diagram in order to evaluate the contribution of various processes to the oceanic CO2 system. The coral reef carbon cycle is mainly driven by two biological processes: organic carbon metabolism (photosynthesis and respiration / degradation) and inorganic carbon metabolism (precipitation and dissolution of calcium carbonate). Photosynthesis and calcification can be represented as changes of CT and AT in the system. In order to correct for the effects of fresh water dilution and evaporation, all measured AT and CT values were normalized to the offshore mean salinity of S = 33.69. The AT-CT plot of the surface samples is shown in Fig-3.
The offshore water is located within the μmol kg in normalized AT and CT, respectively. Offshore water samples have the largest values among all samples, both for AT and CT.
The two points representing reef flat water and the average points of the offshore water make a straight line. The slope of this line corresponds to the photosynthesis / calcification ratio of the community during the daytime. Photosynthesis lowers CT, while calcification decreases AT by two moles for each mole of CaCO3 precipitated16). The slope of the line (ΔAT/ΔCT) is about 0.27; this value corresponds to a photosynthesis / calcification molar ratio of approximately 6. We call this line the "production line" on the active zone of Majuro Atoll.
If calcification occurs without photosynthesis, the ratio of ΔAT/ΔCT equals 2. Therefore, the calcification path is represented by a line whose
Fig-3. Total alkalinity (AT) vs. total carbon dioxide (CT) diagram for the reef - lagoon system of Majuro Atoll. Contours of the seawater PCO2 corresponding to AT and CT values are also shown.
slope is 2 on the AT-CT diagram ("calcification line"). Taking measurement errors into account, the calcification pathway should be represented by a zone rather than a line in Fig-3. If calcification is a major process in the net carbon cycle of the reef system, the chemical composition of seawater within the coral reef should be plotted on the calcification line. In this study, almost all the lagoon samples are plotted around the calcification line. This indicates that calcification reef system, while a rapid turnover of organic matter has no significant net flux. The precipitated carbonate is stable due to its oversaturation in surface seawater. On the other hand, the organic matter is liable to decomposition due to rapid degradation. Therefore, the higher PC02 of the lagoon water can be attributed to calcification.
3-4. Material budgets for Majuro Atoll
(1) Model
Fluxes between the lagoon and ocean waters can be calculated using the equation:
flux(mol d-1) = VL(Co - CL) /τ, (2)
where VL = volume of lagoon(12.9 × 109 m3),CL and CO = concentration of certain components inn the lagoon and ocean, and τ = residence time of lagoonal water(17 days). The average concentrations of all surface lagoon samples and surface offshore samples were used for the representative values for CL and CO, respectively, because the vertical mixing of surface water was relatively good both in the lagoon and the ocean.
(2) Carbon and alkalinity budgets
The resulting carbon and alkalinity budgets for Majuro Atoll show that the entire lagoon is mainly a sink for calcium carbonate(Fig-4). Organic production which might remain in the lagoon is relatively small, compared to calcium carbonate production.
Carbonate production for the entire lagoon(7.7 × l09 mol day-1) can be mostly attributed to the coral-algal community on the reef flat, because the bottom of deep lagoons lacks large calcifying organisms. Carbonate production for the entire lagoon divided by the area of the reef flat equals 8.6 kg CaCO3 m2 year-1. This value is somewhat larger than the range of published values(0.5 - 4 kg CaCO3 m2 year-1)17). This to an underestimation of residence time, resulting from use of the simple tidal prism theory.
The molar ratio of organic / inorganic carbon production can be used as an indicator as to whether coral reefs act as sinks or sources for CO2. When this ratio is larger than approximately 0.6, the PCO2 in seawater decreases18). The ratio for
