The Great Barrier Reef
76
BOX 8.1 HOW TO MEASURE CALCIFICATION
Accurate measurements of coral calcification are critical to any in-depth understanding
of the rate that it plays in reef processes. There are a number of methods that have been
used over the years to measure the rate at which calcium carbonate is deposited by corals and other calcifying invertebrates. A number of these methods are outlined below,
with some comments on their ease of use and efficacy.
Dye to measure linear extension
Corals build their skeletons by incrementally adding layers of calcium carbonate. If the thickness of added calcium carbonate is known, then it is possible to obtain a relative measure of
the rate of calcification. Many studies have used the fluorescent dye Alizarin, which binds
effectively to proteinaceous elements of the coral skeleton. To undertake measurements with
marker dyes, corals are incubated in non-toxic levels of the dye for several hours before being placed back in the field. Following exposure to the dye, which essentially marks the beginning of the period over which skeletal growth will be measured, corals are allowed to
grow under field conditions before being harvested several months to years later. Back at the
laboratory, the corals are killed and their skeletons cut into thin sections to show the annual
banding patterns (exposed using X-ray photography). By measuring the distance between
the skeletal surface (the site of the latest deposition of skeleton) and the fluorescent band
(representing the beginning point), the linear extension of skeleton can be calculated as a
function of time. While this method is easy to use in the field, it can only be used as a relative
measure as growth varies across the colony surface, and hence is not a reliable measure of
the volume of skeletal material that has been formed during the experimental period.
Using radioisotopes to follow the deposition of calcium atoms
Calcium is the cation that is deposited along with carbonate ions as the coral forms a skeleton.
If
45
Ca
2+ (a radioactive isotope of Ca
2+
) is mixed into the water surrounding a coral, it immediately gets taken up by the coral as calcification occurs. By measuring the ratio of radioactive
atoms to non-radioactive atoms of Ca
2+
, the rate at which all Ca
2+ atoms are deposited can be
calculated. This technique is very precise and can yield an accurate measure of the rate of
calcification of corals. The downside of this technique, however, is that it must be done in the
laboratory, is potentially hazardous if the right precautions are not taken, and may not
accurately depict calcification as it might occur on a coral reef (due to its requirements for lab
conditions).
Buoyant weight method
From the difference in density between calcium carbonate and seawater (known entities), it
is possible to convert the weight of an object that has been measured in sea water into an
absolute measure of calcium carbonate deposition. Assumptions made in applying this technique are that the only negatively buoyant part of the coral colony is its calcium carbonate,
which is largely true for the tissues of corals. This technique has the advantage of producing
an accurate measurement of the total calcium carbonate in an experimental coral as well as
having the advantage that corals do not have to be killed during the measurement process.
Many studies use the buoyant weight technique on corals that have been grown on tiles
that can be brought in from the field and periodically weighed.
76
BOX 8.1 HOW TO MEASURE CALCIFICATION
Accurate measurements of coral calcification are critical to any in-depth understanding
of the rate that it plays in reef processes. There are a number of methods that have been
used over the years to measure the rate at which calcium carbonate is deposited by corals and other calcifying invertebrates. A number of these methods are outlined below,
with some comments on their ease of use and efficacy.
Dye to measure linear extension
Corals build their skeletons by incrementally adding layers of calcium carbonate. If the thickness of added calcium carbonate is known, then it is possible to obtain a relative measure of
the rate of calcification. Many studies have used the fluorescent dye Alizarin, which binds
effectively to proteinaceous elements of the coral skeleton. To undertake measurements with
marker dyes, corals are incubated in non-toxic levels of the dye for several hours before being placed back in the field. Following exposure to the dye, which essentially marks the beginning of the period over which skeletal growth will be measured, corals are allowed to
grow under field conditions before being harvested several months to years later. Back at the
laboratory, the corals are killed and their skeletons cut into thin sections to show the annual
banding patterns (exposed using X-ray photography). By measuring the distance between
the skeletal surface (the site of the latest deposition of skeleton) and the fluorescent band
(representing the beginning point), the linear extension of skeleton can be calculated as a
function of time. While this method is easy to use in the field, it can only be used as a relative
measure as growth varies across the colony surface, and hence is not a reliable measure of
the volume of skeletal material that has been formed during the experimental period.
Using radioisotopes to follow the deposition of calcium atoms
Calcium is the cation that is deposited along with carbonate ions as the coral forms a skeleton.
If
45
Ca
2+ (a radioactive isotope of Ca
2+
) is mixed into the water surrounding a coral, it immediately gets taken up by the coral as calcification occurs. By measuring the ratio of radioactive
atoms to non-radioactive atoms of Ca
2+
, the rate at which all Ca
2+ atoms are deposited can be
calculated. This technique is very precise and can yield an accurate measure of the rate of
calcification of corals. The downside of this technique, however, is that it must be done in the
laboratory, is potentially hazardous if the right precautions are not taken, and may not
accurately depict calcification as it might occur on a coral reef (due to its requirements for lab
conditions).
Buoyant weight method
From the difference in density between calcium carbonate and seawater (known entities), it
is possible to convert the weight of an object that has been measured in sea water into an
absolute measure of calcium carbonate deposition. Assumptions made in applying this technique are that the only negatively buoyant part of the coral colony is its calcium carbonate,
which is largely true for the tissues of corals. This technique has the advantage of producing
an accurate measurement of the total calcium carbonate in an experimental coral as well as
having the advantage that corals do not have to be killed during the measurement process.
Many studies use the buoyant weight technique on corals that have been grown on tiles
that can be brought in from the field and periodically weighed.