A. Occurrence of Encrusting Coralline Algae
The current page gives an overview of the occurrences of encrusting coralline algae. The next pages will show the ecological parameters influencing coralline algal distribution, such as substrate and hydrodynamic energy. Finally, you will find an overview of potential palecological implications.
Have a look at the section "growth form features" to read the definitions of growth forms mentioned below.
Coralline algae can form crusts on most substrates in most marine environments. Palecological parameters determine what happens to that crust. High hydrodynamic energy and suitable growth forms may fragment the crust and may lead to the formation of rhodoliths and/or maerl. Suitable conditions may also lead to the formation of continuously overgrowing crusts and finally to huge crustose frameworks.
Algae in Coral Reefs
Most studies suggest that encrusting coralline algae play a crucial role in the formation of coral reef. They are supposed to represent the main binding agent of coral reefs and some say that coral reefs cannot be formed without coralline algae.
In fact, coralline algae are one of the main coral reef constituents. However, recent studies of Macintyre suggest that coral reefs are predominantely bound by early marine carbonate cementation and that encrusting coralline algae only play a minor role.
Reference: Macintyre, 1997
Algal Reefs and Related Crustose Growth Forms
There are four types of Recent crustose coralline algal frameworks: (1) algal ridges, (2) algal cup reefs, (3) trottoir, (4) coralligéne. They can be differentiated by climate: tropical (1, 2) or temperate (3, 4); by the underlying substrate: rock bottom (1 - 4) or soft bottom (4); and by water depth: intertidal to shallowest subtidal (1 - 3), or shallower subtidal down to 160 m (4).
Algal ridges represent a late successional stage of coral reef development forming a constructional cap over coral as the reef approaches sea level.
Algal cup reefs, which are also called boilers or breakers, are intertidal cup-shaped algal bioherms, mostly arising from Pleistocene rocks. They are formed by intergrowing coralline algae and invertebrates, corals occur only sporadic.
Trottoirs are known from the Mediterranean and Northern Atlantic. They are intertidal bioherms, usually growing on steep rock shores, but can also form algal "micro-ridges".
Coralligéne de plateau is known from the Mediterranean and can develop from soft bottoms . It occurs in deeper shelf waters (20 to 160 m) and passes laterally into carbonate sands and gravels or terrigenous sands and muds.
References: Ginsburg & Schröder, 1973; Dean & Eggleston, 1975; Adey & Burke, 1976; Bosence, 1985a; Adey, 1986; Adey, 1978; Thornton et al., 1978; Bosence, 1983d; Bosence, 1984; Steneck et al., 1997.
Rhodoliths are the response of coralline algae to an unstable substrate. Due to the calcified cell walls they are able to withstand abrasion, for example caused by shifting sand.
Living rhodoliths are found from intratidal pools down to more then 200 m. The typical lower limit in the tropics is about 80 m, in colder climates and higher latitudes 20 - 40 m. Water energy is expected to be an important ecological factor for rhodolith distribution. On the one hand, it may influence the occurrence of herbivorous animals. On the other, it controls growth form and taxonomic successions.
One of the most surprising findings is that the lower part of such aggregates - i. e., those parts directly lying on the sand - can grow, which is proved by the dark red color.
Growth form and shape of rhodoliths are expected to be controlled predominantly by the frequency of turning. This is confirmed by the observation that an increasing frequency of rhodolith turning causes a flattening of the branches, which then join together laterally.
Taxonomic successions are, however, not primarily controlled by hydrodynamic energy. They may be the result of a change in rhodolith size (the larger the size, the higher their stability) rather than of environmental change. Another fact is, that coralline algal growth forms do not only depend on ecological factors. Different taxa can show different growth forms!
Several authors suggest that shape and growth form of nodules have little predictive value as indicators of ecological conditions.
References: Bosellini & Ginsburg, 1971; Adey & Macintyre, 1973; Bosence & Pedley, 1982; Bosence, 1984; Steneck, 1985; Adey, 1986; Braga & Martin, 1988; Bosence, 1991; Littler et al., 1991; Wehrmann et al., 1995.
In the turbid, high-energy waters of western Europe, maerl mostly occurs in protected bays at depth of 1 - 10 m. In lower-energy and clear waters of the Mediterranean they are more characteristic in depths down to 40 m. They are also reported from 180 m. While maerl sediments in protected areas are characterised by large, fragile, and branched rhodoliths, those of higher energetic (tidal currents) areas are composed of interlocking coralline branches forming megaripples.
In the northeastern Atlantic, 10 to 30 cm high maerl banks form a loose framework by interlocking branched coralline algae, covering areas up to 0.5 km2. Where currents are low, a mud and sand matrix is deposited between the algal branches. In the northeastern Atlantic, this framework grows in sheltered parts of bays at depths of 1 - 8 m; in the Mediterranean at depths of 30 - 40 m.
References: Bosence, 1983b; Adey, 1986; Scoffin, 1988; Freiwald et al., 1991; Freiwald, 1994.