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Conservation

Plastics increase disease risks for corals

The paper: Lamb, J. B., Willis, B. L., Fiorenza, E. A., Couch, C. S., Howard, R., Rader, D. N., True, J.D., Kelly, L. A., Ahmad, A., Jompa, J., and Harvell, C.D. (2018). Plastic waste associated with disease on coral reefs. Science, 359: 26–29.

Figure 1: A plastic bag ensnared on a coral colony – image from NOAA

In case you’ve missed it, coral reefs are in trouble. These beautiful and important ecosystems have been battling ocean acidification, rising temperatures, and pollution. These stressful conditions have led to mass bleaching events, spikes in diseases, and devastating losses to coral populations worldwide. This is not just a problem for tourists who want to dive into pristine environments; the services we receive from coral reef ecosystems, including fisheries and coastal protection from storms, have been valued at $375 billion USD. Now corals appear to have a new problem: plastics (Figure 1).

It should also come as no surprise that plastic is polluting our oceans. Plastics are known to entangle many animals, limiting their growth and movement and sometimes causing death. They are also a problem for the birds, whales, turtles, plankton, and other animals that end up consuming the floating debris. Even worse, plastics have also been shown to absorb toxins and house pathogenic bacteria – yikes! Corals have been observed ingesting small pieces of plastic in the past. Now, it appears that those hitchhiking, pathogenic bacteria are riding their way to reefs and spelling disaster for corals and the other animals that rely on them.

Researchers wanted to determine just how large of an issue plastic pollution is for corals. Between 2011 and 2014 they surveyed a total of 159 different coral reefs throughout Asia-Pacific (Figure 2). (Note: this research was conducted before the mass bleaching events of 2015). At each reef, the scientists laid down a 10 – 20 m length of tape measure( called transects). They then identified coral colonies along their transects (only for colonies larger than 5 mm), classified the health of the coral colony (healthy or affected by one of six major diseases), and noted when plastic debris was found on coral colonies (either entangling or directly in contact with the coral tissue – Figure 1).

Figure 2: The areas studied in Asia-Pacific is shown in the map (A). Individual countries and reef sites studied are shown in smaller red boxes in B. Maps in B and C are color coded to show the amount of plastic found on coral reefs in 2010 (B) and estimated to be found by 2050 (C). Darker blues indicate higher amounts of plastic debris. From Lamb et al., 2018

The scientists compared the prevalence of disease among coral colonies to the amount of plastic found in the area. In areas where corals were not in contact with plastics, the probability of the coral having a disease was on average 4.4%. However, when corals are in contact with plastic debris, the likelihood of contracting a disease increases to 89.1% – 20 times higher than when corals are not in contact with plastics! Not all coral species were impacted in the same way (Figure 3) and the corals with more structural complexity (“tabular” and “branching”) were 8 times more likely to be in contact with plastic than corals with less complex structures (“massive”). Some of the more structurally complex corals (“branching”) were also far more susceptible to diseases when in the presence of plastics. This can be a problem, as many species that rely on coral reef habitats require a more complex structure provided by these more susceptible corals. Researchers are still not sure just how the plastic increases disease risk, but speculate that plastics could cause injury to the coral tissue and allow pathogens in the area or on the plastic itself to infect the coral colony.

Figure 3 – Not all corals respond in the same way to plastic debris. The graph in A shows that different coral structures are more likely to be in contact with corals (order of complexity from low to high: massive, branching, tabular). The graph on the right shows how the likelihood of contracting diseases increases for each coral type. For comparison, the box B on the left of the graph shows the likelihood of each coral contracting diseases when not exposed to plastics. From Lamb et al., 2018

Based on the amount of plastic researchers found directly in contact with corals, they estimate that 11.1 billion items of plastic exist on coral reefs in Asia-Pacific alone. Some of the largest contributors to oceanic plastic pollution were not even covered by this study (China and Singapore), so this estimate is likely on the low side. If we fail to change our ways, the researchers estimate that by 2025, that figure will increase to 15.7 billion plastic pieces entangling coral reefs in the area of Asia-Pacific they studied.

Coral reefs are already imperiled due to climate change and other human impacts. If we cannot solve our plastic pollution problem, disease outbreaks on coral reefs may continue to rise. This is a big problem for the human population, with over 275 million people globally directly relying on coral reefs for food and jobs. Among other services, corals provide shelter and habitat for many commercially fished species, boost tourism income, and protect coastal areas from devastating storm surges. We need coral reefs, and, now, they need our help too. We can work to protect these vulnerable habitats by incorporating even small changes to our daily routines to reduce our plastic consumption. You can find ideas on how to kick your plastic habit at the links below.

What’s your favorite way to reduce plastic usage? Tell us in the comments below!

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