Corals are a kind of calcareous rock, chiefly made of the skeletons of minute sea organisms called polyps. Coral reefs and atolls are formed due to the accumulation and compaction of the skeletons of these lime secreting organisms. They form one of the most diverse and productive marine ecosystems on earth housing tens of thousands of marine species.
- Sunlight: Corals need to grow in shallow water where sunlight can reach them. Corals depend on the zooxanthellae (algae) that grow inside of them for oxygen and other things and since these algae need sunlight to survive, corals also need sunlight to survive. Corals rarely develop in water deeper than 50 meters.
- Clear water: Corals need clear water that lets sunlight through. They don’t thrive well when the water is opaque. Sediment and plankton can cloud water, which decreases the amount of sunlight that reaches the zooxanthellae.
- Warm water temperature: Reef-building corals require warm water conditions to survive. Different corals living in different regions can withstand various temperature fluctuations. However, corals generally live in water temperatures of 20–32° C.
- Clean water: Corals are sensitive to pollution and sediments. Sediment can create cloudy water and be deposited on corals, blocking out the sun and harming the polyps. Wastewater discharged into the ocean near the reef can contain too many nutrients that cause seaweeds to overgrow the reef.
- Saltwater: Corals need saltwater to survive and require a certain balance in the ratio of salt to water. This is why corals don’t live in areas where rivers drain fresh water into the ocean i.e. the estuaries.
- Abundant Plankton: Adequate supply of oxygen and microscopic marine food, called phytoplankton, is essential for growth. As the plankton is more abundant on the seaward side, corals grow rapidly on the seaward side.
Impact of global warming:
- Coral bleaching: A warming ocean causes thermal stress that contributes to coral bleaching and infectious disease. Photosynthesis pathways in zooxanthellae are also impaired leading to coral bleaching.
- Smothering of corals: Sea level rise may lead to increases in sedimentation for reefs located near land-based sources of sediment. Sedimentation runoff can lead to the smothering of coral.
- Destruction: Changes in storm patterns leads to stronger and more frequent storms that can cause the destruction of coral reefs.
- Impact coral growth: Changes in precipitation, increased runoff of freshwater, sediment, and land-based pollutants contribute to algal blooms and cause murky water conditions that reduce light and thus inhibit coral growth.
- Hampers dispersal: Altered ocean currents leads to changes in connectivity and temperature regimes that contribute to lack of food for corals and hampers dispersal of coral larvae.
- Death of corals: Ocean acidification (a result of increased CO2) causes a reduction in pH levels. Itleads to dissolution of calcium carbonates which form the skeletons of coral.
Coral reefs harbour the highest biodiversity of any ecosystem globally and directly support over 500 million people worldwide. They are among the most threatened ecosystems on earth, largely due to unprecedented global warming and climate changes. Limiting global average temperature to well below 2°C above pre-industrial levels in line with the Paris Agreement provides the chance for the survival of coral reefs globally.