Environmental change is the utmost global threat to coral reef systems. In the previous 30 years, more than 50% of coral reefs have died, and in the coming century, approximately 90% might die worldwide (Gold & Palumbi, 2018). Scientific evidence indicates that the ocean and Earth's atmosphere are warming primarily due to oceanic gases. The frequency of coral bleaching events increases with the temperature rise. Besides that, carbon dioxide absorbed from the atmosphere into the ocean starts to reduce reef-building and reef-associated creatures' calcification levels. Environmental change will negatively affect coral reef systems through altered circulation patterns in the ocean, alterations in the strength and frequency of tropical storms, and sea-level rise. When all these impacts are combined, they intensely alter the ecosystem's functioning and the services and goods that coral reef systems give to persons worldwide. Their numerous reasons the reefs are dying; for this paper, I will focus on coral bleaching and the causes.
The coral reef environment is a varied and intricate assemblage of species that interrelate with one another and the physical atmosphere. The ecosystem is after tropical rainforest. It is 1% of the ocean but house approximately ¼ of all ocean species (Suggett & Smith, 20200. The bedrock of the reef is stony corals which are described by their hard skeleton. Colonies of stony corals are comprised of numerous existing polyps. The polyps can pull liquefied calcium from brine and solidify it into a calcium carbonate structure that performs as the skeletal backing. The expansion of stiff skeletal structures and polyps' growth build up a stable coral reef system with time. Polyps contain zooxanthellae, microscopic algae that exist with it in a symbiotic relationship and are essential for coral calcification (Good & Bahr, 2021). The coral polyps give the algae a home, and in exchange, the algae feed the polyps through photosynthesis. Coral reef systems also provide habitat for many varieties of marine life. They are ecologically linked to nearby mangrove, sea grass, and mudflat communities.
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Climate changes threaten the coral reef systems. Rising seas due to climate change might not threaten coral reefs if it rises slowly enough at a pace that coral growth can keep. But disastrous ice melting leading to the major rise in sea level might greatly affect the coral reef system (Suggett & Smith, 2020). The rise in sea level might also increase sedimentation due to shoreline erosion that could negatively distress coral reefs. Rising planet temperatures are projected to increase the severity and frequency of coral bleaching happenings (Robinson et al., 2019). The temperature rise also increases the outbreaks of coral diseases. Changes in type, frequency, intensity, and amount of precipitation are also occurring. Precipitation patterns show huge natural variability caused by El Niño Southern Oscillation (ENSO) and alterations in atmospheric movement arrays like North Atlantic Oscillation (Parada et al., 2017). A rise in water vapor during warmer climates causes extreme precipitation events and a rise in either flood during rains and drought when there are no rains. El Nino events combined with warming increases the rate of coral bleaching. The cycle of ENSCO and precipitation affect the timing and distribution of droughts and floods, particularly in the tropics leading to influence in coral reef systems.
Unprecedented ocean warming threatens the coral reef ecosystem. For the last three years, coral reefs have suffered from bleaching events due to the rise in global temperature caused by greenhouse emissions (Nielsen et al., 2018). According to the Paris Agreement, the only chances for the existence of global coral reefs are regulating global heat to less than 2 °C beyond pre-industrial intensities. Since pre-industrial periods, emissions from greenhouses have caused a rise in superficial global temperature of about 1°C (Parada et al., 2017). When there is a temperature change, corals eject the symbiotic algae in their tissues, responsible for their color. Sustaining over 1°C temperature in the ocean for many weeks leads to bleaching, the corals turn white (Parada et al., 2017). Corals die if they are bleached for a long time. Large numbers of corals often die due to coral bleaching effects.
Ocean acidification also endangers the existence of coral reefs. The ocean absorbs carbon dioxide from the atmosphere that reacts with seawater and forms carbonic acid. There is an increase in carbon dioxide in the atmosphere; currently, it is at the greatest level than it has been for about 420,000 years (Suggett & Smith, 2020). As a result, the ocean absorbs more carbon dioxide due to the rise in carbon emissions, thus creating more carbonic acid in the seawater. Increased acidification changes the water chemistry. Therefore, seawater can only hold less calcium carbonate. Coral reefs require calcium carbonate to create their skeletons and protective shells. The increasing acidity levels in the water affect coral reefs by making it hard for corals to form their skeletons (Mollica et al., 2019). Particularly, ocean acidification decreases the skeleton's density as it inhibits the thickening process leaving the corals more susceptible to breaking. As carbonate ions and pH drop in ambient water, the concentration of carbonate ions also declines in the calcifying space of the coral reefs (Suggett & Smith, 2020). Hence, coral reefs cannot produce enough aragonite to thicken their skeletons. Consequently, coral reef systems in waters with low pH form thinner skeletons vulnerable to damage from organisms and pounding waves.
Due to the then-emerging El Niño, the coral reef agency predicted a mass bleaching event in late 2014. The El Niño brought unusual warm seawater to the equatorial Pacific that stressed the coral reef system. In January 2015, there were reports of global-scale mass bleaching event between Indonesia and Africa. Over 50% of diverging coral reefs, a few as huge as 2 meters, had begun to indicate the onset of bleaching and colony sizes reduced by 17% (Smith et al., 2019). A study by Smith et al. (2019) showed that ten of thirteen studied colonies declined by 27% in the area. Mass bleaching continued leading to high mortality rates. The mass bleaching events could overwhelm the coral reef system's ability to recover if the events continue to rise in intensity and frequency.
In 2017, coral reef systems in the Persian / Arabian Gulf succumbed to the impacts of global mass bleaching event. The event started in 2015 and negatively impacted the coral reef system at the southern Gulf basin (Burt et al., 2019). The Gulf experienced a long mid-summer with winds rarely exceeding breeze conditions, decreasing heat loss and encouraging dramatic warming. Coral reefs across the Gulf spent almost two months beyond bleaching temperature and approximately two weeks beyond lethal temperatures. Consequently, 94.3% of coral reefs bleached and 73% were lost within a year (Burt et al., 2019). The capacity of recovery and prognosis of the future of the southern Gulf coral reef system is not optimistic due to the increasing mass bleaching and regional warming.
Environmental changes pause more repercussions to the coral communities. Environmental changes like ocean warming stress the coral reef system. Rise in sea temperatures leading to bleaching is one of the greatest threats to coral communities. When seawater is warm, the relationship between the symbiotic algae and the corals is broken down. The symbiosis is disturbed, and they expel the symbiotic algae (zooxanthellae) existing in the coral tissue (Gold & Palumbi, 2018). The coral reefs lose color, turns completely white, and becomes weak. Zooxanthellae decreases due to the disrupted symbiosis. However, warm water is not the only cause of coral bleaching. Coral reefs can endure a bleaching event, but they are more affected and are subject to mortality (Eakin et al., 2019). Often in conceded immune systems, some coral reefs can recover from the bleaching, but they die in most cases.
The negative impact on the coral reef system also affects the coral fisheries. Globally, fisheries on corals support approximately 6 million uninterrupted fishing works and revenues over $6 billion (Robinson et al., 2019). Catches by artisanal and subsistence fisheries constitute over half of the needed mineral intake and protein for above 400 million individuals in the poorest nations in South Asia and Africa (Gold & Palumbi, 2018). The death and decline of the coral reef greatly affect these human benefits linked to coral reef fisheries. The coral reef system provides habitat and support to approximately 1-9 million species (Good & Bahr, 2021). Changes in coral reefs affect the habitat of these species and threaten their existence. The coral reef is also essential to several species as food. The changes in coral reef mainly affect most species whose diet is restricted to coral reef system or organisms that exist on the reef. Some species that spend parts of their life cycle in specific coral are also affected by changes in the coral reef system. However, some coral-associated organisms can change as coral reefs decline.
Changes in the coral reef system also affect human activities. The coral reef safeguards the coastline from erosion and storms. Reduced levels of coral reef recovery and growth increase the rates of erosion on the coastline. The result is a decrease of accretion rates below zero on most coral reefs. Ocean acidification becomes the control variable as it affects calcification and stimulates impacts on reef erosion (Parada et al., 2017). It causes negative accretion at values of aragonite saturation on the subtopics and tropics. Changes in rates of reef accretion vary with the state of aragonite saturation. Therefore, coral reef’s net accretion levels determine whether there is a decrease or increase in coastal protection. Impacts on the coastline by erosion and storms affect the level of income connected to the coral reef (Gold & Palumbi, 2018). Consequently, there are fewer tourism activities that generate global income and job opportunities for the people working in the tourism sector. Thus, other than affecting the coral reef system, environmental changes also distress people.
In conclusion, the coral reef system has many benefits globally. Coral reefs provide recreation opportunities, offer local people jobs, and protect the coastlines from erosion and storms. More than half a billion person rely on coral reefs for protection, income, and food. Snorkeling, diving, and fishing near and on coral reefs add a lot of money to local businesses. The coral reef system is culturally significant to indigenous individuals worldwide. However, climate change raises seawater temperatures that lead to mass coral bleaching. Ocean acidification also threatens their existence as it inhibits the thickening process and reduces the density of corals. Finally, ocean warming changes the temperatures that bleach the coral reefs over time.
References
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