277
effectively dissipated at the reef crest or reef flat. The inner reef is therefore calmer
compared to the outer reef, and often has a lagoon (or “moat” when it is shallow
[typically shallower than 5 m]), which can harbor corals, seagrasses, and mangroves. The outer reef is a high-energy environment and often covered with corals
or coralline algae that are resistant to the high wave energy.
There are three major types of coral reefs: fringing reefs, barrier reefs, and atolls
(lower panel in Fig. 10.3). A fringing reef develops right beside an island, whereas
a barrier reef has a relatively deep lagoon between an island and the reef flat. Atolls
are rings of reefs without a central island; sometimes the reef itself becomes exposed
to air and forms a low-lying island.
The coral/algal reef flats display a wide range of calcification rates (5–126 mol
CaCO 3 m
−2
year
−1
or 0.5–12.6 kg CaCO 3 m
−2
year
−1
; Gattuso et al. 1998). The value
gets progressively higher as the water depths decrease over the reef flat zone (shallowest part of coral reefs) independent of domination by corals or coralline algae
(Sorokin 1993). On average, the shallow seaward reef flat zone produces 40 moles
(or 4 kg) of CaCO 3 m
−2
year
−1
. Considering the “0.6 rule” explained in Sect. 10.2.1,
net primary production on the “average reef” would have to exceed 24 moles (or
288 g) C m
−2
year
−1
to act as a CO 2 sink. Gross primary production (GPP) on reef
flats is quite high, varying from 79 to 584 moles (or 948–7008 g) C m
−2
year
−1
(Gattuso et al. 1998), but community respiration is also high and net primary production is thought to be close to zero. In this case, calcification overwhelms the net
primary production on the reef flat, and the reef flat is believed to act as a source of
CO 2 .
10.2.3 CO 2 Sinks and Sources in Coral Reefs
The debate on whether coral reefs act as a sink or source of atmospheric CO 2 began
in the 1990s. Gattuso’s group (Gattuso et al. 1993, 1996b; Frankignoulle et al. 1996)
reported that most coral reef flats are sources of CO 2 to the atmosphere. Their study
sites were the Tiahura barrier reef (Moorea, French Polynesia) and Yonge Reef
(northern Great Barrier Reef). Both sites are classified as barrier reef flats. Gattuso
et al. (1996b) reported that the Tiahura reef supported abundant corals on the seaward side and abundant macroalgae on the back reef area, whereas the Yonge Reef
flat was mainly dominated by a single community characterized by the coral species
Acropora palifera. In contrast, Kayanne’s group (Kayanne et al. 1995) reported that
coral reef flats act as sinks for atmospheric CO 2 . Their study site was the Shiraho
reef (Ishigaki Island, Japan), which is a fringing reef flat. The dominant coral species around the monitoring site were Montipora digitate, Porites cylindrica, and
Heliopora coerulea (Kayanne et al. 2002), and algal turf and brown algae were on
the offshore side of the reef crest (Kayanne et al. 1995). Since Kayanne et al. (1995)
indicated that coral reef flats are a net sink for atmospheric CO 2 , there have been
many comments and papers published regarding the CO 2 sink–source issue
(Buddemeier 1996; Gattuso et al. 1996a, 1997; Kayanne 1996; Kraines et al. 1997;
Ohde and van Woesik 1999; Bates et al. 2001; Bates 2002; Kayanne et al. 2005),
10 Carbon Dynamics in Coral Reefs
effectively dissipated at the reef crest or reef flat. The inner reef is therefore calmer
compared to the outer reef, and often has a lagoon (or “moat” when it is shallow
[typically shallower than 5 m]), which can harbor corals, seagrasses, and mangroves. The outer reef is a high-energy environment and often covered with corals
or coralline algae that are resistant to the high wave energy.
There are three major types of coral reefs: fringing reefs, barrier reefs, and atolls
(lower panel in Fig. 10.3). A fringing reef develops right beside an island, whereas
a barrier reef has a relatively deep lagoon between an island and the reef flat. Atolls
are rings of reefs without a central island; sometimes the reef itself becomes exposed
to air and forms a low-lying island.
The coral/algal reef flats display a wide range of calcification rates (5–126 mol
CaCO 3 m
−2
year
−1
or 0.5–12.6 kg CaCO 3 m
−2
year
−1
; Gattuso et al. 1998). The value
gets progressively higher as the water depths decrease over the reef flat zone (shallowest part of coral reefs) independent of domination by corals or coralline algae
(Sorokin 1993). On average, the shallow seaward reef flat zone produces 40 moles
(or 4 kg) of CaCO 3 m
−2
year
−1
. Considering the “0.6 rule” explained in Sect. 10.2.1,
net primary production on the “average reef” would have to exceed 24 moles (or
288 g) C m
−2
year
−1
to act as a CO 2 sink. Gross primary production (GPP) on reef
flats is quite high, varying from 79 to 584 moles (or 948–7008 g) C m
−2
year
−1
(Gattuso et al. 1998), but community respiration is also high and net primary production is thought to be close to zero. In this case, calcification overwhelms the net
primary production on the reef flat, and the reef flat is believed to act as a source of
CO 2 .
10.2.3 CO 2 Sinks and Sources in Coral Reefs
The debate on whether coral reefs act as a sink or source of atmospheric CO 2 began
in the 1990s. Gattuso’s group (Gattuso et al. 1993, 1996b; Frankignoulle et al. 1996)
reported that most coral reef flats are sources of CO 2 to the atmosphere. Their study
sites were the Tiahura barrier reef (Moorea, French Polynesia) and Yonge Reef
(northern Great Barrier Reef). Both sites are classified as barrier reef flats. Gattuso
et al. (1996b) reported that the Tiahura reef supported abundant corals on the seaward side and abundant macroalgae on the back reef area, whereas the Yonge Reef
flat was mainly dominated by a single community characterized by the coral species
Acropora palifera. In contrast, Kayanne’s group (Kayanne et al. 1995) reported that
coral reef flats act as sinks for atmospheric CO 2 . Their study site was the Shiraho
reef (Ishigaki Island, Japan), which is a fringing reef flat. The dominant coral species around the monitoring site were Montipora digitate, Porites cylindrica, and
Heliopora coerulea (Kayanne et al. 2002), and algal turf and brown algae were on
the offshore side of the reef crest (Kayanne et al. 1995). Since Kayanne et al. (1995)
indicated that coral reef flats are a net sink for atmospheric CO 2 , there have been
many comments and papers published regarding the CO 2 sink–source issue
(Buddemeier 1996; Gattuso et al. 1996a, 1997; Kayanne 1996; Kraines et al. 1997;
Ohde and van Woesik 1999; Bates et al. 2001; Bates 2002; Kayanne et al. 2005),
10 Carbon Dynamics in Coral Reefs