Majuro Atoll lagoon equals 0.34, a value which is considerably smaller than the critical value for constant PCO2. This indicates that communities in this atoll work as a source of atmospheric CO2.
Fig-4.
Carbon and alkalinity budgets for Majuro Atoll. Concentrations of AT and CT are normalized at offshore salinity(S = 33.69) and represented by the unit of μmol L-1. Production and inputs are shown in mol day-1.
(3) Sink-source behavior of coral reefs
The contribution of suspended particles to the carbon cycling in this reef system is quite small, and we ignore this term in the budgetary calculation. The difference in POC between the lagoon and offshore waters was 1 μmol kg-1,a small value, and thus(Table-2). Excluding measurement errors, organic carbon export as particulate matter would never exceed 0.8 × 106 mol day-1, which is a tenth of the organic production of the entire lagoon. Excess organic carbon produced in the atoll is, clearly, not exported from the reef to the ocean as particulate matter.
Reef sediments contained very small amounts of organic carbon, as did the deep-sea carbonate sediments (Table-2). The chemical composition of reef sediments can also be used as an indicator of sink-source behavior. A reef that is an atmospheric CO2 sink would have to deposit sediments that average more than 6 weight % organic carbon19).
The chemical composition of the reef sediments supports the hypothesis, which was deduced from the AT-CT diagram and the budgetary examination for carbon and alkalinity. The organic matter is liable to decomposition due to respiration and degradation within a relatively short time range. Therefore, we can conclude that calcification plays a major role in the net carbon cycle of this atoll and that this entire reef system works as a source of atmospheric CO2.
CONCLUSIONS
1. The surface water in the lagoon of Majuro Atoll has higher PCO2 than the oceanic water by 25 μatm. These reefs work overall as sources of CO2. even during the daytime. An AT-CT diagram indicates that calcification is mainly responsible for the high PCO2 in reef water. Calcification is a major process in the net carbon cycle of the coral reef.
2. Budgetary examination for carbon and alkalinity indicates that the organic matter of this reef is liable to decomposition due to respiration
Table-2.
Characteristics of suspended particulate material and bottom sediments within and around Majuro Atoll. Concentration of particulate material (μmol kg-1) and composition of sedments (wt.%) are tabulated TC: total carbon, POC: organic carbon. PlC: inorganic carbon, PN: nitrogen, C:N : molar carbon : nitrogen ratio, Org: Inorg-C: molar organic: inorganic carbon ratio. Means±S.D. are shown for particulate matter composition. Weight of percentage of organic matter and calcium carbonate are also given in parentheses for sediments; weight of organic matter is organic carbon multiplied by 1.3.
