Nowadays the worldwide threat to coral reefs is so serious, that unless quick action is taken less than 70% will remain in 20-30 years time. The deterioration of reefs is often a combination of factors of which we will explain the main important ones here.
OVERFISHING AND DESTRUCTIVE FISHING METHODS
A coral reef system is one of the most fragile ecosystems in the world. Like any ecosystem, the species living in it interact with each other and are dependent on each other in different ways. The absence or reduction of one species brings instability to the entire ecosystem.
For instance some herbivorous fish eat algae and clean the corals. If these fish are absent, corals are overgrown by the algae. The result is that the coral can not feed anymore and dies.
Large predators keep the fish populations in balance and healthy. These species are often quite popular on our menu unfortunately. Large fish generate a lot of offspring. Their absence means fish populations dwindle over time, also affecting the corals.
Many fish techniques can be destructive and indiscriminate. We often find big fishing nets trapped and left behind on coral reefs. Blast fishing is one of the most destructive techniques killing all life within a certain radius and destroying the hard coral. Most fishing methods give a lot of bycatch, indiscriminately taking out and killing important reef animals that are not used for consumption.
Stress factors to corals can lead to coral bleaching. This is a process where the symbiotic algae (Zooxanthellae) that give the colour to the corals are released excessively by the coral polyps in an effort to control the amount it contains.
The Zooxanthellae get damaged by increasing temperatures. The polyp increases the amount of expelled damaged Zooxanthellae, and also the intake. If the stressful conditions prevail, accumulation of the damaged symbiotic algae may not maintain the expulsion, which will gradually accumulate in coral tissues. The loss of zooxanthellae and the accumulation of damaged cells results in coral bleaching.
The increasing temperatures of ocean water due to climate change are believed to be one of the major factors of coral bleaching. The already stressed reefs (overfishing, pollution, tourism, etc.) are extra vulnerable for this process. Coral bleaching is occurring worldwide, entire reef ecosystems have already died for this reason.
Coral reefs need clean, clear water to survive. When pollutants enter the water, they smother coral reefs, speed the growth of damaging algae, and lower water quality. Pollution can also make corals more susceptible to disease, impede coral growth and reproduction, and cause changes in food structures on the reef.
Mosts sunscreens are considered dangerous for corals and other marine life. The chemicals (especially oxybenzone, octinoxate and methyl paraben) in most sunscreens are having bleaching effects on the corals, amongst others. Both bathing guests and the sewage are sources of this chemical input to the oceans. Mineral sunblocks including zinc oxide and titanium dioxide that are ‘non-nano’ in size are considered less harmful than chemical sunscreens (ps formulations below 100 nm (nm=nanometers) are nano, thus also harmful). Given the fact that chemical sunscreens are not healthy for humans either, the best option is to wear sun protective clothing.
Absorption of carbon dioxide from the atmosphere causes the oceans pH to decrease or acidify. Calcium carbonate minerals are the building blocks for the skeletons of corals and shells of many marine organisms. Continued ocean acidification is causing many parts of the ocean to become undersaturated with these minerals, which is likely to affect the ability of organisms to produce and maintain their shells.
This acidification also causes algae to die off. These algae are needed to aid in the building of coral reefs.
Terrestrial runoff by e.g. building projects can pose a severe threat to the health of surrounding coral reefs. Fine terrigenous sediment entering the nearshore ocean during runoff events affects corals in two ways: suspended in seawater, the sediment drastically reduces the amount of light reaching coral reefs and other shallow benthic systems. And secondly, as the sediment settles, it can bury corals or cause them to expend a large amount of energy keeping their surfaces clean. Although a clear link exists between high sediment loads and coral-reef degradation, the mechanisms responsible for coral decline are not well quantified.