Describe the conditions which provide the best environment for the growth of coral and

explain how coral may be threatened by changes to those conditions. [10]

Corals are colonial marine invertebrates. They typically form compact colonies of many
identical individual polyps. Coral species include the important reef builders that inhabit
tropical oceans and secrete calcium carbonate to form a hard skeleton. A few of the
conditions required for coral to grow well are : temperature, depth, light, salinity, sediment,
wave action and exposure.

No reefs develop where the mean annual temperature of the water is below 18°C. The best
conditions for reef development occur in water temperatures between 23-25°C. Above 27°C
causes problems for the health of the reef. Rising (or even falling) water temperatures can
stress coral polyps, causing them to lose algae (or zooxanthellae) that live in the polyps’
tissues. This results in “coral bleaching,” so called because the algae give coral their colour
and when the algae “jump ship,” the coral turns completely white. The algae also give coral
polyps the food they need to survive. While a bleached coral is not dead, and corals can
survive bleaching events, they are under greater stress, are less resistant to other threats such
as disease, and are thus subject to mortality.

Most reefs grow in depths of 25m or less on the margins of continents or islands. Deeper
reefs are currently considered to be less susceptible to local and global disturbances, such as
overfishing, pollution and climate change, compared to shallow reefs. Thus, they may act as
refuges and sources of propagules for shallow threatened reefs. The most well-known corals
are shallow water species that host photosynthetic algae to obtain energy. However, some
species of coral are actually found at great depths, and don't require sunlight to survive.
These deep water corals don't host the algae symbiont, and instead catch food in the water
column. They're generally found at depths between 160 and 1500 feet, but have even been
found at 20,000 feet. Much is still unknown about these corals, and new deep sea reefs
continue to be discovered as we map the ocean floor.

As light is needed for photosynthesis for the tiny algae (called zooxanthellae) which live in
corals and in return for space to live, supply the corals with 98% of their total food
requirements, coral reef growth is restricted to shallow waters. The coral polyps (animals)
provide the algae (plants) a home, and in exchange the algae provide the polyps with food
they generate through photosynthesis. Because photosynthesis requires sunlight, most reef-
building corals live in clear, shallow waters that are penetrated by sunlight. When polluted
runoff and wastewater enter the marine environment, they carry chemicals, nutrients, and
bacteria that can be harmful to coral reefs and spur the growth of algae that competes with
corals for space. Most reef-building corals depend upon zooxanthellae (tiny little algae that
grow inside of them) to photosynthesize and provide food. If the water becomes cloudy or
murky, or if corals are covered in sediment, the sunlight can’t get to the zooxanthellae and
the corals lose that important food source.

Corals are marine creatures which are intolerant of water which is less than 32-42psu, but
surprisingly they will tolerate highly saline conditions such as those in the Red Sea or Persian
Gulf. Globally, corals occur in a salinity range between 32 and 40 PSU (practical salinity
units), although coral reefs also thrive in exceptionally high and low salinity conditions. More
important than average salinity concentrations, however, are sudden decreases in salinity
due to high freshwater input by rivers or torrential rains. Decreased salinity is seen as a
second factor for the disappearance of massive coral reefs in front of large river mouths.

In conclusion, if coral reefs are not in the correct environment where there is sufficient
sunlight, salinity depth and temperature they are subject to bleaching where there chances
of living and recovering are very slim.