It is not a surprise that plastic is harmful to our marine ecosystems. According to the United Nations Environment, between 60% and 80% of the rubbish in our oceans is plastic. Furthermore, it impacts over 800 aquatic animals. Whilst floating plastic, such as those seen in the Great Pacific Garbage Patch, are estimated to only make up 1% of global ocean plastic. The rest sinks to the seafloor. There’s a lot of debate about the extent of the damage these plastics are causing on our oceans in general, but what about coral reefs in particular?

Coral reefs are exceptionally vulnerable and are currently facing many challenges due to human activities. Threats include overfishing, the introduction of non-native species, nutrient pollution, and global climate change. But what about the impact of plastic pollution? Research published in Science (Lamb et al., 2018) discusses the danger of plastic pollution to coral reefs, highlighting the extent and seriousness of the problem we face.

The research conducted by Lamb et al. (2018) examined 159 different reefs from across the Asia-Pacific region over a timescale of four years. The fundamental finding was that in waters where they found higher volumes of plastic pollution, the coral were not as healthy. Let’s break the research findings down to explore how plastic pollution impacts coral reef health.

Plastic Increases The Chance Of Coral Disease

When corals come face to face with plastics, the risk of an outbreak of disease rises from 4% to a staggering 89%. In fact, the presence of plastic significantly increased the likelihood that corals would become infected with a disease. Overall, if corals come into contact with plastics, they are 20 times more likely to be hit by disease than those that do not.

Plastic Promotes The Growth Of Harmful Pathogens

Scientists have several hypotheses about why plastic-contaminated reefs will be more susceptible to becoming infected. In the first instance, plastic will prevent light and oxygen from accessing the coral. There are two elements that are crucial in order for corals to survive.

Plastic is also thought to carry harmful pathogens and transport these into coral reefs, thereby promoting the growth and spread of disease. Plastics do this as they are abrasive to the surface of the corals, creating cuts and new openings. It’s essentially like ripping the skin of the coral and which can allow an infection to enter from anywhere if there are lots of rips in and tears in the protective barrier of the coral skeleton. More troubling is that when coral loses tissue, it grows back exceptionally slowly. Leaving a greater chance of being infected from new pathogens, leaving them very vulnerable to diseases spreading throughout their structure. 

Complex Corals Are Particularly At Risk

Analysis has shown that some species of coral are more likely to have problems with plastic pollution than others. Structurally complex corals, such as staghorn coral and other forms of coral that have delicate branching forms, are eight times more likely to become entangled in plastics. This is a problem when it comes to marine ecosystems and the associated diversity of life that lives within coral reefs. This is because these types of coral, whose intricate and delicate structures offer a number of hiding places for juvenile fish, are usually ecologically important environments, especially as nursery habitat.

Corals Eating Plastic

Whilst corals are cut open by plastics, parts of the plastic get lodged into the coral skeleton. Although this isn’t necessarily intentional, research has found corals were ingesting the plastic and in some cases actually actively seeking plastic out as a food source. No, the corals weren’t accidentally mistaking the microplastics as food. Researchers found that corals were deliberately eating pieces of plastic if they floated by. It’s currently thought that the coral might be tasty to the corals due to the chemical compounds that the plastics are made of. This relationship needs to be investigated further, as if these plastics are carrying contaminants or disease, this would be a very toxic relationship.

Billions Of Plastic Pieces On Coral Reefs

In the Asia-Pacific region alone, it is estimated that there are around 11.1 billion pieces of plastic entrapped and entangled on coral reefs. What’s worse, this is expected to jump by 40% by 2025, to over 15.7 billion pieces of plastic. Coral reefs are already vulnerable to several other threats, such as the impacts of climate change. So the overwhelming amount of plastic waste that is now threatening these ecosystems is of huge concern.

Plastic Breaks Down Into Our Food Chain

Plastic pollution is not just an issue for our coral reefs, it impacts the wider ocean environment and the very food chain we rely on. Although huge bits of plastic waste and discarded fishing nets are among the obvious evidence of the destructive effect that humans have on sea life, what is even more disturbing is what happens when it breaks down. When plastic waste is carried from populated areas by rivers, tides or storm drains, it is eventually broken down into smaller parts through direct sunlight and heat exposure. When exposed to ultraviolet sunlight, there is a chemical reaction in the plastic that leads to the separation of large polymer molecules and the eventual deterioration of the substance.

Once plastics are less than 5 mm in size, they are referred to as microplastics because, rather than floating on the surface of the ocean, they permeate into the water column. Apart from the harmful chemicals that are used in the manufacturing process of these plastics, they also accumulate contaminants. These processed toxins are first consumed by fish and other aquatic animals until they are incorporated into the food chain and biomagnified.  In some cases, these food chains contain humans, with tiny bits of plastic contained within our food being consumed at an ever growing rate.

What Can Be Done?

Tourism is one of the most main causes of ocean pollution. Yet even our actions at home is important to protect coral reefs from damage. It’s hard to imagine the local impact we have as individuals when pictures of  plastic bags littering coastal habitats several hundred miles away are so commonplace. But these images are a much-needed reminder that even though you live nowhere near a coral reef, the plastic waste can be swept away on a remote shoreline and damage the marine life there instead.

We all have the option to avoid single-use plastics, reduce our use of plastics, reuse plastics as much as possible and recycle them. Although corals face many challenges beyond our individual influence (such as ocean acidification and the impact of climate change), the changes we will bring to mitigate local pressure on coral reefs can help them become more resilient to other threats.