User:Winde068/Ocean acidification in the Great Barrier Reef

Ocean acidification threatens the Great Barrier Reef by reducing the viability and strength of coral reefs. The Great Barrier Reef, considered one of the seven natural wonders of the world and a biodiversity hotspot, is a 1,429 mile stretch of coral reef located off the east coast of Australia in the Coral Sea. Similar to other coral reefs, it is experiencing degradation due to ocean acidification. Ocean acidification results from a rise in atmospheric carbon dioxide, which is taken up by the ocean. As the oceans take up more carbon dioxide, it can increase sea surface temperature, decrease calcium carbonate, and lower the pH of the ocean. As humans have slowly been increasing carbon dioxide emissions via the burning of fossil fuels, the oceans are forced to absorb more carbon dioxide which results in ocean acidification.

Calcifying organisms are under risk, due to the resulting lack of calcium carbonate in the water and the decreasing pH. A decrease in pH occurs when ocean water combines with carbon dioxide to make carbonic acid and hydrogen ions, effectively acidifying the water. This process blocks many calcifying organisms from utilizing the carbonate ion in carbonic acid to make calcium carbonate (aragonite). Calcium carbonate is what the calcifying organisms use to make their shells and skeletons. Carbonate ions bond to the excess hydrogen ions in acidified waters, so the calcifying organisms slowly lose the ability to make their shells. The organisms work harder to rebuild their shells, which leads to less energy and time being allocated for reproduction.

This decreased health of coral reefs, particularly the Great Barrier Reef, can result in reduced biodiversity. Organisms can become stressed due to ocean acidification and the disappearance of healthy coral reefs, such as the Great Barrier Reef, is a loss of habitat for many taxa whose livelihood depends on healthy coral reef habitats.

Biodiversity
The Great Barrier Reef is a biodiversity hotspot, blossoming with over 5480 different species, including species of coral, fish, sponges, sea snakes, marine turtles, mollusks, echinoderms, and marine algae. However, the Great Barrier Reef is threatened by ocean acidification which causes an increase in pH and reduces levels of calcium carbonate. Ocean acidification can indirectly affect any organism by increasing stress, reducing photosynthesis and reproduction, or making organisms more vulnerable to disease. Some elasmobranchs are vulnerable to the effects of ocean acidification with clear effects on body condition, growth, and behavior being shown in previous studies in benthic species. Rare and endemic species, such as the porcupine ray, are at high risk as well. A predator to coral reefs in the Great Barrier Reef, the Crown of Thorns sea star, has experienced a similar death rate to the coral on which it feeds. Any increase in nutrients, possibly from river run-off, can positively affect the Crown of Thorns and lead to further destruction of the coral. Increasing temperature and pH is also affecting the behavior and fitness of the common coral trout, a very important fish in sustaining the health of coral reefs.

Coralline algae hold together some coral reefs and are present in multiple ecosystems. As ocean acidification intensifies, however, coralline algae calcifications rates reduce which could damage the viability and structural integrity of coral reefs. Additionally, as coral reefs become stressed due to increasing temperatures and pH, they expel algae which are in symbiotic relationships with the coral. Without the algae to provide nutrients for the coral, they lose their color and become bleached.

Not only can ocean acidification affect habitat and development, but it can also affect how organisms view predators and conspecifics. Studies on the effects of ocean acidification have not been performed on long enough time scales to see if organisms can adapt to these conditions. However, ocean acidification is predicted to occur at a rate that evolution cannot match.

Mass bleaching events of the last five years
According to the Australian Government Great Barrier Reef Marine Park Authority, 60% of the Great Barrier Reef (GBR) had bleached between 2019 and 2020 (Link 1). This was one of several mass coral bleaching events that have occurred with increasing frequency in the last two decades (link 2). In the last five years, the GBR has undergone three mass bleaching events, one that was included in a global coral bleaching event.

The mass coral bleaching event of 2016 did not just affect the GBR, as widespread bleaching was seen in the Pacific, Indian, and Atlantic ocean basins (link 3). NOAA declared it the third global coral bleaching event in 2015. Later on, another bleaching event was seen in 2017, although it was not as severe as the 2016 bleaching. The 2016 bleaching was the most widespread and severe bleaching event the GBR had ever experienced. This bleaching was dominant in the northern and central regions of the reef. In 2017, the central region of the reef was significantly affected, leaving two-thirds of the reef affected between 2016 and 2017.

The final massive GBR bleaching event of the last five years occurred in 2020, which is when it was found that 60% of the GBR had moderate to severe bleaching. While this event was not as severe as the 2016 event, it did have a more substantial impact on the reef than the 2017 event. Unfortunately, the southern portion of the reef, which remained unaffected by the two prior bleachings, was damaged in the 2020 event. The southern region had experienced extensive bleaching for the first time, as well as moderate bleaching occurrences being observed in the central and northern regions of the reef (link 4). Despite 40% of the reef having no or minimal bleaching, 35% was moderately bleached and 25% was severely bleached (link 1 again).

Evaluating reef health and current status/future hope
In order to evaluate the health of the reef, a multitude of variables are commonly observed. The most common factor to observe is coral appearance. If the coral appears to be colorful and shows no signs of disease or bleaching (white color), it is likely healthy. Whereas if it is white, translucent, or showing indications that it is bleaching, it is not healthy. To differentiate between disease and bleaching, the transition from color to white can be analyzed. A coral’s color fading from dark to light/white indicates bleaching, whereas a clear cut line between the colorful and white sections indicates signs of a disease (link 5). Another indicator of coral reef health is fish abundance and a large diversity of fishes (link 6). For example, the presence of sharks is a substantial source of indication that a coral ecosystem is healthy. This is because sharks aid in maintenance of food webs and nutrient cycling, as well as helping to control invasive species and the spread of disease (link 7).

While only 40% of the reef was intact and healthy in 2020, there is optimism for a prosperous and healthy future for the Great Barrier Reef. Despite a multitude of bleaching events and lethal disturbances to the GBR over the last two decades, as of 2021 coral cover is rising in the northern, central, and southern regions of the reef (link 8)! Since coral bleaching has been linked to climate change, addressing anthropogenic impacts could allow for recovery of bleached corals. Decreasing water pollution and implementing controls to eliminate overfishing, as well as reducing emissions that drive temperature increases, will provide significant opportunities for coral recovery (link 9). Furthermore, increasing the global area of ocean waters that are considered MPAs (marine protected areas) and increasing human regulations and restrictions in these areas, will help preserve coral health. If anthropogenic activity is left unchecked, it is predicted that by 2050 almost all coral reefs will be threatened (link 10). Furthermore, corals that are dead (different from being bleached) are not able to recover. While recovery is possible, it has its limits, introducing a sense of urgency in solving this issue.