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CORAL GROW TH

F8

What

What Is It?

BENCHMARK

Corals are the structural building blocks of reefs,

To

To be developed.

and calcification (deposition of calcium carbonate)

Is the fundamental process supporting coral growth.

TA R G E T

Coral growth is a combination of two processes: (1)

development of new polyps, and (2) accumulation of

To be developed.

skeletal mass. Growth rates are typically characterized

in terms of the extent of carbonate accumulation (in

cm) per year.

RED FL AG

Corals generally grow very slowly, at rates as low as

To be developed.

0.5 cm/yr though sometimes as high as 10 cm/yr.

Branching, shallow-water corals (acroporids) often

have the highest growth rates, followed by finger

or damage by humans, and global environmental

corals (e.g., Porites porites), mound or head corals

change. Increasing atmospheric CO2 is changing ocean

(e.g., Montastraea, Diploria) and then platy corals

chemistry in ways expected to make calcification

(e.g., agaricids). Acropora palmata grows ~10 cm/yr,

more difficult for corals. By 2100, ocean acidification

Montastraea annularis grows ~1 cm/yr, and Diploria

could cause a reduction in calcification of 17-35%

labyrinthiformis grows only ~0.5 cm/yr30.

compared to pre-industrial levels, and could result

Coral growth rates are influenced by light availability,

in weaker coral skeletons, growth rate reductions,

nutrient

concentrations,

sedimentation

and

increased susceptibility to erosion, and reduced ability

disturbances (e.g., chemicals). Generally, growth rates

to maintain reef growth31.

decrease with increasing water depth (i.e., decreasing

How

How Do We Measure It?

light levels).

Why

Why Do We Measure It?

Various methods are available for assessing coral

growth. To study growth bands, scientists use an

Coral growth rate can be an indicator of reef condition

underwater drill to take a coral core. The cores are

and the processes of accretion and bioerosion. (Coral

then taken to a laboratory where an electric saw

growth rate should not be confused with measures of

cuts the coral parallel to the maximum growth axis.

coral size. As an indicator, coral size is more applicable

On x-ray images of the slab, scientists then mark the

to studies of community dynamics or the status of

annual density bands (i.e., yearly growth rings). From

a coral population). Growth rates have important

the distances between dated bands, annual linear

repercussions for overall reef accretion.

extension rates can be estimated.

Coral growth rates, combined with skeletal density

Growth rates can also be measured by staining with

measurements, yield calcification rates.

This

alizarin red dye, direct visual measurement by tracking

information, together with the amount of burrowing

pins placed in the coral for reference, weighing or

seen in coral cores, can be used to infer bioerosion

volumetric determination.

rates.

To calculate average annual calcification rates, the

Sclerochronology is the analysis of growth patterns

annual extension rate is multiplied by the average

(somewhat akin to tree rings) in animal "hard parts,"

annual skeletal density.

such as coral skeletons or gastropod shells. Such

Geochemical studies of stable isotopes and trace

studies -- for which corals are especially useful

elements in seasonal and annual bands can also be

-- have yielded much information about historical

used to determine calcification profiles. This approach

environmental conditions on reefs.

provides an independent chronology to complement

Coral growth rates can be reduced by disease,

visual studies of annual banding.

bleaching, competition/predation effects, hurricanes,

excessive nutrients and sediments, direct removal

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