Calcareous algae thrive, but corals are uncommon, on parts of reefs exposed to extreme wave action. Where there is some protection and where the water is clear, Indo-Pacific reef flats are usually dominated by Acropora. Unconsolidated rubble accumulations occur primarily where conditions for coralline algae growth are poor – where wave action is weak, the water turbid, or the temperature frequently falls below 18°C. Conditions for coral growth, however, may be good in such places.
Substrate availability, sedimentary regimes, bathymetry, tidal regimes, turbulence, water quality, nutrients and biotic factors all affect reef development by controlling coral growth. These operate on local scales in shallow tropical seas primarily by affecting substrate conditions and light availability. On larger scales of space and time, sea levels and temperature are the most important limiting environmental parameters.
Because coral reefs built by zooxanthellate corals can exist only in very shallow water, they are continually affected sea-level changes due to the land or sea rising or falling. Darwin recognised this long before the phenomenon of changing sea levels per se was actually discovered, yet even now it is not generally appreciated that coral reefs have not always looked like they do today. In times past, sea-level changes have been so rapid and so frequent that what are reefs today would have alternated between being exposed limestone islands and submerged limestone pinnacles. The strongly zoned reefs of today are artifacts of an unusually long period of relatively stable sea level.
Reef-building corals and the reefs that they build have similar, tropical, distributions. This creates an interesting dilemma: which comes first, the coral or the reef? There have been many theories, all assuming some effect of ocean temperature. There is no correlation between coral diversity and reef formation (see here), however there is a clear correlation between ocean temperature and zooxanthellate coral distribution and also between temperature and formation of highly consolidated reefs. Reefs only occur where sea temperature does not fall below 18°C for extended periods of time, however approximately half of all coral species occur where temperatures regularly fall to 14°C. The likely solution to the dilemma is that temperature controls the formation of reefs through ecological processes. Reef building requires carbonate production and that requires coral-dominated ecosystems. In the tropics, coral ecosystems exist because they are able to out-compete algae-dominated ecosystems, but this is not the case in higher latitudes. Thus, the highest latitude coral reefs in the world all have mixtures of coral and algal communities which compete, and the corals struggle to get enough light and space to survive.
If corals grow in sufficient quantity and the rate of both coral calcification and algal cementation exceeds that of erosion, the resulting accumulation of calcium carbonate can form limestone reefs.
The success of the process depends on speed, which is why reef-building corals depend on symbiotic zooxanthellae to harness the energy of the sun to power the process. Why do these organisms invest so much energy into building something that is dead? One aspect is that skeletons are needed to form colonies and colonies are needed to build large wave-resistant reefs.
The two evolutionary innovations of colony formation and algal symbiosis clearly go together and have very likely evolved together. The importance of this is demonstrated by the fact that most living Cnidaria that reap the advantages of reef-building are both colonial and symbiotic. The two major groups of extinct reef-building corals (rugose and tabulate corals) are also colonial, however it is not known if they were also symbiotic.
Corals are the most prominent reef-builders today because, due to symbiotic zooxanthellae, they can harness the energy of the sun to make building material sufficiently quickly that the rate of reef accretion outstrips that of erosion. However, not all zooxanthellate corals contribute to reef-building; perhaps half of all species do not grow in environments suitable for reef growth, especially where the water is too turbid or too cold or where there is limited hard substrate and others have skeletons which are fragile and easily removed by wave action.
Symbiosis Reefs of today are an ancient phenomenon without parallel in Nature – a specific result of a symbiotic relationship between plants and animals. Just what sort of plants and animals is not critical, but the relationship between them is. Symbiosis allows the limitless resources of sunlight and seawater carbonates to be harnessed and used to construct an ecosystem. In effect, symbiosis allows reefs to be built by animals because it gives them the energy-generating capacity of plants. What the animal gives the plant is a medium in which to live that is stable yet exposed to sunlight. What the plant gives the animal is an enhanced capacity to remove metabolic waste, an enhanced capacity to calcify, a direct nutrient source and a capacity to concentrate and re-cycle limited nutrients including nitrates and phosphates. Symbiosis thus allows corals to exist in an almost nutrient-free environment, including the nutrient deserts of the emptiest oceans.
Clearly, the evolutionary advantage of symbiosis is very great, but the evolutionary cost is also great. Zooxanthellate corals are constrained to live near the ocean surface, the most hostile of marine environments both physically and biologically. Most importantly, symbiosis constrains corals to live in places where they must compete with macroalgae. Coral-algae symbiosis is therefore ultimately responsible for the geographic constraints of reefs as well as their construction.
Speed of coral growth is critical to both coral survival and reef-building once basic environmental needs are satisfied. This is because corals and coralline algae must build reefs fast enough to outstrip the rate of erosion.
Rates of reef growth (accretion) can be measured directly from cores taken from reefs or by a wide range of indirect measures of carbonate production. Normally, the maximum rate of reef growth is about 0.6 metres per century although rates in optimal conditions may reach three times this. These optimal conditions only occur where the water is shallow and clear and currents are strong, the probable reason why continuous areas of reefs occur only where the tidal range is great and the ocean floor is shallow. These environments provide high light levels combined with continuous flushing and nutrient transport.
Rates of reef erosion determines the appearance and existence of reefs every bit as much as rates of growth because accretion must out-strip erosion for any reef to form and survive. There are no reliable estimates of rates of reef erosion because they are too slow to be directly measurable, however best estimates suggest that around 90% of all calcium carbonate produced by coral calcification is removed by erosion. There are three main mechanisms of reef erosion: physical erosion, chemical erosion (including rainwater dissolution), and bioerosion.
Physical erosion leads to the formation of grooves seen in most reefs from the action of waves moving rubble back and forth. This commonly develops into the ‘spur and groove’ structures common along reef fronts exposed to strong wave action.
Chemical erosion occurs primarily in shallow reefs exposed to rainwater, which is acidic. Rainwater dissolution of exposed reefs commonly results in ‘rill weathering’, a process that creates interlocking knife-like edges on the surface of limestone outcrops. Depending on the chemical composition of the limestone, some aerially exposed reefs endure this process much longer than others.
Bioerosion is a greatly under-recognised process, yet can be very active in shallow water where it not only erodes the limestone surface but prevents the growth of newly recruited corals and coralline algae on available substrates. Over thousands of years, the actions of many types of bioeroders such as urchins would probably be capable of keeping pace with slow sea-level falls, leaving no reef exposed above high tide level. There are many studies of the rate at which different organisms (such as sponges, urchins, limpets, chitons and parrot fish) ingest limestone (typically up to 18cm3 per animal per year for intertidal invertebrates such as sea urchins), however these cannot be reliably translated into rates at which these animals might plane-off whole reef surfaces.
The notion that corals build reefs in order to have shallow substrates where they can out-compete macro-algae correctly implies that corals are the key structural components of reefs. However, herbivorous fish that eat these algae are the key functional components. If fish were removed from most reefs for just a few years, the corals would be smothered by algae. Both animal groups ultimately dependent on the presence of healthy reef ecosystems.