Coral reefs are an essential cornerstone of the marine ecosystem. These reefs are homes to thousands of aquatic species and provide protection to coastlines. There has been a phenomenon occurring over the past two decades known as coral bleaching. Coral bleaching occurs when water becomes warm which causes algae to be released from their tissues.
This causes coral to turn white and put them at high risk of catching disease and eventually dying. Fish that use coral reefs for habitat also become more susceptible to predators when the reefs are bleached. Massive bleaching events occurring from 2014-2017 have caused major coral loss in the Great Barrier Reef and the Japanese reefs.
The major impacts of this mass bleaching have already been seen with 75% of fish biodiversity lost. While there are many different factors that go into the bleachings, they are mainly caused by climate change and anthropogenic causes. If the remaining healthy reefs continue to receive thermal stress, a domino effect could ensue potentially catastrophic for the marine ecosystem.
The ocean comprises of different ecosystems, one of which, is the coral reefs. Coral reefs are a diverse aquatic ecosystem that are made up of calcium carbonate. While they only cover 0.1% of the oceans, coral reefs provide living space for 25% of all the marine species (Jones & McCormick 2009). Not only do they provide aquatic species with a habitat, but they provide protection to coastlines and also deliver an aesthetic value to tourism as well. Coral reefs are typically found in shallow waters in tropical areas.
Coral bleaching is a phenomenon that occurs when there is a breakdown in the relationship between scleractinian corals and their algal symbionts (Guest & Baird 2012). Coral bleaching is driven by a few different factors. Pollution, rising ocean temperatures and storm runoff are some of the major causes for coral reef degradation.
It is a common misconception that reefs are dead when they become bleached. This is not the case, reefs only become more susceptible to diseases when they are bleached. Diseases such as red-band and black-band are what kill the coral and make them unrecoverable.
Massive bleaching events have been occurring at an alarming rate over the last ten years. Reefs in Australia, Japan and the Caribbean’s have all seen mass bleaching events negatively affecting their coral reefs. There isn’t one concrete reason for coral bleachings, but there are an assortment of factors believed to be generated by anthropogenic causes.
Some of these anthropogenic factors are direct like overfishing and pollution while others are indirect such as climate change and thermal stress. The consequences for coral bleaching have been seen to produce a catastrophic domino effect that can cause suffering for aquatic organisms and entire marine ecosystems.
This paper will provide further analyses on causes of coral bleachings as well as how the phenomenon can cause a negative impact on the entire marine ecosystem. Studies that have been conducted on major reefs will be used to support the notion that these reefs have been harmed by anthropogenic causes as well as natural causes.
Effect Coral Bleaching has on the Entire Marine Ecosystem
The effect coral bleaching on the entire marine ecosystem is nearly impossible to quantify. Coral reefs are home to many different fish throughout the ocean that depend on the reef for survival (Jones & McCormick 2009). While it is easy to say that fish depend on the reefs for survival it is important to take a look at the relationship between the fish and the coral reefs.
75% of reef fish species declined in abundance and 50% declined to less than half of their original numbers (Jones & McCormick 2009. In a study by Jones and McCormick, they surveyed the feeding models and habitat use by species in 20 reef fish families. Results also showed that ~11% of 538 species had an obligate association with living corals. This would mean these species would have to adapt to exist without the reefs or that these species would inevitably die off.
Another effect that coral bleaching has on the marine ecosystem is the impact it can have on coastlines. The structure of the coral reefs helps provide a buffer that protects the coastlines from waves and flooding events. Australia saw their reefs go through a massive bleaching event and the coastline received some damage as a result. The result of a damaged coastline will raise the risk of having flooding events or serious damage from natural hazards such as a coastal storm (Hooidoink 2013).
How Climate is Affecting the Coral Reefs
Climate change has emerged as the main factor that has been contributing to coral reef bleaching. Climate Change has contributed to the warming waters, which causes the corals to expel algae and results in the white color. Bleaching has been consistent among coral taxa and there was a mass bleaching event in South East Asia.
This can be telling about how areas more susceptible to warmer weathers can negatively impact the health of the coral reefs (Guest & Baird 2012). Thermal stress is considered to be any change created in temperature that can lead to the degradation of material. In the case of coral reefs, it is one of the main factors that has been contributing to bleaching. There have also been studies done that examine how coral reefs adapt to thermal variance.
Understanding the adaptation to climate will further provide more information on how coral reefs handle different temperatures. In 2010, a study was conducted by researchers to see if coral reefs in more thermally variable environments less susceptible to severe bleaching during episodes of elevated sea temperatures (Guest and Baird 2012).
Based on their study, they concluded that reefs in thermally variable environments would be less susceptible to elevate sea temperatures if they have already undergone a massive bleaching event. Corals are seen to have low evolution potential and scleractinian are evidence of this since they have exhausted their capacity to adapt to rising sea temperatures.
Another climate factor that contributes to coral reef thermal stress is ocean acidification. Ocean acidification is where carbon dioxide is dissolved into the ocean and decreases the pH of the ocean. It creates a negative impact by destroying reefs, as well as the calcium carbonate shells of several marine species. One way in which ocean acidification damages coral reefs is that it reduces coral diversity (Hoegh & Bruno 2010).
Increasing emissions of CO2 in the atmosphere cause global temperatures to rise which results in more frequent coral bleaching. Recent data suggests that the thresholds will soon be exceeded for acidities correlated with atmospheric CO2. If atmospheric CO2 rises above 450ppm it could push coral reefs into a negative carbon balance (Hoegh-Guldberg & Bruno 2011). A negative carbon balance would be bad for the oceans because it means that the global temperatures would be rising.
The 400-450ppm range has been identified in other key ocean components such as loss of polar sea ice, melting of Greenland and the melting of Western Antarctic ice sheets. These thresholds can be used to predict large scale consequences for oceanic ecosystems. If the atmospheric CO2 rises above the threshold it can trigger the system into a wide amount of irreversible changes which could leave permanent damage on the coral reefs (Hoegh-Guldberg & Bruno 2011).
While ocean acidification a major problem for the coral reefs it is seen as more of a problem long term for the health of the reefs. Studies have shown that when doubling the atmospheric CO2 to 560 ppm, coral calcification decreases almost 40% through restriction of aragonite formation (Hoegh-Guldberg & Bruno 2011). It has been projected that ocean acidity will decrease by as much as 0.4 pH by the end of the century, this will be detrimental to coral reef accretion because pH levels will drop below the required levels to sustain the reefs. The acidity can be different from region to region.
For example, the Great Barrier Reef in Australia can attain high levels of aragonite quicker than any region around the United States. The reason aragonite and pH cause a problem for the coral reefs is that the reefs are made of a high-magnesium calcite skeleton. This means that they are very sensitive, which causes skeleton losses. It can also weaken coral growth growth that can harm the settlement of the coral (Hoegh-Guldberg 2007).
Bleaching locations can be accurately predicted by using degree heating week index. It can show how much thermal stress is being accumulated in an area, and whether or not it is significant to the coral’s health. To date, the DHW index has been nearly perfect, but conservative, predictor of bleachings around the world.
For 23 of 24 virtual stations monitored, bleaching has been reported in all cases when the NOAA has issued a coral-bleaching alert (Eakin, Lough & Heron 2009). In 1998, a reef in the western Indian Ocean was negatively associated with the DHW index. The areas were seen to have a massive loss of coral cover (Mccalanahan & Weil 2009). The DHW index will continue to help researchers predict events where coral loss can occur, and find patterns throughout different regions to understand why other areas are more under stress than others.
Human Contributions and Impact on Bleaching
While there are few natural factors that play a role in coral bleaching, there are many anthropogenic factors that contribute to the bleachings of the coral reefs. One contribution humans have to the degradation of the coral reefs is by in putting plastic waste into the ocean. There was plastic debris on 17 genera from eight families of reef-forming corals.
When the corals come in contact with the plastic debris, the chance of getting a disease increased by 4% across all eight of the regions (Lamb et al. 2018). This is concerning considering how increased plastic is contributing to ocean pollution. It is projected by 2025 that the cumulative quantity of plastic waste entering the marine ecosystem will increase by one order of magnitude (Lamb et al. 2018).
Reefs are seen to be more susceptible to various diseases when interacting with plastics. This mechanism is still being investigated but the influence that plastic debris has on development has been seen throughout regions where plastic dumping is high.
Human population size around coastal regions and waste management systems also play a part in plastic contamination. It has been estimated that 80% of the plastic debris that enters the ocean is dumped in via land (Lamb et al. 2018). Four countries (Australia, Myanmar, Thailand and Indonesia) were studied based on the levels of plastic waste that they were contributed to the ocean.
This study was conducted from 2011-2014 and the model showed that 88% of mismanaged plastic waste was entering the ocean from these Asian-Pacific countries. These levels added up to 804,214 metric tons of plastic over the course of the years. Nine of these Asian-Pacific countries ranked in the top ten when it came to plastic polluting countries globally (Lamb et al. 2018).
If this trend continues, 11.1 billion items of plastic debris will be put onto the coral reefs along the Asia-Pacific coast lines. This is also seen as more of a generous estimation since it does not include China and Singapore since they fall outside the range of the model.
Another anthropogenic factor that contributes to coral bleaching is overfishing. Overfishing occurs when more fish are caught than the system can support. While it might not seem like it would directly impact coral reefs, some fish play a major role in maintaining coral reefs ecosystem processes (Burke & Reytar 2011).
This becomes a serious issue when it is seen from an economic standpoint because most under developed countries around shore lines need to catch fish for their local businesses and countries’ economy. Overfishing can result in a chain reaction that would be catastrophic for these economies due to declining coral, and in turn would cause habitat loss for other species. Benthic communities along the coast of North America have suffered the most from these events.
Besides plastics, there are other forms of pollution that can play a big role in degradation of the coral reefs. Certain events such as fertilizer and nutrient run off into the oceans can damage the reefs. Runoff from agricultural supplies can result in microalgae blooms, resulting in a reduction of abundance and impairment of growth in coral reefs. This also reduces competitive reef behavior rand ultimately push reefs to ecologically collapse (Burke & Reytar 2011).
Agriculture is a key part of society along the coastlines so fertilizer run off is to be expected but it continues to harm reef communities. One of the main problems that reef communities have run into due to fertilizer addition is hypoxia. The coral tissues become deprived of oxygen, which ultimately damages the coral. Finding a solution is tricky since many communities rely on agriculture to keep their economy going.
How Major Reefs Around the World Have Been Impacted
To understand the severity of the coral bleaching, the major reefs around the world can be used as examples to show how the reefs have been affected from bleaching events over the last two decades. One of the biggest and famous reefs is the Great Barrier Reef in Australia. This reef is home to some of the most diverse species in the oceans and serves as as a major tourist attraction for people all over the world. In 2016, a proportion of the reef experienced extreme bleaching.
Around >60% of the coral was bleached, which was four times higher compared to 1998 or 2002 (Hughes 2017). This signifies how bad the bleachings has gotten over time, as well as how corals are more likely to undergo another bleaching event. Only 8.9% of corals escaped the 2016 bleaching unharmed while 42% of corals were unharmed in 2002 and 44% in 1998 (Hughes 2017). This trend shows how the Great Barrier Reef handles repeated bleaching events over time. The Great Barrier Reef is at high risk for being progressively more damaged as time goes on if another bleaching event occurs.
Reefs in the Caribbean have also experienced a massive decline over time. Elevated water temperatures set off a mass bleaching event in 2005. This caused around 90% of the corals to undergo thermal stress and start to appear completely white. Before bleaching in 2005, the mean coral cover among sites was 21.4% with three sites having more than 20% coral cover. Through sampling in 2006, mean coral cover among the sites dropped to 10.3% with no site having over 20% coral cover (Miller & Muller 2009).
It is also not uncommon to see corals regain color as time passes after a bleaching event but it is still likely for the reef to be very vulnerable to another bleaching event. For six years the coral cover was monitored in the Caribbean region and researchers used the Spearman Rank Correlation to see if there was a correlation between coral disease and water temperatures. The results before a bleaching event were statistically significant as the Mennebeck Reef increased by 23% in 2000 to 26.7% in 2004.
The S. Fore reef increased from 17% in 2002 to 19.6% in 2005. After the bleaching event the Newfound Reef decreased coral cover from 18% to 13% over the course of six years (Miller & Muller). This significant loss in coral cover is a serious concern for the Caribbean region since their economy is heavily reliant on the marine ecosystem. Degradation of these reefs could lead to a major economic collapse if a protection plan is not put into place.
The third major reef that has seen serious impacts from mass bleaching events are the reefs near the Japanese coastlines. The Ryukyu reefs experienced extensive bleaching in 2016 that saw the Ishigaki Island had 90% bleached of its coral bleached. Sekisei Lagoon and Shiraho Reef saw 40% and 55% of their coral’s bleached respectively (Kayanne & Suzuki 2016). The event in 2016 started in the Mariana Islands, made its way to the Great Barrier Reef and finished on the Japanese coastlines.
While this had a major impact on reefs, this was not the first mass bleaching event that occurred throughout these areas. Between 1997-1998, these areas went through a strong El Niño event that ended up going through the Ryukyu Islands (Kayanne & Suzuki 2016). The National Oceanic Atmospheric Administration (NOAA) was able to confirm the coral bleaching via DHW, the highest value being the one in Ishigaki. The Japanese reef, along with the other reefs, help provide significant data to reinforce the notion that many of the major reefs around the world are under risk of serious ecological collapse.
Analyzing these three major reefs provide the data to support the idea that the reefs are in danger of collapsing. Figure 1 compares three major coral reef sites cover loss from 2000-2008 and 2009-present. The uptick in cover loss for each reef site supports the idea that sequential bleaching events destroy the coral cover.
The extent of these bleachings vary and as stated earlier, corals in different regions are more prone to bleachings after they have already occurred in the past. The major reefs being negatively impacted by bleachings show how serious this issue is in the marine ecosystem.
Using different studies, projections, and data it can show how important reefs are important to the entire biosphere. As stated before, consequences are still unfolding on how bleaching is continuing to negatively affect coral reefs all around the world. The chain reaction of coral reefs collapsing not only just impacts fish people and different species all around the world. To help minimize this problem, underdeveloped countries must be taught about that importance of coral reefs so they can protect and be more conscious about them. Another solution would be to create regulations for plastic waster and overfishing.
For example, putting fishing quotas for coastal countries would be an important step in salvaging the remainder of coral reefs. Maintaining anthropogenic causes is also crucial since there are natural factors that will continue to put stress on the reefs such as warming waters. The reefs are tolerant of some stress but when it becomes more than one factor, the reefs begin to breakdown. Proper education of the consequences of coral bleaching should be implemented globally to raise awareness of why this large scale problem.
There are conservation efforts being put into play to help spread awareness amongst the public and take care of current reefs.. There has been a group formed known as the Coral Restoration Foundation and their main objective is to protect reefs as well as create nursery’s to help maintain reefs that are growing.
The NOAA has also created their own conservation program known as the Coral Restoration Program which is similar to the Coral Restoration Foundation in that they both aim to maintain the health of current reefs. If projections hold up, the reefs will be completely gone around the year 2025 and it could collapse the entire marine ecosystem.