obtained far north of Majuro Atoll(St. E; 7°17'N; 171°12'E; 3991 m)by the R/V Hakurei-Maru, when she was leaving Majuro Harbor just after the survey in the lagoon. Sediment samples for CHN determination were stored at-20℃ in a freezer on the research ｖessel and dried prior to analysiｓ. Again, a Yanaco MT-5 CHN analyzer was used, with the vapor method14) emplopoyed for removal of carbonates.
3. RESULTS AND DISCUSSION
3-1. Circulation and residence time of Majuro Atoll
The salinity pattern, increasing from northeast to southwest, indicates that the main flow was from northeast to southwest, with evaporation causing the salinity increase(Fig-2). There was no evidence of upwelling around Majuro Atoll.
Residence time can be estimated on the basis of tidal range and water depth by using the simple tidal prism theory15). Assuming that all water movement is tidally driven and that there is complete mixing of water during each tidal cycle, the proportion of reef water replaced on each tide will be equal to the tidal range divided by the mean depth of the lagoon. This calculation gives the lower limit of residence time for the entire lagoon, because part of a given flood tide may contain a portion of the proceeding ebb tide. The tidal range was predicted to be about 1.8m spring tide and the averaged tide range was 1.2m. Since the mean depth of the lagoon was estimated to be 39m, the residence time for the averaged tidal range was 16.9 days.
The residence time of the lagoon can also be estimated theoretically by using the salinity difference between the lagoon and the outer ocean. However, the averaged salinity difference between the surface water of the lagoon and that of the outer ocean was quite a small value of 0.07. Therefore, residence time estimation using the salinity budget approach was believed to be impossible here.
Thus, in the subsequent discussion, the estimate of 17 days obtained by the tidal prism theory is used.
3-2. Distribution of CO2 system parameters in seawater
We divided the waters within and around Majuro Atoll into four water masses: offshore, east lagoon, west lagoon, and reef flat waters (Table-1).
The offshore surface samples had a uniform composition. Calculated PCO2 in the offshore water was 345 μatm and almost equal to the equilibrated value with the atmosphere.
Lagoon water varied widely in composition. In general, there was a greater difference between values of the west lagoon and offshore values than between those of the east lagoon and offshore. Also, samples showing the highest PCO2 values were found in the west lagoon.
The reef flat water was characterized by an extremely high pH value, although it should be noted that only two samples were measured. These samples were taken around noon. Calculated PCO2 also had an extremely low value of 292 μatm.
In order to ascertain the net effect of reef systems on air-sea CO2 exchange, it is necessary to examine the difference between the PCO2 of the seawater inside and outside the atoll. The mean PCO2 in the lagoon was 25 μatm higher than that in the offshore water. This indicates that the biological ecosystem worked as a source of CO2 even during the daytime, when photosynthesis tends to reduce PCO2 in the surface water.
3-3. Total system analysis based on an AT-CT diagram
We employ a graphic approach, using an AT-CT
Table-1.
Composition of major categories in Majuro Atoll. Mean values for each category are tabulated, with standard deviations in parentheses. For total alkalinity(AT) and total carbon dioxide(CT), normalized values at S=35 are also shown. Partial pressure of CO2 in seawater(PCO2) is calculated from pH and AT values at in-situ temperatures.
