Rising Ocean Temperature
Corals are sensitive to changes in temperature, and an increase of just two degrees Celsius over a few weeks to months can be enough to cause catastrophic coral bleaching. When corals are stressed from the warmer water, they expel their photosynthetic zooxanthellae algae which are critical to their survival. Once the zooxanthellae algae leaves, the corals turn a ghostly white colour and it is difficult for them to survive. When corals are bleached, harmful algaes start taking over the corals. And with the loss of herbivorous fish, these algae can quickly out-compete and kill the coral.
Coastal development is one of the biggest threats to coral reefs, this includes construction of hotels, resorts, and homes along the coastline, as well inland mining and logging especially on smaller islands. This development often stirs up sediments which cover and kill corals. For inland activities, once trees are removed, sediments can easily enter the ocean after rainfall. During mining activities, sediments and harmful chemicals can enter the ocean through freshwater streams or runoff.
Overfishing and Destructive Fishing
Coral reefs support artisanal, national, and international fisheries, with hundreds of millions of people worldwide relying on fisheries for food and as a primary source of income. Last year, over 80 million tonnes of fish was collected from the ocean. Overfishing, especially the loss of algae-eating fish such as parrotfish, continues to be a threat to coral reefs. Along with overfishing, destructive fishing techniques like the use of dynamite can severely damage the complexity of a coral reef. This reduces the amount of habit and space for juvenile fish to hide.
Coral reefs and tropical fish are often the first reason people try scuba diving and snorkelling. The underwater world is full of strange and wonderful creatures which draw us closer to the action. But tourisms, including scuba diving, snorkelling, and boat activities can cause physical damage to coral reefs.
Increased carbon dioxide (CO2) in the atmosphere eventually makes its way into the ocean. As this CO2 is absorbed, the pH of the ocean drops, resulting in a more acidic environment. Normal seawater is between 7.5 and 8.5 pH and anything below 7 is considered acidic. Corals use calcium carbonate to build their solid skeletons, but in an acidic ocean, this skeleton becomes weaker and coral have more difficulty building their skeleton. While this is not such a concern for mature colonies, ocean acidification may be affecting the growth and development rates of baby corals, causing deformities in their skeletons.
Plastics and other marine debris are becoming an all too familiar sight. Plastics which enter the ocean are eaten by fish and other marine life, or worse, they break apart into millions of microscopic pieces that absorb chemicals from the water before they are eaten. Larger debris like nets, plastic bags and fabrics can also become tangled around corals causing the coral tissue to die. It is estimated that more than 250 million tons of plastics w ill be making their way into our ocean by 2025.