Assignment Task
What environmental factors most significantly influenced the extent of coral bleaching in the Great Barrier Reef and Ningaloo Reef during the 2016-2017 bleaching events?
Introduction
Coral bleaching has been documented since the 1870s, as reported by Glynn in 1988. Bleaching refers to the phenomenon of coral appearing pale due to external stress. This can occur due to various factors such as the loss of zooxanthellae, the loss of photosynthetic pigments (known as “photobleaching”), the degradation of the cells containing the zooxanthellae, or a combination of these factors (Glynn. P. W. 1998). The host corals exhibit a pale appearance when the aragonite limestone skeleton becomes visible through the translucent tissues of the polyps (Hoegh-Guldberg and Smith, 1989).
The mass coral bleaching events that struck the Great Barrier Reef (GBR) and Ningaloo Reef (NR) in 2016-2017 were primarily driven by anomalously high sea surface temperatures (SSTs), which were significantly influenced by the prevailing El Niño conditions (Hughes et al., 2017). Increasing solar irradiance aggravated coral bleaching in addition to thermal stress, especially in shallow reef locations where corals are more exposed to sunlight (Baker et al., 2017). Furthermore, it has been determined that variables compromising coral resilience to thermal stress include water quality concerns, such as increasing sedimentation and nutrient runoff, especially in the GBR (Wooldridge, 2016; Brodie et al., 2017). It is also impossible to overlook the importance of ocean currents, since their fluctuations affect the distribution of stressors and the temperature of the water, which may be a factor in the geographical variability of bleaching in both reef systems (Zhang et al., 2017).
Around 2,500 species of stony corals, which are distinguished by their hard calcium carbonate skeletons, make up the phylum Cnidaria , which is home to sessile corals (Veron, 2000). Because corals are sessile, they cling to the ocean floor indefinitely, “taking root” as other plants do (CRE, 2019) . Coral reefs, which are among the planet’s most prolific and diversified ecosystems, and are made up of these skeletons (CRE, 2019) . Zooxanthellae, microscopic algae that inhabit coral tissues, coexist with the corals in a symbiotic relationship. (NOAA. 2019) For the corals to meet their energy requirements, the zooxanthellae supply them with organic molecules made by photosynthesis (Muscatine & Porter, 1977). For their part, the algae receive the chemicals required for photosynthesis as well as a safe environment from the corals (Nama, S.et al, 2023). Coral reefs, which are found on less than 1% of the ocean floor, support about 25% of all marine species at some time in their life cycles. As such, the health of this symbiotic connection is critical to the wellness of coral reefs (Spalding et al., 2001).
Apart from their biodiversity, coral reefs are important for protecting coastlines, supporting human populations, especially in the tourism and fishing industries, and providing economic benefits (Moberg & Folke, 1999). Coral bleaching episodes and ocean acidification, which can have disastrous effects on coral health, are brought on by overfishing, pollution, and climate change, endangering these ecosystems (Hoegh-Guldberg, 1999).
Concerns over the global coral reefs’ steady deterioration are growing. The great vulnerability of reef-building corals to rising seawater temperatures has resulted in decreased mortality and limited growth. Other major anthropogenic risk factors include ocean acidification, dredging, overfishing, destructive fishing, and coastal development (De’ath, G., et al. 2012). These biological structures, which are mostly found in tropical ocean waters, are most frequently linked to the shallow, warm, transparent, and clear waters of the Indo-Pacific area, which includes the world’s largest coral reef system, the Great Barrier Reef, off the coast of Australia (Hughes et al., 2017).
Ningaloo Reef, located off the coast of Western Australia, is one of the longest near-shore reefs in the world (Lawson K, 2022). In contrast to numerous other coral reefs situated in deeper waters, Ningaloo Reef is notable for its close closeness to land; in certain regions, it is merely a few hundred meters away from the shore (Jones et al., 2018). This reef serves as a vital habitat for a variety of marine life, such as whale sharks, humpback whales, and sea turtles. It is home to approximately 200 kinds of hard coral and over 500 species of fish (Smith & Johnson, 2021). In addition, the United Nations Educational, Scientific and Cultural Organization ([UNESCO], 2011) has designated Ningaloo Reef as a World Heritage Site, a designation that highlights the reef’s remarkable biodiversity and conservation significance. According to Williams et al. (2015), the management strategies that prioritize reducing human influence and promoting sustainable tourism are responsible for the health of the reef.
A serious problem that is closely related to the Sustainable Development Goals (SDGs), especially SDGs 13 and 14 (Climate Action and Life Below Water), is coral bleaching. Since rising global temperatures are a major contributor to coral bleaching events, SDG 13 emphasizes the need for immediate action to counteract climate change and its effects (United Nations, 2015). The symbiotic relationship between corals and zooxanthellae is stressed by rising sea surface temperatures, which frequently results in the algae’s ejection and the corals’ subsequent bleaching (Hughes et al., 2017).
In addition, SDG 14 calls for the conservation and sustainable use of marine resources, including the oceans and seas, including the preservation of coral reefs (United Nations, 2015). Reducing marine pollution, which can speed up the bleaching process, and controlling fishing methods that harm coral structures are two actions under this aim that have a direct impact on coral bleaching (Rogers et al., 2018). SDG 14 also calls for addressing ocean acidification, another effect of climate change that is critical to maintaining healthy coral reefs since it interferes with corals’ capacity to calcify and form their skeletons (Hoegh-Guldberg et al., 2007).
The connections among these objectives emphasize the necessity of comprehensive strategies to reduce the causes of coral bleaching. For example, achieving SDG 13 and SDG 14’s goal of increasing coral reefs’ resilience to climate change calls for coordinated efforts to cut greenhouse gas emissions and manage regional stressors through sustainable coastal development and effective management of marine areas (United Nations, 2015).
Methods
A conceptual diagram of coral bleaching with various stressors (factors) that contribute to bleached corals. These factors lead to stressed corals, which may respond by expelling the symbiotic algae (zooxanthellae) living in their tissues. This expulsion results in the coral turning white, a state known as bleaching.
Increased Temperatures (T): Warm water can put stress on coral polyps, causing the symbiotic zooxanthellae that feed the corals through photosynthesis to flee (Hoegh-Guldberg, 1999).
Storm Runoff and Pollution (P): Excessive nutrients and pollutants carried by runoff can deteriorate water quality and put additional strain on coral systems (Fabricius, 2005).
High Solar Irradiance (I): Strong sunlight, particularly when paired with warm water, can aggravate the bleaching process and cause thermal stress (Lesser, 1996).
Bleaching Severity Equation ( BS ) = f ( T , P , I )
Where:
- BS is a measure of bleaching severity.
- f is a function that represents the complex relationship between the stress factors and the bleaching response.
- T is a variable representing increased temperatures.
- P is a variable representing the impact of storm runoff and pollution.
- I is a variable representing the effect of high solar irradiance.
Based on the Degree Heating Weeks (DHW) measure, which totals the thermal stress that corals endure over time, is one of the more widely used models to forecast coral bleaching (Smith, J. A et al. 2023).
DHW=1 /7 ∑n/(i=1 ) (SSTi-SSTmean)
Where:
- DHW is the Degree Heating Weeks, a measure of the accumulated heat stress that corals experience over time.
- SSTi is the Sea Surface Temperature for week i .
- SSTmean is the mean baseline temperature threshold for the coral reef, typically the maximum monthly mean temperature.
- n is the number of weeks over which the temperature anomaly persists.
Baird et al. (2018) found that the reduction in temperature inhibits the active component of the reaction centres, leading to a drop in carbon fixation. Carbon fixation in coral bleaching is the conversion of carbon dioxide (CO₂) from the water into organic compounds by the symbiotic algae (zooxanthellae) within coral tissues through photosynthesis (NOAA. 2019) This mechanism is crucial for the well-being of corals since it supplies necessary nutrients. During coral bleaching, the normal functioning of this system is interrupted as a result of stress factors such as increased water temperatures (Hughes et al., 2017). This interruption causes the expulsion of zooxanthellae and the decline in their ability to carry out photosynthesis, which greatly reduces the coral’s capacity to convert carbon and acquire energy (Hughes et al., 2017).
The phenomenon of coral bleaching centres around this important concept. The mechanism underlying this phenomenon is thought to entail the inactivation of the Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO, Lilley et al. (2010).
Zooxanthellae, the symbiotic algae found in coral tissues, rely on RuBisCO to carry out photosynthesis (Fabricius, K. E. 2005). Although RuBisCO does not directly cause coral bleaching, its carbon fixation activity plays a crucial role in the survival of algae and corals (Doo, S et al. 2012). Thermal stress diminishes the efficiency of photosynthesis, particularly the mechanism mediated by RuBisCO. Coral bleaching can occur due to the production of reactive oxygen species (ROS) and the breakdown of the coral-algae symbiotic relationship (Lesser, 1997).
Field research suggests that bleaching occurs in response to the climatological value unique to each reef site (Liu et al., 2014), indicating that corals possess a mechanism to adapt to local conditions. To express the inhibition of carbon fixation as a function of temperature, we recommend using the following equation for practical purposes:
α*Qox = (1 – exp(-(2 -T))) = (1 – exp(-2))
The temperature anomaly (∆T) is determined by subtracting the model bottom temperature from the climatological temperature at that depth, which varies both geographically and temporally (Ridgway and Dunn, 2003).
The Equation above was derived from a logical argument that suggests that bleaching stress starts when there is a temperature anomaly of 1◦C (the NOAA bleaching index considers a temperature anomaly of 1◦C above climatology, which would result in a reduction of ∗Qox in the Equation to 0.73) (Baird, M. E et al. 2012). Additionally, it suggests that sustained exposure to a temperature anomaly of 2◦C (equivalent to 16-degree heating weeks) over a period of 2 summer months causes the maximum level of stress (a∗Qox = 0).
Results
Unfortunately, the current condition of coral reefs does not have a straightforward solution (NOAA. 2019). The degree of damage inflicted upon the world’s coral reefs varies, and several reefs have undergone a process of recovery (NOAA. 2019). Nevertheless, the majority present a bleak perspective (Burke, L. and Wood, K. 2021). Approximately 50% of the world’s coral reefs are presumed to have undergone degradation as a result of climate change, pollution, and overfishing (Burke, L. and Wood, K. 2021). There has been a substantial decrease in hard coral cover in certain areas, accompanied by a noticeable alteration in the composition of coral communities, characterized by the disappearance of vulnerable coral species and a reduction in overall diversity (Burke, L. and Wood, K. 2021).
Discussion
Coral reef ecosystems are present in over 100 nations and territories. Despite occupying just 0.2% of the seafloor, they provide habitat for at least 25% of marine species and serve as the foundation for the safety, coastal defence, welfare, sustenance, and economic stability of hundreds of millions of individuals (Souter, D et al. 2021). Coral reefs are susceptible to human-induced pressures, which encompass both global threats like climate change-induced ocean acidification, and local threats such as coastal development, the introduction of nutrients and sediments from agriculture, pollution from the sea, overfishing, destructive fishing practices, and other activities (Souter, D et al. 2021).