275
addressed. Finally the relationships between coral reefs and other ecosystems,
especially with seagrass beds and mangrove forests, are discussed to highlight the
necessity of regarding these connected ecosystems together as a blue-carbon
ecosystem.
10.2 Carbon Cycling, Storage, and Export in Coral Reefs
10.2.1 Basic Carbonate-Chemistry Changes
Due to Calcification and Primary Production
We first briefly explain the basics of carbonate chemistry alterations due to calcification and photosynthesis to provide the background necessary to understand this
chapter. For further reading, please refer to Gattuso et al. (1999) or Zeebe and WolfGladrow (2001).
The calcification reaction, which releases CO 2 , is commonly expressed by the
following equation:
Ca
HCO
CaCO H O CO
2
3
3
2
2
2
+
+
®
+
+
-
–
(10.1)
The reverse of Eq. (10.1) is the reaction for the dissolution of CaCO 3 .
Photosynthesis fixes CO 2 and is expressed by the following equation:
CO H O CH O O
2
2
2
2
+
®
+
(10.2)
The reverse of Eq. (10.2) is the reaction for aerobic respiration and decomposition.
There is a so-called “0.6 rule” for seawater, where about 0.6 moles of CO 2 is
liberated (not the expected 1 mole) when 1 mole of CaCO 3 is produced by Eq.
(10.1) (Ware et al. 1991; Frankignoulle et al. 1994). In contrast, 1 mole of CO 2 is
fixed when 1 mole of organic C (CH 2 O) is produced by Eq. (10.2). Therefore, when
the rate of photosynthesis is greater (less) than 60% of the calcification rate, CO 2 is
fixed (liberated) and the system acts as a sink (source) of CO 2 . The value 0.6 in the
0.6 rule is due to the buffering capacity of seawater, and it changes with ocean acidification (the trend of increasing CO 2 in seawater). This value, termed Ψ (Frankignoulle
et al. 1994), is 0.6 when pCO 2 (partial pressure of CO 2 ) in seawater is 350 μatm,
salinity is 35, and water temperature is 25 °C, but it rapidly increases to 0.78 when
pCO 2 reaches 1000 μatm under the same conditions. Ψ also changes with temperature (see Fig. 10.2). At constant pCO 2 , Ψ decreases as water temperature increases.
For example, seawater at higher latitudes generally has higher Ψ values and lower
buffering capacity.
10 Carbon Dynamics in Coral Reefs
addressed. Finally the relationships between coral reefs and other ecosystems,
especially with seagrass beds and mangrove forests, are discussed to highlight the
necessity of regarding these connected ecosystems together as a blue-carbon
ecosystem.
10.2 Carbon Cycling, Storage, and Export in Coral Reefs
10.2.1 Basic Carbonate-Chemistry Changes
Due to Calcification and Primary Production
We first briefly explain the basics of carbonate chemistry alterations due to calcification and photosynthesis to provide the background necessary to understand this
chapter. For further reading, please refer to Gattuso et al. (1999) or Zeebe and WolfGladrow (2001).
The calcification reaction, which releases CO 2 , is commonly expressed by the
following equation:
Ca
HCO
CaCO H O CO
2
3
3
2
2
2
+
+
®
+
+
-
–
(10.1)
The reverse of Eq. (10.1) is the reaction for the dissolution of CaCO 3 .
Photosynthesis fixes CO 2 and is expressed by the following equation:
CO H O CH O O
2
2
2
2
+
®
+
(10.2)
The reverse of Eq. (10.2) is the reaction for aerobic respiration and decomposition.
There is a so-called “0.6 rule” for seawater, where about 0.6 moles of CO 2 is
liberated (not the expected 1 mole) when 1 mole of CaCO 3 is produced by Eq.
(10.1) (Ware et al. 1991; Frankignoulle et al. 1994). In contrast, 1 mole of CO 2 is
fixed when 1 mole of organic C (CH 2 O) is produced by Eq. (10.2). Therefore, when
the rate of photosynthesis is greater (less) than 60% of the calcification rate, CO 2 is
fixed (liberated) and the system acts as a sink (source) of CO 2 . The value 0.6 in the
0.6 rule is due to the buffering capacity of seawater, and it changes with ocean acidification (the trend of increasing CO 2 in seawater). This value, termed Ψ (Frankignoulle
et al. 1994), is 0.6 when pCO 2 (partial pressure of CO 2 ) in seawater is 350 μatm,
salinity is 35, and water temperature is 25 °C, but it rapidly increases to 0.78 when
pCO 2 reaches 1000 μatm under the same conditions. Ψ also changes with temperature (see Fig. 10.2). At constant pCO 2 , Ψ decreases as water temperature increases.
For example, seawater at higher latitudes generally has higher Ψ values and lower
buffering capacity.
10 Carbon Dynamics in Coral Reefs