Coral growth is only part of a complex equation that describes the history of modern reefs. Equally important are the subsequent grinding away of the reef by organisms seeking food or shelter, and the redistribution of the resulting sediment. The following section will examine each element of this formula, and finally integrate them into a single model. We will focus on corals because they contribute the greatest volume to the "carbonate budget" of modern reefs. We ignore other organisms not because they are unimportant, but rather to keep the discussion a bit simpler.
























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.

Coral growth - the constructive stage

The most diverse and abundant components of modern reefs are the stony corals. In effect they are both plant and animal. The individual polyp is an invertebrate of the phylum Cnidaria. The body is soft and tubular, with an oral opening surrounded by six (or multiples of six) tentacles. At night, these are extended to entrap plankton . The body wall consists of three layers, the innermost of which (gastrodermis) contains photosynthetic dinoflagellates called zooxanthellae . The details of the relationship between these symbionts and the coral polyp are not completely understood, but their importance in meeting the coral's metabolic demands is well established. Nearly 90% of the carbon fixed by zooxanthellae is released to the coral host primarily as glycerol. Nitrogen and phosphorous derived from captured plankton are shared between symbiont and host. The contribution made to the calcification process is of pivotal importance to this discussion. While this link is well known, the precise pathways along which it occurs remain the subject of considerable discussion.

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.

Terminology

Not all modern corals possess zooxanthellae. All deep-water corals, and some in shallow water rely solely on a regular supply of plankton to meet their energy demands. The two groups (those with and without algal symbionts) differ greatly in their ability to produce calcium carbonate and, therefore, to build 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.

Carbonate Production by Corals

The world ocean is saturated with the three major polymorphs of calcium carbonate (aragonite, calcite and magnesian-calcite). Yet, free precipitation in seawater is rare. As a result, biologically mediated carbonate production by corals is the most important contributor to the carbonate budget of modern reefs. Coral polyps absorb calcium ions from seawater and move them to the site of calcification, where they are deposited as aragonite. Minute crystals are formed in the outer cell layer of the polyp and pass to the skeleton where they act as nuclei for continued growth.

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).

Coral Growth Rate

The growth rate of modern corals varies among species, but in general, declines in deeper water. The highest growth rates are associated with shallow-water, branching corals (i.e. Acropora palmata, Acropora cervicornis), followed by "finger" corals (i.e. Porites porites), "head" corals (i.e. Montastrea annularis), and finally platy corals (i.e. Agaricia spp). Within species, growth rate responds to a variety of factors including temperature, Glynn & Stewart, 1973 sedimentation, Hubbard, et al., 1986 nutrient levels and light, Dustan, 1975 as discussed earlier.

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.

Bioerosion - the destructive stage

While corals and coralline algae are capable of producing massive structures over time, most other organisms living in and on the reef counter that process in their quest for food (grazers) or shelter (borers). This process was termed bioerosion by Neumann, 1966 and has subsequently been recognized as a major factor in both the biological and geological development of reefs. Hutchings, 1986; Kiene, 1988

Grazers and Predators

All dead surfaces of the reef are rapidly overgrown by a thin film of filamentous green algae. These form broad algal turfs that are a favorite diet of many fishes and urchins. Some algae bore tiny but ubiquitous holes into the reef surface. These endolithic algae can weaken the substrate, making it more susceptible to damage by grazers. As these organisms die, their borings are usually filled by mud-sized sediment (micrite). Repeated episodes of algal infestation, micritic infilling and cementation forms a thin and usually darkened rind around the edges of virtually all carbonate grains. This reworked "skin" is called a micrite envelope.

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.

Borers

Many organisms degrade the reef structure in the process of creating homes. A variety of worms, bivalves and sponges bore into the reef for shelter. Of these, sponges are generally the most pervasive. In the Caribbean, several species of the genus Cliona aggressively attack dead substrate and can, on occasion, totally destroy any evidence of original structure. Cliona excavates its gallery by chemically dissolving bits of carbonate away from the surrounding walls. The result is a silt-size chip that has a diagnostic shape when viewed in SEM. Moore and Shedd 1977 found a link between nutrient levels and boring by sponges that thrive on nutrients. Wilkinson 1987 proposed that nutrient availability drives the pattern of world-wide sponge abundance, and this likely applies to these burrowing species as well. Thus, the same high nutrient levels that might damage reef corals initially will encourage the subsequent infestation by sponges that break down the structure that has been built. Also, high nutrients encourage algae that attract grazers and can occupy available space faster than coral larvae.

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.

Rates of Bioerosion

While grazers occasionally leave scrapes and gouges that can be identified in the geologic record, it is impossible to determine the volume of material that has been removed. In contrast, infaunal bioeroders leave an excellent record of their presence. Each borer produces a gallery that is distinctive in both its size and its shape. Furthermore, the abundance of various gallery types can be directly converted into volumes of carbonate removed.

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.