Since the beginning of coral reef studies algal formations of various sizes and shapes, made of coralline algae and other organisms and generally associated with coral reefs, have been described under various names (algal ridges, crests, mounds, reefs, etc.). They were described as being developed upon the windward edge of reefs, but mention of algal mounds, crusts or rims developed on rock also exist in the early literature. A detailed description of algal rim structure was given by Tracey et al. (1948) on Bikini atoll.
Locations were mainly Pacific, but analogous formations were also described in the Atlantic, such as the “boilers” of Bermuda (Agassiz 1895). Later papers (Kempf and Laborel 1968; Gessner 1970; Glynn 1973; Adey and Burke 1976; Focke 1978; Jindrich 1983; Bosence 1984) dealt with algal and animal populations associated with Atlantic rims, and demonstrated their identity with Indo-Pacific rims, and their independance from coral reefs proper, since algal rims can develop directly upon littoral rock, or in regions where corals do not thrive.
Definition and Morphology
Algal rims are reef-like structures (bioherms or biostromes, following their size, development, and influence on local sedimentation processes) developing on the outer edge of reef-flats as well as on rocky coasts submitted to permanent, strong surf (trade winds). They exist both in Atlantic and Indo-Pacific regions but rim-like formations have also been described in the Mediterranean (Blanc and Molinier 1955).
They are built mainly by massive or encrusting coralline algae (mostly Porolithon), associated with Hydrocorals (Millepora), and Vermetid Gastropods (Dendropoma), with a few sturdy corals. Specific composition varies with exposure, slope, and nature of substrate (Focke 1978); coralline algae replacing vermetids when wave energy increases.
Rims are often built-up upon the inner parts of spur-and-groove systems by fusion of algal heads (Tracey et al. 1948). In extreme surf conditions, upward water movement leads to algal terraced pinnacles or blowholes.
Rim populations gradually mingle seaward with populations of the outer slope, rich in coralline algae down to several meters deep but enriched in corals (such as Atlantic Acropora palmata).
In weaker surf conditions, the rim gradually passes into a spur-and-groove reef edge or to thin rim-like formations along rocky coasts. Algal rims or equivalent structures may develop in tideless as well as in tidal conditions.
An often overlooked fact is that structures with algal rim or rimmed terrace morphology may be formed not by constructional evolution but by an erosive process. The Bermudian “boilers”may thus be divided into “erosive boilers”generated by erosion of an emerged stack of soft rock, the edge of which is protected by a thin organic cover (original description of a Bermudian boiler) and into “constructive boilers”obtained by the upward growth of a coral patch, covered by a surface layer of vermetids (Ginsburg and Schroeder 1973).
Relation with Past Sea Levels
Some components of algal rims (Dendropoma vermetids, some Lithophyllum) have a narrow repartition range around MSL and their presence in cores or on elevated coastlines is a precise indicator of ancient sea levels, with an approximation of less than 1 m (Jindrich 1983; Laborel 1986; Laborel et al. 1994). Conversely, corals such as Acropora palmata, have less precise indicators (Lighty et al. 1982) unless a true rim may be recognized in the core.
Further study of algal rims is nevertheless needed to obtain better biological indications of past sea level stands.
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