The Great Barrier Reef
154
calcium carbonate (CaCO 3 ). Crustose calcareous algae
(CCA, e.g. Porolithon and Peyssonnelia) are important
framework builders and framework cementers in coral
reefs, whereby they bind adjacent substrata and provide a calcified tissue barrier against erosion. This process is particularly important on the reef crest
environments of the GBR. Crustose calcareous algae
are also important in deeper areas at the edge of the
continental platform in the southern GBR (80–120 m),
where they form large algal frameworks of several metres high. Deposition of calcium carbonate within the
tissues of CCA (as calcite as well as high magnesium
calcite) can be up to 10.3 kg CaCO 3 m
2 yr
1 in some
parts of the GBR (e.g. Lizard Island).
Upright calcareous algae such as Halimeda, Udotea,
Amphiroa and Galaxaura contribute to the production of
marine sediments that fill in the spaces between corals.
The white sand of beaches and reef lagoons is largely
the eroded calcium carbonate skeletons of these algae.
Calcium carbonate is deposited as aragonite in Halimeda
with an estimated production of around 2.2 kg CaCO 3
m
2 yr
1
. Calcification may be an adaptation to inhibit
grazing (a defensive mechanism), resist wave shock,
and to provide mechanical support.
Facilitation of coral settlement
Crustose coralline algae induce settlement and metamorphosis of coral larvae and a range of other invertebrates in the GBR, thus playing a critical role in reef
resilience. Some evidence supports the idea that this
interaction seems to be mediated by chemicals released
by the alga.
Roles in reef degradation
Macroalgae play important roles in reef degradation,
particularly in ecological phase shifts, where abundant
reef-building corals are replaced by abundant fleshy
macroalgae. Reductions in herbivory due to overfishing, and increases in nutrient inputs leading to eutrophication (e.g. sewage and fertiliser), have been
suggested as causes of increased abundance of fleshy
macroalgae leading to coral overgrowth and reef degradation. Coral bleaching, crown-of-thorns starfish
outbreaks, extreme low tides, coral diseases, cyclones,
and so on result in coral mortality, providing an
environment that is rapidly colonised by diverse algal
communities (Fig. 15.2N). Such disturbances, and particularly those due to climate change (e.g. bleaching),
may lead to an overall increase in the total amount of
macroalgae (see Chapters 9 and 10 for further details).
Dominance by thick mats or larger, fleshy macroalgae
may contribute to reef degradation by overgrowing
corals, inhibiting coral recruitment, contributing to
coral diseases (e.g. Fig. 15.2P), and thereby decreasing
the aesthetic value of reefs.
Bioerosion
Endolithic algae that live within the skeletons of both
healthy and dead corals as well as other calcareous
substrates contribute to reef erosion and destruction.
These algae are generally filamentous and microscopic
but form a thin dark green band visible to the naked
eye underneath the coral and crustose algal tissue
(Fig. 15.2L). Some examples of carbonate-boring algae
include the greens Ostreobium spp., cyanobacteria
Mastigocoleus testarum, Plectonema terebrans, and Hyella
spp. and some red algae. Endolithic algae penetrate
and dissolve the calcium carbonate, weakening the reef
framework and thus hasten other erosive activities.
Studies at One Tree Island on the GBR have show rates
of bioerosion by endolithic algae to range between
20–30 g m
2 yr
1 . For more information on bioerosion
see Chapter 9.
ADDITIONAL READING
Algae of Australia Series (2007). (Eds) ‘Algae of Australia:
Introduction.’ (Australian Biological Resourc es Study:
Canberra/CSIRO Publishing: Melbourne.)
Australian Marine Algal Name Index. Available at
http://www.anbg.gov.au/abrs/online-resources/
amani/ [Verified 26 February 2008].
Borowitzka, L. J., and Larkum, A. W. D. (1986). Reef
algae. Oceanus 29, 49–54.
Clayton, M. N., and King, R. J. (Eds) (1990). ‘Biology of
Marine Plants.’ (Longman Cheshire: Melbourne.)
Cribb, A. B. (Ed.) (1996). ‘Seaweeds of Queensland: A
Naturalist’s Guide.’ (The Queensland Naturalist’s
Club: Brisbane.)
154
calcium carbonate (CaCO 3 ). Crustose calcareous algae
(CCA, e.g. Porolithon and Peyssonnelia) are important
framework builders and framework cementers in coral
reefs, whereby they bind adjacent substrata and provide a calcified tissue barrier against erosion. This process is particularly important on the reef crest
environments of the GBR. Crustose calcareous algae
are also important in deeper areas at the edge of the
continental platform in the southern GBR (80–120 m),
where they form large algal frameworks of several metres high. Deposition of calcium carbonate within the
tissues of CCA (as calcite as well as high magnesium
calcite) can be up to 10.3 kg CaCO 3 m
2 yr
1 in some
parts of the GBR (e.g. Lizard Island).
Upright calcareous algae such as Halimeda, Udotea,
Amphiroa and Galaxaura contribute to the production of
marine sediments that fill in the spaces between corals.
The white sand of beaches and reef lagoons is largely
the eroded calcium carbonate skeletons of these algae.
Calcium carbonate is deposited as aragonite in Halimeda
with an estimated production of around 2.2 kg CaCO 3
m
2 yr
1
. Calcification may be an adaptation to inhibit
grazing (a defensive mechanism), resist wave shock,
and to provide mechanical support.
Facilitation of coral settlement
Crustose coralline algae induce settlement and metamorphosis of coral larvae and a range of other invertebrates in the GBR, thus playing a critical role in reef
resilience. Some evidence supports the idea that this
interaction seems to be mediated by chemicals released
by the alga.
Roles in reef degradation
Macroalgae play important roles in reef degradation,
particularly in ecological phase shifts, where abundant
reef-building corals are replaced by abundant fleshy
macroalgae. Reductions in herbivory due to overfishing, and increases in nutrient inputs leading to eutrophication (e.g. sewage and fertiliser), have been
suggested as causes of increased abundance of fleshy
macroalgae leading to coral overgrowth and reef degradation. Coral bleaching, crown-of-thorns starfish
outbreaks, extreme low tides, coral diseases, cyclones,
and so on result in coral mortality, providing an
environment that is rapidly colonised by diverse algal
communities (Fig. 15.2N). Such disturbances, and particularly those due to climate change (e.g. bleaching),
may lead to an overall increase in the total amount of
macroalgae (see Chapters 9 and 10 for further details).
Dominance by thick mats or larger, fleshy macroalgae
may contribute to reef degradation by overgrowing
corals, inhibiting coral recruitment, contributing to
coral diseases (e.g. Fig. 15.2P), and thereby decreasing
the aesthetic value of reefs.
Bioerosion
Endolithic algae that live within the skeletons of both
healthy and dead corals as well as other calcareous
substrates contribute to reef erosion and destruction.
These algae are generally filamentous and microscopic
but form a thin dark green band visible to the naked
eye underneath the coral and crustose algal tissue
(Fig. 15.2L). Some examples of carbonate-boring algae
include the greens Ostreobium spp., cyanobacteria
Mastigocoleus testarum, Plectonema terebrans, and Hyella
spp. and some red algae. Endolithic algae penetrate
and dissolve the calcium carbonate, weakening the reef
framework and thus hasten other erosive activities.
Studies at One Tree Island on the GBR have show rates
of bioerosion by endolithic algae to range between
20–30 g m
2 yr
1 . For more information on bioerosion
see Chapter 9.
ADDITIONAL READING
Algae of Australia Series (2007). (Eds) ‘Algae of Australia:
Introduction.’ (Australian Biological Resourc es Study:
Canberra/CSIRO Publishing: Melbourne.)
Australian Marine Algal Name Index. Available at
http://www.anbg.gov.au/abrs/online-resources/
amani/ [Verified 26 February 2008].
Borowitzka, L. J., and Larkum, A. W. D. (1986). Reef
algae. Oceanus 29, 49–54.
Clayton, M. N., and King, R. J. (Eds) (1990). ‘Biology of
Marine Plants.’ (Longman Cheshire: Melbourne.)
Cribb, A. B. (Ed.) (1996). ‘Seaweeds of Queensland: A
Naturalist’s Guide.’ (The Queensland Naturalist’s
Club: Brisbane.)