Overfishing is damiging to coral reefs, because on reefs around the world it has created something called the top-down effect. That is where fish at the top of the food chain have been so over-fished that the primary produces at the bottom of the food chain become over abundant. That is not ideal because the increase in primary producers puts more nutrients into the water, causing algae blooms that smother new coral polyps. Also, the depletion of large predatory fish in the Caribbean can cause large numbers of Hump Head parrot fish that can consume five tons of coral a year. Another problem is the tropical fish industry, and the way they catch fish to sell. One terrible method is using cyanide, which is a very toxic poison. They pour cyanide into the water to stun the fish and then break the coral up to get to the fish. Both the cyanide and the smashing of the coral to get to the fish kill the coral. The cyanide also kills 90% of the surrounding fish that do not get taken to sell. Another way is using dynamite to stun the fish. It sends a shock wave through the water, killing coral polyps and other animals.
Sewage dumping into water
The water quality of reefs has been seriously degraded due to soil erosion and fertilizers from agriculture, coral mining, dredging, drilling for oil, coastal development, oils and sewage from boats. The biggest factors that affect water quality are erosion of soil and fertilizers from agriculture, coastal development, and oils and sewage from boats. These three factors are detrimental to coral because they create an increase in nutrients and sediment in the water. The increase in nutrients causes large algae blooms that smother new coral polyps and makes it more difficult for corals to get light; it can also spread or start diseases in corals and invertebrates. Furthermore, sediment that runs into the ocean from coastal development smothers new coral polyps. An example of disease started by dumping of sewage is in the Caribbean. Long-spine sea urchins that are algae grazers had a 99% die-off due to a virus or bacteria that was brought over through the Panama Canal in the bilge water of a ship. Another example is in the Great Barrier Reef, where the increase of nutrients in the water allowed more Crown-of-Thorns sea stars to live past larval stage and eat large quantities of coral.
Dead and broken Elk Horn Coral
Lots of reefs are badly damaged or destroyed each year by storms, such as hurricanes. Although storms are natural, they have become more frequent and more severe because of global climate change and increased greenhouse gasses. We are polluting the water and putting the coral under an extreme amount of stress, not allowing them to be able to regrow and repopulate dead and damaged reefs that have been effected by storms.
Global Climate Change
Global climate change has impacted about half of the world coral reefs in the last 25 years. Problems that it has caused are increased acidity in oceans, more frequent and severe storms, and many coral bleaching events. Coral bleaching is when the rise in water temperature causes the corals symbiotic relationship with zooxanthellae to be no longer. What happens is the algae leaves the coral and turns it white. Without the zooanthellae the corals cannot get energy through photosynynthesis causing them to die. However corals can recover from bleaching events and storms but because of the pollution and destruction of natural reef food webs it has become much more difficult for corals to do so.
Coral calcification is when coral produce calcium carbonate, or CaCo3 to construct their skeleton. What is happening to the coral is that because there is an increase in co2 in the water the PH drops not allowing the aragonite or calcium carbonate to dissolve in the water for the coral to consume. This is bad because it does not allow the hard coral to grow. Hard coral are the building blocks of reefs and provide a base for other corals to grow on top of. Without these coral natural reefs would not be able to expand. Another big issue is that the rise in co2 is causing the construction rates of reefs to decrease below the natural rate of reef destruction from hurricanes and other things. That does not include human activities that are destroying reefs such as pollution.
Black Band Coral Disease
Coral disease is brought on by stress, which weakens and eventually kills the coral. There are many different kinds of coral disease such as Black band, Yellow band, White band, Red band, Neoplasm, and White pox . The disease Black band creates a ring or band around the coral with dead skeleton on one side and living coral flesh on the other. Disease can affect many different kinds of coral and because there are so many different kinds of disease the damage coral, it can do can be very widespread.