Growth and Bioerosion

Growth

The rate at which corals and reefs grow and the sizes they can grow to can be limited by the surrounding environment. Currents, nutrients/food, temperature, salinity and other biological and physical processes affect the amount a coral can grow vertically.

Cold-water corals tend to grow at a much slower rate than tropical corals. Estimations have shown that the growth rate for L.pertusa is around 4-25mm per year (Rogers 1999; Reed 1981), this is extremely low compared to that of tropical corals containing zooxanthellae which grow at around 100-200mm per year (Buddemeier & Kinzie 1976). Observations of adult Lophelia pertusa colonies have shown that when a daughter polyp is produced, it increases in size at a high rate whereas the parent growth rate decreases (Freiwald et al. 2004). It is presumed that a new polyp is produced every year depending on environmental factors, as the daughter polyp is produced the coral forms a new calcareous layer, this layer then produces what is known as a ‘tree ring’ (Freiwald et al. 2004). The term ‘tree ring’ is taken from the rings that can be seen from a cross-section of a tree, each ring represents a year of growth. These rings can be observed to age corals.

Studies have shown that adult L.pertusa rarely have more than 20 polyps which means that they have a rough lifespan of about 20 years, in these 20 years L.pertusa can reach up to 35cm in height (Freiwald et al. 2004). Studies carried out by Andrews et al. (2002) show that the lifespan of some gorgonian corals, for example Paragorgia sp and Primnoa resedaeformis are between 100 and 200 years. Some cold-water coral reef structures have been known to grow over 40m in height up to a number of kilometres in length (Davies et al. 2008).

Bioerosion

Bioerosion is the process where corals are broken down as a result of the activity of other organisms such as worms, bivalves, sponges, herbivorous fish and other benthic organisms. The process of bioerosion considerably weakens reefs, resulting in the colonies becoming unstable (Boerborn et al. 1998). For corals to develop into large, complex reef colonies then first they have to overcome bioerosion.

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