278
and the debate has been reviewed extensively (Gattuso et al. 1999; Suzuki and
Kawahata 2004). Here we summarize the points of the debate and list some of the
more recent literature published after those reviews.
Table 10.1 summarizes the CO 2 sinks and sources in coral reefs, separately listing reef flats and lagoons. Positive CO 2 flux values indicate sources and negative
values indicate sinks. Although the values vary widely, the majority of reef flats
studied acted as sources of CO 2 , with a median CO 2 flux value of 2.2 mmol m
−2
day
−1
. The majority of relatively deep lagoons also act as sources of CO 2 with a
median value of 0.7 mmol m
−2
day
−1
. These values are relatively small and comparable to the CO 2 flux for open ocean in most of the subtropical and tropical areas
where coral reefs exist, which typically exhibit fluxes of −1 to 1 mol m
−2
year
−1
(or
−2.7 to 2.7 mmol m
−2
day
−1
, Takahashi et al. 2009). Some reefs absorb CO 2 from
the atmosphere, but many researchers insist that the reef flats absorbing CO 2 are
mainly fringing reefs (see Fig. 10.3 and Sect. 10.2.2). The hypothesis is that fringing reefs receive substantial amounts of nutrients from adjacent lands, conditions
more favorable for the growth of seagrasses or macroalgae than corals. With the reef
area covered by these autotrophs, net primary production exceeds calcification and
the system overall absorbs CO 2 . Gattuso et al. (1997) tested this hypothesis by measuring community metabolism and air–sea CO 2 fluxes on a fringing reef at Moorea.
The results showed that this reef flat was a sink for CO 2 up to 10 mmol m
−2
day
−1
,
whereas the neighboring barrier reef flat was a CO 2 source (Gattuso et al. 1993,
1996b; Frankignoulle et al. 1996). These contrasting results from the fringing reef
and the barrier reef of Moorea led to the concept that “algal” reefs absorb CO 2
whereas “coral” reefs emit CO 2 to the atmosphere.
More recently, different views have been offered regarding the air–sea CO 2 flux
in coral reefs resulting from long-term or continuous monitoring at the same sites.
For example, Kayanne et al. (2005) showed that the Shiraho fringing reef became a
source of atmospheric CO 2 following coral bleaching
1
in 1998, although the reef
was a CO 2 sink during other non-bleached periods. Massaro et al. (2012) presented
continuous CO 2 data covering 2.5 years in southern Kaneohe Bay, Hawaii, a semienclosed tropical coral reef ecosystem. They showed that local climatic forcing
strongly affected the biogeochemistry, water-column properties, and air–sea CO 2
gas exchange. Large drawdowns of CO 2 following storms occasionally caused the
bay waters to switch from a CO 2 source to a sink. These results indicate that even
the same reef can dynamically shift from sink to source depending on reef conditions (e.g. coral and macroalgal coverage) as well as on external forcing (e.g. storms
and subsequent supply of nutrients). These kinds of dynamic features can be more
important than the static sink–source views of coral reefs under rapidly changing
reef conditions due to global climate change and local environmental changes.
1 Coral bleaching: Corals have symbiotic algae called zooxanthellae inside their tissue. When corals are stressed from high water temperature or other causes, they release or digest their zooxanthellae and lose their color, making the white coral skeleton visible. This phenomenon is called
coral bleaching.
A. Watanabe and T. Nakamura
and the debate has been reviewed extensively (Gattuso et al. 1999; Suzuki and
Kawahata 2004). Here we summarize the points of the debate and list some of the
more recent literature published after those reviews.
Table 10.1 summarizes the CO 2 sinks and sources in coral reefs, separately listing reef flats and lagoons. Positive CO 2 flux values indicate sources and negative
values indicate sinks. Although the values vary widely, the majority of reef flats
studied acted as sources of CO 2 , with a median CO 2 flux value of 2.2 mmol m
−2
day
−1
. The majority of relatively deep lagoons also act as sources of CO 2 with a
median value of 0.7 mmol m
−2
day
−1
. These values are relatively small and comparable to the CO 2 flux for open ocean in most of the subtropical and tropical areas
where coral reefs exist, which typically exhibit fluxes of −1 to 1 mol m
−2
year
−1
(or
−2.7 to 2.7 mmol m
−2
day
−1
, Takahashi et al. 2009). Some reefs absorb CO 2 from
the atmosphere, but many researchers insist that the reef flats absorbing CO 2 are
mainly fringing reefs (see Fig. 10.3 and Sect. 10.2.2). The hypothesis is that fringing reefs receive substantial amounts of nutrients from adjacent lands, conditions
more favorable for the growth of seagrasses or macroalgae than corals. With the reef
area covered by these autotrophs, net primary production exceeds calcification and
the system overall absorbs CO 2 . Gattuso et al. (1997) tested this hypothesis by measuring community metabolism and air–sea CO 2 fluxes on a fringing reef at Moorea.
The results showed that this reef flat was a sink for CO 2 up to 10 mmol m
−2
day
−1
,
whereas the neighboring barrier reef flat was a CO 2 source (Gattuso et al. 1993,
1996b; Frankignoulle et al. 1996). These contrasting results from the fringing reef
and the barrier reef of Moorea led to the concept that “algal” reefs absorb CO 2
whereas “coral” reefs emit CO 2 to the atmosphere.
More recently, different views have been offered regarding the air–sea CO 2 flux
in coral reefs resulting from long-term or continuous monitoring at the same sites.
For example, Kayanne et al. (2005) showed that the Shiraho fringing reef became a
source of atmospheric CO 2 following coral bleaching
1
in 1998, although the reef
was a CO 2 sink during other non-bleached periods. Massaro et al. (2012) presented
continuous CO 2 data covering 2.5 years in southern Kaneohe Bay, Hawaii, a semienclosed tropical coral reef ecosystem. They showed that local climatic forcing
strongly affected the biogeochemistry, water-column properties, and air–sea CO 2
gas exchange. Large drawdowns of CO 2 following storms occasionally caused the
bay waters to switch from a CO 2 source to a sink. These results indicate that even
the same reef can dynamically shift from sink to source depending on reef conditions (e.g. coral and macroalgal coverage) as well as on external forcing (e.g. storms
and subsequent supply of nutrients). These kinds of dynamic features can be more
important than the static sink–source views of coral reefs under rapidly changing
reef conditions due to global climate change and local environmental changes.
1 Coral bleaching: Corals have symbiotic algae called zooxanthellae inside their tissue. When corals are stressed from high water temperature or other causes, they release or digest their zooxanthellae and lose their color, making the white coral skeleton visible. This phenomenon is called
coral bleaching.
A. Watanabe and T. Nakamura