Will coral reefs survive the next 50 years, and what impact might their loss have if they don’t?
An objective piece of writing that is designed to inform
What are coral reefs?
A coral is a colony of tiny genetically identical polyps that function as a superorganism. The colony grows by polyps regularly cloning their organs and separating into two (known as “budding”). This process is slow, meaning most corals grow at around 11/2 centimetres per year (Lewis, 2018). The different types of corals are divided into “soft”, “hard”, “warm/shallow water” and “cold/deep water”. Most reef building corals are hard and warm water, but hard, cold/deep water reefs also exist (see figure 1).
In hard, cold water corals, the polyps collect calcium carbonate from ocean currents and secrete it as a skeleton underneath (NOAA, 2017). Warm water corals do the same but also have a mutual symbiotic relationship with algae which supply them with sugars from photosynthesis. In return, the algae are provided with nutrients, like nitrogen, that are usually scarce in coral waters. These algae (known as “Zooxanthellae”) are also what gives corals their colour (Carilli, 2013). During the night, when their algae are not active, polyps use a combination of stingers and tentacles to catch small fish, invertebrates, plankton or detritus for their nitrogen (Weis, 2008).
In order for a healthy reef to form, the environment has to meet certain conditions. Sunlight is an essential source of energy for photosynthesis and therefore warm water corals need to be in shallow, clear water where the sunlight can easily reach them. For this reason, they rarely develop in water deeper than 165 feet (50 metres). Deeper cold water corals have evolved to cope without the algae that need the sun, but as a result, grow much slower. However, warm water corals are much less adaptable to changes in temperature, confining them to more stable waters that stay within 20–32°C (Coral Reef Alliance, 2017). Temperatures outside this limit can send their Zooxanthellae into distress, causing them to release toxins as a defence mechanism and stop producing sugars. As a result, the polyps are forced to excrete the algae and usually starve within a couple of days (Weis, 2008).
On one night a year, the corals go through mass spawning. All the polyps release packets containing either egg or sperm cells which spill open after reaching the surface of the water. The sperm and egg cells then have to find each other, fuse together (to form “Planulae”) and then swim and attach themselves to a solid surface to begin growing(NOAA, 2017). Reproducing sexually in this way, increases the genetic diversity in the population, so that they are less susceptible to disease (Chornesky & Peters, 1987). However, reproducing this way can be problematic as a lot is up to chance. Planulae will often sit at the surface of the water for some time (up to a couple of weeks) until they find a suitable surface to attach themselves to, leaving them at risk of being eaten or swept away by currents and storm (NOAA, 2016). In addition, corals only spawn within a short time period and only when the conditions are right, making it hard to harvest eggs, sperm or Planulae and grow baby corals for conservation.
In terms of addressing the main question of this article, there are multiple factors that make corals (especially warm/shallow water corals) vulnerable to change. While there are examples of corals adapting to more extreme environments (such as inter-tidal pools that can rapidly fluctuate in temperature) the fact that coral waters have remained “unusually stable” for the past 300 million years (Lewis, 2018) means they will be unlikely to effectively cope with a fast rate of change.
Why are they important?
Economically, it is estimated that today’s coral reefs bring in a value of US$33 trillion per year due to their contributions towards food, tourism and medicine, and as a buffer against storm surge (Marhaver, 2015). Over 4000 species of fish and their food are reliant on coral reefs that, in turn, prop up 12% of the entire world’s fisheries. These fisheries provide income, jobs and nutrition for millions of people. This is especially the case in developing countries, where they provide one-quarter of the total fish catch (Cesar, H., Burke, L., & Pet-Soede, L. (2003), while covering “less than one tenth of one percent of the ocean floor” (Costanza et al.,1997). In terms of tourism, at least 94 countries benefit directly from reefs, with reef tourism contributing to more than 15% of GDP in 23 of these and at an estimated total tourism value of $9.6 billion (Burke et al., 2011). Coral reefs are also “considered the medicine cabinets of the 21st century”, with potential in multiple areas of human disease (NOAA, 2017). This stems from the sheer diversity of species (which exceeds that of rainforests (Ray, 1988)) and the numerous chemical pathways they use. For example, Secosteroids (a subclass of steroids used by corals to fight disease), have anti-inflammatory properties and are used to treat asthma and arthritis in humans. Similarly, other compounds found in reef organisms have shown potential in treating HIV, cardiovascular diseases, ulcers and cancer (Fenical, 1996).
Ecologically, reefs provide a source of nutrition and shelter right up the food chain, and are hence estimated to support 25% of all marine life (Lewis, 2018). The nooks created by their 3D structures allow animals to hide themselves and their eggs from predators, making them ideal nurseries for young fish (Nagelkerkenab et al., 2000). The solid structure also acts as an anchor for other organisms requiring a fixed base (such as anemones) and as a buffer against storms (NOAA, 2017). It is for this reason that “the more complex the structure of the reef is... the more life it supports” (Lewis, 2018). The presence of reefs can also have knock on effects for species in neighbouring environments. For instance, larger, roaming predators (such as sharks and turtles) regularly benefit from native parasite eating fish (such as the Cleaner Wrasse). This attention helps control the spread of disease, reduces stress, and increases the lifespan of these larger animals (Grutter, Murphy & Choat, 2003). In addition, several organisms consume coral Planulae as a source of food and a few species (such as parrotfish, butterfly fish and crown-of-thorns sea stars) will eat adult coral (Miller & Hay, 1998). Because of the unique ecosystem they provide, coral reefs are also a rich source of endemic species (species that are not found anywhere else). For instance, out of the 43 species of Wrasse fish known to inhabit Hawaiian waters, 13 are only found on coral reefs in that area, and these would completely disappear if their reefs were to die (Vitousek, Loope & Stone, 1987).
For the main question of this article, it seems the loss of coral reefs would have much wider implications than their effect on just the immediate surroundings. The value gained by processes which degrade local reefs, such as dredging to prevent groundings of marine shipping traffic, is likely to be far outweighed by the consequences of losing them, in terms of jobs, GDP and coastal erosion. It is hard to predict how many other species will become critically endangered or extinct if coral reefs continue to die off, but it is considered by some to be the beginning of “an ecological collapse, that may not stop at just species, but entire classes [4 orders above species] of organisms as well” (Chasing Coral, 2017).
So far, my research has concluded that the disappearance of coral could have a devastating effect ecologically. This not only affects species endemic to coral reefs, but also any outsiders that interact with these organisms, either through the food chain, through symbiotic relationships (such as the cleaner fish) or species who are temporary visitors to the reefs and use them as a nursery or breeding ground. Economically, fishing, tourism, and coastal cities directly benefit from the presence of reefs and their products as mentioned before, but may also benefit indirectly from the beneficial effect reefs have on their neighbouring environments (through these temporary visitors), known as the “spill over effect” (Lewis, 2018).
Why are they at risk?
In the past 100 years, average sea surface temperatures have already increased by almost 1°C and are on track for a 4°C increase by the end of the century (Lewis, 2018). Whilst corals/reefs are capable of recovering from bleaching events, a full recovery will usually take around 10 years. So ultimately, it’s the frequency of the bleaching events that matters. For a bleaching event to occur, the high water temperature has to be sustained over a series of days or weeks so that the polyps are not able take in new Zooxanthellae and recover. The leftover skeletons are usually overgrown by algae, which any new baby corals will have to fight off if the reef is to be re-colonised (Hoegh-Guldberg, 1999).
It is now recognised that this increase in sea temperature is due to human emissions. The combustion of fossil fuels releases greenhouse gases (including carbon dioxide: CO2) into the atmosphere which forms an insulating layer around the planet. Without some form of atmosphere, the world would be far too cold for any life to exist, but with too much it will heat up; this is known as the greenhouse effect (NOAA, 2017). Although CO2 is the most well-known, levels of all greenhouse gases have consistently risen over the past century, as shown below.
In 2016, the Earth’s CO2 average passed 400 parts per million (ppm) for the first time in several million years (NOAA, 2017). It is believed that the ocean absorbs 30% of this carbon dioxide (Lewis, 2018) which can degrade a coral’s calcium carbonate skeleton in the following reaction:
CO2 + CaCO3 + H2O ⇒ Ca(HCO3)2 (aq)
Or in simple terms: carbon dioxide + calcium carbonate + water ⇒ calcium bicarbonate (‘aq’ means it is dissolved in water).
However, the larger problem is that it disrupts the natural pH (acidity or alkalinity) buffering system of the world’s oceans. This buffering system works by having a large amount of calcium carbonate naturally dissolved in seawater (from the erosion of limestone and other rocks over time) which will neutralise any acid added to the water, making seawater unusually stable in its acidity (Lewis, 2018). The reaction above removes this buffer. Unfortunately, carbon dioxide also reacts with water to form carbonic acid (represented as H2CO3) (Doney et al., 2009):
CO2 (aq) + H2O ⇒ H2CO3 (aq)
This Carbonic Acid decreases the overall pH of the ocean (makes it more acidic). All organisms evolve to function within a certain pH threshold (for instance, our own blood pH must remain within 7.35 to 7.45 for our bodies to function). Whilst animals on land are able to control their internal pH fairly easily, corals (and their Zooxanthellae) are reliant on the natural stability of the seawater surrounding them. If this passes a critical level, then their cellular structures may break down or be damaged and key enzymes that keep the organism alive will stop functioning (The Royal Society, 2005). In 2014, the pH of ocean surface water had already decreased by 0.1 (making it roughly 26% more acidic) and this is only likely to continue with our current rate of emissions (Pachauri & Meyer, 2014).
To date, mass bleaching events have occurred annually in Hawaii, Bermuda and the Great Barrier Reef (Chasing Corals, 2017). I would say that if we do not dramatically reduce our emissions, corals skeletons will begin to be dissolved faster than they can grow (due to the more acidic oceans) and will not have sufficient time to recover from the continuous bleaching. It is hard to see how they would survive the next 50 years if emissions continue as before, especially since over 50% of coral reefs have been lost already (Chasing Corals, 2017).
Rising sea levels
Increasing global temperatures add another challenge as they causes land-based glaciers near the poles to melt, and this displaces the surrounding water causing it to rise. As mentioned before, warm water corals rarely develop in water deeper than 50 metres (where the sunlight can reach) and are slow growing (especially with coral bleaching and ocean acidification stunting any growth even if they survive it), so areas in deeper water may become uninhabitable (Lewis, 2018).
Fertiliser from farmland can also leak out into the oceans and cause algal blooms, which compete with corals for resources (Scannone, 2016). This fertiliser runoff also causes a spike in the number of ‘crown-of-thorns starfish’ larvae, potentially causing an outbreak. The starfish feed on coral and a spike in their numbers can be devastating to a reef (Cooper, 2017) as it has a compounding effect. Corals that have open wounds from the starfish are more exposed to infection and are also less resistant to bleaching events. On the other hand, when it has been possible to decrease local stresses like this, the change has been shown to increase reef resilience to bleaching and potentially buy the coral some time (Lewis, 2018).
The fish within and around coral reefs exist as part of a much larger food network and changes in their numbers can have wider, ripple effects. For instance, some fish clean algae off corals, keeping them healthy (BBC, 2017). While others, like the parrot fish, will eat small chunks of coral and recycle the nutrients as sand (Lewis, 2018). If (due to overfishing) one of these were to change in numbers, the internal balance within the food chain would be lost, causing a sudden rise or drop in the numbers of predators and prey, affecting their own predators and prey in turn. This process is known as a “trophic cascade” (Encyclopedia Britannica, 2010). Additionally, deep sea dredging for fish damages and kills deep sea corals, which take even longer to grow back due to their slow growth rate (Lewis, 2018).
So far, we have produced ‘9.1 billion tons’ of plastics (Scharping, 2017) with only 9% of that being recycled (Parker, 2017). Plastics are getting into our seas from illegal dumping into the ocean, from wind blowing plastics off beaches, from plastic dropped into rivers, from microplastics in our toothpaste and beauty products going down the sink, and from synthetic fibres being washed. Even plastic that makes it to landfill is still at risk of being blown away into the surrounding rivers and oceans (Casson, 2017). The problem with plastics for corals is that the microplastics in particular can create cuts in the coral’s polyps which can become infected. A study using 124,000 reef-building corals from 159 reefs in the Asia-Pacific region showed that “89% of those fouled by plastic were suffering disease. On plastic-free reefs, only 4% of the corals were diseased.” (Carrington, 2018). This is a shocking statistic, especially when you combine this issue with the others that corals face, it seems to become incredibly unlikely that any corals will remain in the seas naturally. Even if a coral survives a bleaching, survives ocean acidification, survives a crown-of-thorns starfish outbreak, they will still have to survive plastics causing disease; something that only 11% of corals in the Asia-Pacific region could avoid.
So how does this relate to the overall question of this project? There is research which suggest that reducing local stresses on corals, such as fertiliser, plastic and fishing does contribute to a reefs ability to survive mass bleaching events (Lewis, 2018). Ultimately, though, there needs to be a global effort to tackle climate change if reefs are to ever fully recover.
Will coral reefs recover and survive?
Protecting coral reefs can seem like a monumental task, because it requires every country to collaborate on combating climate change as well as each country’s lawmakers protecting reefs in their local area. During the 2015 Paris Climate conference, 175 out of 195 countries agreed to targets that would keep the average temperatures from rising above 2°C, marking a huge step forward in global collaboration on this issue (Pachauri & Meyer, 2014). But, will this be enough to save coral reefs?
In 2014, the International Panel on Climate Change (IPCC) used computer modelling to see what it would take to keep the earth below 2°C, up as far as 2100.
In simple terms, they created a series of best and worst-case possible scenarios that could happen in the next century (called “Representative Concentration Pathways” - or RCPs), these were:
- RCP 2.6 - Best case scenario (show in blue below), with strict laws on total emissions and aggressive scaling back of fossil fuels, so that CO2 levels would not pass 480 ppm
- RCP 8.5 - Worst case scenario (in red), no cap on emissions and business as usual, meaning CO2 levels would exceed 1000 ppm
- RCP 4.5 and 6.0 - (orange and light blue) intermediate scenarios capped at 580 and 720ppm.
ppm = parts per million
The results are shown below in figure 7
From these graphs we can conclude that:
- Only the scenario with significant scaling back of fossil fuels (RCP 2.6) guaranteed staying under 2°C of warming
- Even in this best-case scenario there could still be extensive ice loss, sea level rise and a drop in pH.
In reality, due to the movement of water by currents and the weather, the effects of this will be worse in some locations than others, shown in figure 8 below.
From these images models we can say:
- That in a worst-case scenario, the sea surface temperature will be consistently too high for corals in a large portion of current reefs.
- That the resulting change in pH will be at its worst closer to the North Pole (affecting cold water corals the most), but even near the equator a pH change of 0.35 should be expected.
- This is likely to mean only the reefs most isolated from these factors (and human activity in general) will survive.
From my research, I believe that it is possible, but unlikely that the world’s major coral reefs will survive the next 50 years, especially if we continue at our current rate of emissions. Some smaller, more isolated reefs might survive and offer the potential for re-establishing larger reefs in the future. But the outlook for reefs in general is bleak. The effects of losing them will likely be severe, as we will lose an entire ecosystem that supports 25% of all marine life, 12% of the entire world's fisheries and a huge economic asset. If we want to avoid this, it will take a much more aggressive scaling back of fossil fuels as well as local law enforcement (to help the resilience of reefs to the changing climate).
By Elynor Todd-Jones
An A level student with a keen interest in Biology and the natural world. She is passionate about making science more accessible to readers of all backgrounds and unpacking global environmental issues.
In answering my question, I have been surprised at the vastness and variety of areas that coral reefs impact and how important they are, not only to our ecosystems but to our economy. This project is nothing new to its field, but my findings are completely new to me and almost everyone I have spoken to, including friends and family. So perhaps this project could be used as a way of spreading the word and influencing people to help save coral reefs. Ultimately, conservationists can do their best to save individual coral reefs, but they cannot stop the underlying, global issues (like climate change) if the rest of the world does not get involved. Additionally, people will generally not rally behind a cause they do not understand. This project hopefully will help educate non-experts, who may feel put off by the jargon in scientific papers.
I have learnt a lot during this project and hope to continue to learn and educate people on this subject. Doing this project has allowed me to learn much more about a subject that I would have otherwise not had the chance to study in detail. While the answer to my question may seem slightly depressing, I do feel there is still a chance to save at least some of these reefs. Alternatives to fossil fuels and ideas for local and international law already exist, it’s just a case of implementing them. If we can do this, climate change, annual mass bleaching and ocean acidification can be stopped. Under this scenario, any corals that were left would be able recolonise old reefs and continue to support their local environment. We made this mess, and it is up to us to clean it up.
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