions. Philos. Trans. R. Soc. Lond. B. Biol. Sci. 364, 2013–2025 (2009).

5. Wright, S. L., Thompson, R. C. & Galloway, T. S. The physical impacts of microplastics on marine organisms: A review. Environ. Pollut. 178, 483–492 (2013).

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