B. Ecological Parameters Influencing the
Distribution of Present-Day Coralline Algal 

Salinity

Encrusting coralline algae are predominantly restricted to normal marine waters. Only a few species can live in waters with low or high salinity, respectively.

The single species are mostly stenohaline, which means that they show low tolerance for variation in salinity

Light / Depth

Due to their red pigments, coralline algae use the blue light spectrum for photosynthesis. Therefore, they can live in deeper waters than most other algae and are abundant in shallow water cryptic environments. 

Coralline algae are known from intertidal pools down to 258 m water depths! But the typical limits in tropical environments are ca. 80 m and in colder environments ca. 20 - 40 m.

Many species can occur in a wide depth range, but several species show restricted depth (i. e., light) distributions.

Obviously, it is not the depth as such that influences the distribution of algae. Those species which prefer low-light conditions can occur in greater depths - but they can also occur in cryptic shallow-water environments, such as reef caves. Finally, the water turbidity influences the penetration of light and therefore influences the depth distribution of algae.

Read more about photosynthesis at the Smithonian homepage.

Climate / Temperature

Coralline algae occur in a wide temperature range. They are known from tropical to arctic/antarctic environments and tolerate temperatures down to >0!

Some trends seem to be recognizable. For example, the genus Phymatolithon is most prominent in non-tropical environments and some say that Sporolithon is more typical for warm waters (see the list of taxa for informations on these genera).

Hydrodynamic Energy

One of the great competitional advances of coralline algae is that they can occur in very low to extremely high energetic environments.

They cannot occur in stagnant waters, because a certain hydrodynamic energy is required for the delivery of nutrients and the removal of photosynthetic products.

Due to calcified cell walls and heavy cementation to hard substrate, they can live in extremly high energetic environments, such as reef crests. The great advantage is that the high energy keeps away other macroalgae and grazing animals, such as gastropods.

Hydrodynamic energy is also responsible for the turning of rhodoliths (but: see below), which is crucial for the growth into all directions, as well as for the formation of maerl. However, if the energy is too high, the algal thalli are obviously fragmented and destroyed.

Studies on relations between water motion and rhodolith movement are studied at the Moss Landing Phycology Lab.

Biotic Interactions

Several studies showed that turning of rhodoliths is not only caused by hydrodynamic energy, but frequently by the activities of food searching animals, such as fishes.

Another aspect of biotic interactions is space competition. Fleshy macroalgae, such as brown algae, can grow much faster than calcified coralline algae and therefore should have a particular advantage with respect to space competition. However, coralline algae can deal with this problem in several ways. On the one hand, corallines can occur in high energetic environments which are not suitable for fleshy macroalgae (see above); on the other, the grazing activities of animals removes the fleshy algae, but hardly affect the coralline algae, which are protected from grazing by calcified cell walls. Moreover, corallines can live under low light conditions. This implies that they may receive sufficient light for photosynthesis even when they are shaded by overgrowing fleshy algae.

Read more about space competition within corallines and about grazing effects at the homepage of Derek Keats.

Or read about sea urchins in rhodolith beds and kelp canopy shading (Moss Landing Phycology Lab)

Substrate

Coralline algae occur on hard and soft substrates.

On hard substrate they can form simple crusts and reef-type buildups.

Stable soft substrate, such as sand, causes partial encrustations of clasts, such as coral or mollusc fragments and frequently uni-directional coralline algal growth. This is because the encrusted bioclasts are stabilized and are not turning in order to allow rhodolith formation. The Mediterranean Coralligene de Plateau frequently forms on stable soft substrate.

Instable soft substrate enables the formation of rhodoliths, because the corallines can grow into all directions. It also enables the formation of maerl.

Muddy substrate is highly unfavorable for coralline algae, except there are bioclasts to be encrusted. In contrast to some peyssonneliacena red algae, coralline algae seem unable to grow directly on mud.

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