Like trees, reef-building corals depend on photosynthesis: they use the unlimited resources of solar energy and air to produce food. However, about half of all Scleractinia (azooxanthellate species) do not have symbiotic algae. Some azooxanthellate corals live on coral reefs, especially under overhangs or in caves, but with the exception of a few species that are both symbiotic and non-symbiotic, all zooxanthellate corals need light, and it is only these corals that build reefs. As a result, reefs are restricted to shallow sunlit waters. Azooxanthellate corals are not limited by light or by temperature, nor are they confined to shallow sunlit water; they live in the vast expanse of the ocean depths where there is less competition for space. These taxa cannot build reefs and must therefore live without food from photosynthesis: food can only come from the chance capture of passing plankton.
Corals growing in very shallow water such as reef flats have sunscreens (chemical agents in their tissues) to reduce the amount of light reaching their zooxanthellae, because if this is not controlled the zooxanthellae can produce toxic amounts of oxygen, the principal cause of mass bleaching.
Water depth, turbidity and latitude
Any factors which alter light in the marine environment will have a significant effect on calcification rates and reef development. Even so, the depth to which zooxanthellate corals can grow has been greatly understated in most literature because corals go below depths accessible to scuba divers. Nevertheless, only a few zooxanthellate corals live below 100 metres, even where the water is very clear and the substrate does not slope so steeply that it is shaded. Leptoseris commonly forms extensive beds to at least 160 metres in the Red Sea and Hawaii and there are several records of moderately diverse coral communities at depths of over 100 metres elsewhere.
Turbidity has a dominant role to play in controlling light levels in all except clear-water habitats. Where the water is not very clear, as is the case with most reefs near major land masses, coral diversity decreases sharply below about 50 metres. Where the water is particularly muddy, such as occurs along many coastal zones, the depth limit for any coral can be as little as 5 metres. Turbidity, especially that caused by fine clay particles which are easily re-suspended by wave action, has other effects on corals besides reducing light.
Temperature sets limits on the latitudinal spread of corals throughout the world. A different temperature constrains the spread of reefs. The difference between these two – constraints on corals and on reefs – has created havoc in palaeoclimatic reconstructions of past reef environments as well as studies of reef growth because it is so widely misinterpreted.
Low temperature limits for reef and coral growth
It has been known for decades that reefs do not form where the ocean temperature regularly goes below 18°C for intervals of weeks to months. Reef geologists concerned with the history of reefs refer to this well-established fact, yet in so-doing they often assume that lower temperatures kill corals. This is seldom the case.
Reef-building allows entire ecosystems to exist, a process that can only happen if the growth of macro-algae is held in check. This requires a lot of uninterrupted energy, which is why reef-building corals are dependent on symbiotic algae. This symbiosis requires exposure to sunlight, which is why reef building corals are restricted to shallow water. Around 18°C corals are able to produce calcium carbonate fast enough to fulfill their guild role as producers of building materials. This is achieved by creating three dimensional habitats where herbivores, especially fish, can control algae for them. At lower temperatures algae usually outgrow corals; however the corals themselves are not affected by temperatures lower than 18°C. This is best seen along the Ryukyu Islands of Japan where the southern islands have extensive reefs yet further north the sea temperature progressively decreases until it reaches the critical 18°C point. It is here that reef development fails. The corals, however, do not: nearly half of all coral species regularly tolerate prolonged exposure to 14°C. A few tolerate 12°C although seldom less (azooxanthellate corals excepted).
It was once believed that corals in cold high latitude regions have an ephemeral existence, neither reproducing nor growing like their tropical counterparts, however this is not so. As far as is known, corals reproduce normally in high latitudes. There are some taxonomic variations in this, the most important being in the massive colonies of Porites seldom do well in temperate waters, neither do some other taxa, notably fungiid (mushroom) corals. It is not known why.
High temperature limits to reef and coral growth
Low and high temperature limits do not mirror each other. This is because corals are highly dependent on upper temperature limits. Nevertheless, high temperature per se has little direct negative effect on corals. The warmer the water the faster most metabolic processes become and the faster calcification could become if it were not for its effect on zooxanthellae. Faster metabolic rates for zooxanthellae mean faster photosynthesis which, in turn can result in oxygen being produced at rates where it becomes toxic. Corals are forced to expel their increasingly poisonous zooxanthellae and ‘bleach’, a response to temperature and light acting in concert (see mass bleaching).
When it comes to high temperature limits, those of coral growth and reef growth are more-or-less the same for they have the same upper limit, linked to the upper limit of the ocean in which they occur. This link is an evolutionary one and appears to have always existed for there is no interval in geological time where high temperature has excluded reefs from equatorial regions. General points about the effects of high temperature on coral and reefs are:
- Although the Coral Triangle has an equatorial position it is unlikely that high temperature alone leads to this region’s high diversity. High diversity is not seen in other tropical regions and there are other reasons for it in the Coral Triangle including habitat diversity and the close interlinking of surface currents.
- Higher temperatures lead to higher coral growth rates up to the point where oxygen toxicity becomes an issue. This is best observed by measurement, for these higher rates are normally associated with weaker skeletons, as if the corals tissue outgrows its ability to form its own skeleton.
- There seems to be no good correlation between temperature and the rate of reef growth. Perhaps the higher rate of production of skeletal calcium carbonate is offset by the production of weaker skeletons which are more quickly eroded; however, there are too many facets to this issue to give it unambiguous support.
Substrate, turbulence and mechanical effects of turbidity
There is a strong ecological gradient across the Great Barrier Reef from a muddy inshore coastal zone to a clear-water offshore reef zone. A similar gradient is likely to occur in most other regions where reefs are situated well offshore. Substrate type and water clarity are always closely linked, especially when depth and turbulence are factored in. White calcareous sand, although typically coarse-grained, is light and therefore readily moved around by wave action, in which case it is capable of burying corals if suspended in sufficient quantity. However, it is clay from rivers that adversely affects corals, for not only does it attenuate light, it requires cleaning away – which corals do by using cilia on their tentacles as well as by several other methods – a costly activity in terms of metabolic energy.
Substrate is also of paramount importance to settling larvae, for these will not settle on sand of any sort, or on substrates that are coated with bacterial slime as commonly develops on reefs that have been degraded.
One major effect of turbulence on coral skeleton formation is that wave action produces dense skeletons. The corals on a high-energy reef front typically have extremely hard, dense skeletons, whereas those in a protected lagoon have light, brittle skeletons. This is partly because of the differences in species that occupy these habitats, yet even within the same species this effect is pronounced.
The term ‘water quality’ is commonly used in connection with the health of the marine environment. Water quality that is good for particular coral reefs or coral communities is assumed to have tolerable levels of sediments and nutrients and environmental contaminants. The term is therefore used in the context of the health of reefs and possible degradations of that health by human activities. Otherwise, it is used as a descriptor of ‘normal’ environmental conditions. In this context, normal does not mean permanent, for there are no baselines for coral reefs, only intervals of time when the environment appears not to change.
Salinity is an aspect of water quality that has not been adequately studied. Corals appear to be sufficiently tolerant of high salinity that lethal levels seldom, if ever, occur naturally. The opposite commonly applies to low salinities for these play a large role in creating areas where there is little or no coral or reef growth. Corals commonly grow in and around mangrove areas, either by being tolerant of low salinities or because these places have no freshwater intake other than directly from rain which remains on the ocean surface until removed by wind or tide.
There are other environmental controls on reef-building hidden in water chemistry that may not overtly limit reef distribution today but which may have been important in the geological past and are destined to become so in future. Oceans are normally so well buffered that chemical changes are infinitesimally slow, providing plenty of time for organisms to evolve adaptations to any alteration. However, sometimes the rate of change exceeds physical or biological thresholds and cannot be tolerated by any except the most specialised organisms. This can happen when large tracts of ocean become anoxic, hydrogen sulphide concentrations become toxic, pH alters beyond tolerable limits for calcification, or other contaminants make the water uninhabitable.