The Great Barrier Reef
72
be sloughed of corals at the end of the day (after extensive photosynthesis has occurred). Corals on the intertidal reef flat at Heron Island exude up to 4.8 litres of
mucus per square metre of reef area per day, and, of
that, up to 80% dissolves in the reef water. While the
dissolved component stimulates a burst of metabolic
activity in the sediments where it is largely metabolised, the remaining particulate proportion is eaten by
fish and other particle feeders on the reef crest. This
transfer of energy is thought to represent a major
trophic exchange of energy, and relative to other marine food webs is fairly unique.
In summary, this chapter explored the production
and flow of energy through coral reefs, which are highly
productive ecosystems that prosper in the nutrient
poor waters of the tropics. While tropical oceans that
surround most coral reefs, they have a primary
productivity that is close to zero, the coral reefs that
they bathe often have levels of primary productivity
that are among the highest in the ocean. This productivity is a manifestation of the efficient photosynthetic
processes and recycling that occurs within the warm
and sunlit setting of coral reefs. We also examined one
of the key nutrient cycles of coral reefs, that of nitrogen,
observing that nitrogen is regenerated by nitrogen fixation and that it cycles between the different organisms
within the food web of coral reefs along with the energy of organic carbon bonds. The ‘wall of mouths’
clouds of small fish and other particle feeders that forage at the interface of coral reefs and the open ocean
play an important role in acquisition of energy. As well,
the efficiencies of mutualistic symbioses like those seen
between corals and symbiotic dinoflagellates have
huge benefits to a wide range of organisms in the dilute
nutrient conditions of tropical seas.
Probably no single factor can explain why coral reefs
are so productive. The answer probably lies in combinations of characteristics and mechanisms that generate
and recycle nutrients. There is one important take-home
message that may not be obvious at first: it is a mistake
to think that much of the energy generated can be harvested as a net product of the system. Tight recycling of
nutrients and energy means that the majority of primary production is rapidly recycled back into the ecosystem by the many pathways elucidated in this chapter.
This has been likened to a ‘beggar’s banquet’ where the
table is set for a feast that looks at first glance to be generous and abundant yet very little can be eaten or taken
away from the table. This situation appears to be fundamentally different than that seen in marine ecosystems
such as kelp forests where the rate of primary production can be large seasonally and substantial amounts of
energy and organic carbon are exported out of the kelp
ecosystem. Another way of understanding this is to
compare the measures of P G and P N on a community
basis. The rate of gross photosynthesis of the community is large in both coral reefs and in kelp forests, but
P N community of coral reefs is far less than P N community of kelp
forests. These differences strike at the heart of the unique
nature of coral reefs and may drive other emergent features such as the sensitivity of coral reefs to small
changes in environment that surrounds them.
ADDITIONAL READING
Anthony, K. R. N., and Hoegh-Guldberg, O. (2003).
Variation in coral photosynthesis, respiration and
growth characteristics in contrasting light microhabitats: an analogue to plants in forest gaps and
understoreys? Functional Ecology 17, 895–899.
Benson, A., and Muscatine, L. (1974). Wax in coral
mucus – energy transfer from corals to reef fishes.
Limnology and Oceanography 19, 810–814.
Darwin, C. R. (1842). ‘The Structure and Distribution of
Coral Reefs.’ (Smith Elder and Company: London.)
Hamner, W. M., Jones, M. S., Carleton, J. H., Hauri,
I. R., and Williams, D. (1988). Zooplankton, planktivorous fish, and water currents on a windward
reef face: Great Barrier Reef, Australia. Bulletin of
Marine Science 42, 459–479.
Hatcher, B. G. (1988). Coral reef primary productivity:
a beggar’s banquet. Trends in Ecology and Evolution
3, 106–111.
Hoegh-Guldberg, O. (1999). Coral bleaching, climate
change and the future of the world’s coral reefs.
Marine and Freshwater Research 50, 839–866.
Hughes, T. P., Baiard, A. H., Bellwood, D. R., Card,
M., Connolly, S. R., Folke, C., Grosberg, R., HoeghGuldberg, O., Jackson, J. B. C., Kleypas, J., Lough,
72
be sloughed of corals at the end of the day (after extensive photosynthesis has occurred). Corals on the intertidal reef flat at Heron Island exude up to 4.8 litres of
mucus per square metre of reef area per day, and, of
that, up to 80% dissolves in the reef water. While the
dissolved component stimulates a burst of metabolic
activity in the sediments where it is largely metabolised, the remaining particulate proportion is eaten by
fish and other particle feeders on the reef crest. This
transfer of energy is thought to represent a major
trophic exchange of energy, and relative to other marine food webs is fairly unique.
In summary, this chapter explored the production
and flow of energy through coral reefs, which are highly
productive ecosystems that prosper in the nutrient
poor waters of the tropics. While tropical oceans that
surround most coral reefs, they have a primary
productivity that is close to zero, the coral reefs that
they bathe often have levels of primary productivity
that are among the highest in the ocean. This productivity is a manifestation of the efficient photosynthetic
processes and recycling that occurs within the warm
and sunlit setting of coral reefs. We also examined one
of the key nutrient cycles of coral reefs, that of nitrogen,
observing that nitrogen is regenerated by nitrogen fixation and that it cycles between the different organisms
within the food web of coral reefs along with the energy of organic carbon bonds. The ‘wall of mouths’
clouds of small fish and other particle feeders that forage at the interface of coral reefs and the open ocean
play an important role in acquisition of energy. As well,
the efficiencies of mutualistic symbioses like those seen
between corals and symbiotic dinoflagellates have
huge benefits to a wide range of organisms in the dilute
nutrient conditions of tropical seas.
Probably no single factor can explain why coral reefs
are so productive. The answer probably lies in combinations of characteristics and mechanisms that generate
and recycle nutrients. There is one important take-home
message that may not be obvious at first: it is a mistake
to think that much of the energy generated can be harvested as a net product of the system. Tight recycling of
nutrients and energy means that the majority of primary production is rapidly recycled back into the ecosystem by the many pathways elucidated in this chapter.
This has been likened to a ‘beggar’s banquet’ where the
table is set for a feast that looks at first glance to be generous and abundant yet very little can be eaten or taken
away from the table. This situation appears to be fundamentally different than that seen in marine ecosystems
such as kelp forests where the rate of primary production can be large seasonally and substantial amounts of
energy and organic carbon are exported out of the kelp
ecosystem. Another way of understanding this is to
compare the measures of P G and P N on a community
basis. The rate of gross photosynthesis of the community is large in both coral reefs and in kelp forests, but
P N community of coral reefs is far less than P N community of kelp
forests. These differences strike at the heart of the unique
nature of coral reefs and may drive other emergent features such as the sensitivity of coral reefs to small
changes in environment that surrounds them.
ADDITIONAL READING
Anthony, K. R. N., and Hoegh-Guldberg, O. (2003).
Variation in coral photosynthesis, respiration and
growth characteristics in contrasting light microhabitats: an analogue to plants in forest gaps and
understoreys? Functional Ecology 17, 895–899.
Benson, A., and Muscatine, L. (1974). Wax in coral
mucus – energy transfer from corals to reef fishes.
Limnology and Oceanography 19, 810–814.
Darwin, C. R. (1842). ‘The Structure and Distribution of
Coral Reefs.’ (Smith Elder and Company: London.)
Hamner, W. M., Jones, M. S., Carleton, J. H., Hauri,
I. R., and Williams, D. (1988). Zooplankton, planktivorous fish, and water currents on a windward
reef face: Great Barrier Reef, Australia. Bulletin of
Marine Science 42, 459–479.
Hatcher, B. G. (1988). Coral reef primary productivity:
a beggar’s banquet. Trends in Ecology and Evolution
3, 106–111.
Hoegh-Guldberg, O. (1999). Coral bleaching, climate
change and the future of the world’s coral reefs.
Marine and Freshwater Research 50, 839–866.
Hughes, T. P., Baiard, A. H., Bellwood, D. R., Card,
M., Connolly, S. R., Folke, C., Grosberg, R., HoeghGuldberg, O., Jackson, J. B. C., Kleypas, J., Lough,