areas that have significant coral rubble can have high net
production. Thus, much of this zonation of production
and consumption is dependent on the nature of the substrate. Sand and mud have a tendency to be heterotrophic
(negative net production), while areas exposed to high
water motion and hard substratum with algae tend be autotrophic (positive net production). Rich coral areas and
knolls usually have high gross production, but a net production that is close to zero. Seaward areas with
a relatively high net production can be sustained by
dissolved nutrients in the incoming ocean waters, and
where high water motion can support higher nutrient
uptake and photosynthetic rates than calmer areas (Hearn
et al., 2001; Carpenter and Williams, 2007). Nutrient
uptake is proportional to nutrient concentration and water
velocity, with a coefficient of proportionality that is
directly related to the friction of the water flowing over
the bottom communities (Atkinson and Falter, 2003).
The energy dissipated as bottom friction helps drive
net photosynthesis and net production of carbon, and
is of order 1,000 kJ m
À2 d
À1 (Hearn et al., 2001), or about
10% of the energy in sunlight for typical cross-reef currents. Organic carbon production has high ratios of carbon:nitrogen:phosphorus (C:N:P), reflecting dominant
production by macrophytes (Atkinson and Grigg, 1984).
Fluxes of planktonic carbon
Coral reef communities take up suspended planktonic
organic matter (detritus, phytoplankton, zooplankton) as
a source of “new” carbon – i.e., carbon they do not fix
themselves via photosynthesis (Ayukai, 1995; Ribes
et al., 2003; Yahel et al., 1998). Reported rates of particulate carbon uptake are, however, relatively low (<40
mmol C m
À2 day
À1
), compared to rates of gross primary
production and community respiration (Table 2).
Suspended organic matter is thus relatively unimportant
as a source of carbon for many hard and soft coral communities. However, it is an important source of specific essential nutrients for many communities, and food for some
(Fabricius et al., 1998; Sebens et al., 1997).
Fluxes of dissolved organic matter
Dissolved organic carbon comes from microbial decomposition of plant and animal detritus and fecal material.
It is ubiquitous in water over coral reefs, and it typically
occurs at concentrations much greater than those of particulate organic matter (50 mmol m
À3
). Dissolved organic
carbon is taken up and released by a variety of organisms,
including corals and sponges (Schlichter and Liebezeit,
1991; Hoegh-Guldberg and Williamson, 1999; Yahel
et al., 2003). Rates of dissolved organic matter metabolism
can be either significant or insignificant in the context of
total metabolism. Much of the metabolism has been attributed to symbiotic bacteria (Ferrier-Pages et al., 1998;
Yahel et al., 2003), but the cycling of dissolved organic
matter at the community and ecosystem scales remains
poorly understood. The metabolism of specific organic
compounds occurring in low concentrations, such as
steroids, can have substantial impacts on the biology of
specific organisms (Tarrant et al., 2004).
Reef waters also contain dissolved organic nitrogen; it
is typically exported from reef communities (Wilkinson
et al., 1984). The nature of dissolved organic nitrogen
and the rate kinetics of its uptake are also unknown, making it difficult to establish their rates of uptake or
recycling. Like dissolved organic compounds, generally,
dissolved organic nitrogen may be resistant to chemical
breakdown in seawater and require bacterial or sponge
communities for re-mineralization; its uptake and release
by the benthic community will thus strongly depend on
the composition and abundance of the benthic biota.
Fluxes of sediment carbon
Sediments in coral reefs typically contain <1% organic
carbon, indicating little sequestration of organic carbon
into these systems. By contrast, rates of inorganic carbon
deposition (as calcium carbonate skeletons) are large, its
production accounting for approximately 10–20% of
gross primary production. Communities with high gross
primary production tend to have the highest calcification
rates (Table 2). Calcification is positively correlated to
light and net photosynthesis (Gattuso et al., 1999); the
activation energy to produce a typical amount of calcium
carbonate represents only 1% of the energy in gross primary production (activation energy for carbonate precipitation is $4 kjoule m
À2 d
À1 for 0.1 mol CaCO 3 m
À2 d
À1
).
The rate of calcification is positively correlated to carbonate ion concentration in the sea water (Atkinson and Cuet,
2009); projected decreases in carbonate over the next
60 years from rising atmospheric carbon dioxide may
reduce coral calcification by up to 30% (Smith and
Buddemeier, 1992; Kleypas and Langdon, 2006). Dissolution of carbonates occurs naturally inside coral heads
(Entsch et al., 1983), in interior pore-spaces of coral reef
sediments (Tribble et al., 1990), and from the erosion
action of boring organisms (Tribollet, 2008). Historically,
rates of dissolution have been much slower than rates of
biogenic precipitation (<10%; Tribble et al., 1990), but
it is now suggested (Hoegh-Guldberg et al., 2007) that
ocean acidification will reverse the relative rates.
Pore-water carbon
Coral reef frameworks are partially lithified carbonate
structures on which reef communities grow. Pore-waters
of reef frameworks are mostly anoxic and contain elevated
levels of dissolved nutrients (Sansone et al., 1990). This
combination of low oxygen and high nutrients is
a common feature of many coral reefs and a direct result
of oxidation of organic matter in the interstitial spaces
(Tribble et al., 1990). The subsequent production of carbonic acid from the oxidation of organic matter lowers
pore-water pH and reduces the activity of the carbonate
ion, thus facilitating in situ dissolution of carbonate (primarily aragonite). Reef pore-waters become anoxic at
CARBON FLUXES OF CORAL REEFS
183
production. Thus, much of this zonation of production
and consumption is dependent on the nature of the substrate. Sand and mud have a tendency to be heterotrophic
(negative net production), while areas exposed to high
water motion and hard substratum with algae tend be autotrophic (positive net production). Rich coral areas and
knolls usually have high gross production, but a net production that is close to zero. Seaward areas with
a relatively high net production can be sustained by
dissolved nutrients in the incoming ocean waters, and
where high water motion can support higher nutrient
uptake and photosynthetic rates than calmer areas (Hearn
et al., 2001; Carpenter and Williams, 2007). Nutrient
uptake is proportional to nutrient concentration and water
velocity, with a coefficient of proportionality that is
directly related to the friction of the water flowing over
the bottom communities (Atkinson and Falter, 2003).
The energy dissipated as bottom friction helps drive
net photosynthesis and net production of carbon, and
is of order 1,000 kJ m
À2 d
À1 (Hearn et al., 2001), or about
10% of the energy in sunlight for typical cross-reef currents. Organic carbon production has high ratios of carbon:nitrogen:phosphorus (C:N:P), reflecting dominant
production by macrophytes (Atkinson and Grigg, 1984).
Fluxes of planktonic carbon
Coral reef communities take up suspended planktonic
organic matter (detritus, phytoplankton, zooplankton) as
a source of “new” carbon – i.e., carbon they do not fix
themselves via photosynthesis (Ayukai, 1995; Ribes
et al., 2003; Yahel et al., 1998). Reported rates of particulate carbon uptake are, however, relatively low (<40
mmol C m
À2 day
À1
), compared to rates of gross primary
production and community respiration (Table 2).
Suspended organic matter is thus relatively unimportant
as a source of carbon for many hard and soft coral communities. However, it is an important source of specific essential nutrients for many communities, and food for some
(Fabricius et al., 1998; Sebens et al., 1997).
Fluxes of dissolved organic matter
Dissolved organic carbon comes from microbial decomposition of plant and animal detritus and fecal material.
It is ubiquitous in water over coral reefs, and it typically
occurs at concentrations much greater than those of particulate organic matter (50 mmol m
À3
). Dissolved organic
carbon is taken up and released by a variety of organisms,
including corals and sponges (Schlichter and Liebezeit,
1991; Hoegh-Guldberg and Williamson, 1999; Yahel
et al., 2003). Rates of dissolved organic matter metabolism
can be either significant or insignificant in the context of
total metabolism. Much of the metabolism has been attributed to symbiotic bacteria (Ferrier-Pages et al., 1998;
Yahel et al., 2003), but the cycling of dissolved organic
matter at the community and ecosystem scales remains
poorly understood. The metabolism of specific organic
compounds occurring in low concentrations, such as
steroids, can have substantial impacts on the biology of
specific organisms (Tarrant et al., 2004).
Reef waters also contain dissolved organic nitrogen; it
is typically exported from reef communities (Wilkinson
et al., 1984). The nature of dissolved organic nitrogen
and the rate kinetics of its uptake are also unknown, making it difficult to establish their rates of uptake or
recycling. Like dissolved organic compounds, generally,
dissolved organic nitrogen may be resistant to chemical
breakdown in seawater and require bacterial or sponge
communities for re-mineralization; its uptake and release
by the benthic community will thus strongly depend on
the composition and abundance of the benthic biota.
Fluxes of sediment carbon
Sediments in coral reefs typically contain <1% organic
carbon, indicating little sequestration of organic carbon
into these systems. By contrast, rates of inorganic carbon
deposition (as calcium carbonate skeletons) are large, its
production accounting for approximately 10–20% of
gross primary production. Communities with high gross
primary production tend to have the highest calcification
rates (Table 2). Calcification is positively correlated to
light and net photosynthesis (Gattuso et al., 1999); the
activation energy to produce a typical amount of calcium
carbonate represents only 1% of the energy in gross primary production (activation energy for carbonate precipitation is $4 kjoule m
À2 d
À1 for 0.1 mol CaCO 3 m
À2 d
À1
).
The rate of calcification is positively correlated to carbonate ion concentration in the sea water (Atkinson and Cuet,
2009); projected decreases in carbonate over the next
60 years from rising atmospheric carbon dioxide may
reduce coral calcification by up to 30% (Smith and
Buddemeier, 1992; Kleypas and Langdon, 2006). Dissolution of carbonates occurs naturally inside coral heads
(Entsch et al., 1983), in interior pore-spaces of coral reef
sediments (Tribble et al., 1990), and from the erosion
action of boring organisms (Tribollet, 2008). Historically,
rates of dissolution have been much slower than rates of
biogenic precipitation (<10%; Tribble et al., 1990), but
it is now suggested (Hoegh-Guldberg et al., 2007) that
ocean acidification will reverse the relative rates.
Pore-water carbon
Coral reef frameworks are partially lithified carbonate
structures on which reef communities grow. Pore-waters
of reef frameworks are mostly anoxic and contain elevated
levels of dissolved nutrients (Sansone et al., 1990). This
combination of low oxygen and high nutrients is
a common feature of many coral reefs and a direct result
of oxidation of organic matter in the interstitial spaces
(Tribble et al., 1990). The subsequent production of carbonic acid from the oxidation of organic matter lowers
pore-water pH and reduces the activity of the carbonate
ion, thus facilitating in situ dissolution of carbonate (primarily aragonite). Reef pore-waters become anoxic at
CARBON FLUXES OF CORAL REEFS
183