Fagerstrom 1991 proposed that we consider the relative roles of the organisms involved in the accretionary process of reef building. He identified five basic "guilds" into which reef organisms can be placed:
The constructors provide the building blocks of the reef, whatever their ultimate fate; they can be overgrown and bound together by algae, forams and other members of the binder guild.
Bafflers are those organisms that affect accretion by interrupting the flow of water, thereby encouraging sedimentation. Destroyers include grazers and borers that break down the primary framework in various ways. Dwellers are usually passive inhabitants that contribute to the ecologic diversity of the reef but often have little to do with the actual accretionary process, except to help "fill in the spaces" within the reef interior.
Corals vary in their dependence upon photosynthesis. Those with larger polyps are well adapted to the active capture of plankton from the water column. Porter, 1976 Corals whose polyps have high surface areas relative to their volume are morphologically adapted to more efficient light reception. This relative dependence on photosynthesis plays perhaps the greatest role in determining reef zonation and depth-related patterns of calcification in modern coral reefs.
we prefer to use the terms zooxanthellate and non-zooxanthellate to distinguish between the two groups. The terms hermatypic (mound-building) and ahermatypic are often used, and have unfortunately become synonymous with zooxanthellate and non-zooxanthellate to many authors. This is a critical error, because many deep-water corals that are totally lacking in zooxanthellae, are quite capable of building mounds. Conversely, there are corals that are part of the mound-building process and do not contain endosymbionts. We, therefore, return the terms to their root origins. Any coral that has built reef-like topography (i.e. a bioherm) is considered as hermatypic. Regardless of its ability to build such a structure, it is histologically classified as zooxanthellate or non-zooxanthellate (alternately, azooxanthellate) based on the presence or absence of algal symbionts.
The chemical process is a complicated one in which the building blocks for calcium carbonate can be provided from several sources. Complete treatment of the calcification process, either in the open ocean or within individual organisms is outside the scope of this chapter, and the reader is referred to Bathurst for a general review on the subject. The most important processes in the marine system can be described by the formula:
Ca++ + 2HCO3- <=> CaCO3 + CO2 + H2O
The vigor with which aragonite will form is thus related to the abundance of free calcium (Ca++) and HCO3-. The addition of CO2 to water ultimately makes both of these available through the following process:
CO2 + H2O <=> H2CO3 <=> H+ + HCO3- + Ca++
Free H+, left over from the calcification process lowers the pH (i.e. makes the solution acidic). Conversely, dissolution of carbonate will increase pH. The ability of various organisms to regulate pH within their tissues, and drive the reaction toward the precipitation of aragonite, may be an important factor in biologically-mediated calcification.
Marine organisms secrete all three calcium carbonate polymorphs. Aragonite forms a crystal with a more open structure (orthorhombic) and, therefore, is more susceptible to chemical breakdown than calcite and magnesian calcite with stronger crystal bonds (hexagonal crystals). The only difference between the latter two is the inclusion of magnesium as an impurity within the crystal lattice (mg-calcite is defined as any calcite containing greater than 4 mole-percent magnesium).
Growth rates of individual colonies have been measured by weighing, volumetric determination, staining with Alizarin Red dye and direct measurement along inert pins placed in the coral for reference. The most commonly used technique has been X-radiography . Corals secrete skeletal material of varying density depending on temperature change, light level and intensity of reproduction. While the precise link between density banding and various physical-oceanographic conditions is still a point of debate, the regular pattern that is visible on X-rays provides a calendar upon which the development of an individual colony can be charted. In Montastrea annularis, the banding pattern has been shown to usually be annual, and is likely linked to seasonal changes in water temperature as well as reproductive patterns. During periods of warmer temperatures, the coral lays down a denser band that is reflected in the darkened band on an X-ray positive.
While some grazers (i.e. damselfish) selectively pluck turfs from the substrate, and actively " farm " the turfs within their territories, most grazers are less selective. Some have evolved specialized systems that allow them to ingest wholesale patches of turf along with sections of the supporting substrate. Parrotfish bite off pieces of substrate and pass them through a rasping structure, the pharyngeal mill, which produces a mixture of algae and sediment. The algae are digested, and the remainder is passed through the gut, mostly as sand. Urchins similarly rasp away substrate along with the algae they ingest. The sedimentary by-product is a roughly equal mixture of sand and mud. Ogden, 1977; Frydl & Stearn, 1978 While most of these grazers attack dead and algal-covered substrates, some are known to also feed on live coral. Along the Great Barrier Reef, the Crown-of-Thorns starfish (Acanthaster plancii) has been the focus of national concern each time its population reaches epidemic proportions and devastates large areas of live coral. In the Caribbean, coralliophyla (coral devoring snails) are becoming larger and more common and the number and size of fire worms is increasing. Both of these feed on coral.
In many localities, the dominance of Cliona is matched by boring bivalves. Most important among these is the genus Lithophaga and several boring chitons. Lithophaga can reach 30 cm in length and, in isolated instances, over 50 individuals per cubic meter can be found within a patch of reef. In addition to destroying substrate, the resulting borings significantly reduce the resistance of the overall structure to other forms of biological breakdown and physical damage.
Our best estimates of bioerosion come from controlled experiments in both the laboratory and the field. Based on these, grazers appear to be responsible for better than half of the bioerosion in Caribbean reefs. Ogden 1977 proposed a rate of 0.49 kg/m2-yr for a small reef system on the north side of St. Croix (Caribbean Sea). This was computed from the amount of sediment produced by an "average" fish (determined by divers collecting "samples" from numerous fishes) multiplied by the number of defecations per fish and the number of fish on the reef. At many locations, urchins produce larger amounts of sediment (up to 5 kg/m2-yr; avg ~ 2kg/m2-yr, equally split between sand and mud. The relative importance of sponge boring was determined for St. Croix by Moore and Shedd 1977 who measured rates averaging near 1.25 kg/m2-yr, with 90% of this being mud. MacGeachy, 1977 Rates exceeding 4 kg/m2-yr are certainly possible.