The reef water CO2 system of Majuro Atoll, the Marshall Islands, in the northern equatorial Pacific, was examined in order to ascertain the net effect of the reef and lagoon systems on air-sea CO2 exchange. Coral-algal communities on the reef flat reduced the partial pressure of CO2 in seawater (PCO2), despite their rapid calcification during the day. However, the mean PCO2 in the lagoon was 25 μatm higher than that in the surrounding ocean, even during the day. Alkalinity depletion in the lagoon water of Majuro Atoll suggests that this high PCO2 is the result of the calcification of reef organisms, together with the decomposition of organic matter in the lagoon. The chemical composition of the lagoonal water and sediments also indicates that calcification plays a major role in the net carbon cycle in this reef system, which has no significant accumulation of organic matter due to its rapid turnover rate. These reefs appear to act overall as a source of atmospheric CO2
1. INTRODUCTION
The role of coral reefs in the global carbon cycle, especially concerning whether reefs serve as sinks or sources for atmospheric CO has been the subject of considerable discussion1,2). The process of marine photosynthesis fixes atmospheric CO2. However, calcification shifts the chemical equilibrium of the CO2 system in seawater (equation 1) and results in a net increase of the partial pressure of carbon dioxide in seawater(PCO2)3). In this case, CO2 is released from the ocean to the atmosphere.
Ca2++2HCO3-→CaCO3+H2O+CO2↑ (1)
It has been pointed out that coral reefs are a potential source of atmospheric CO2, because of the high carbonate production rates that occur there4). Gattuso et al. reported direct measurements of air-sea CO2 flux on the Polynesian barrier reef and concluded that the reef was a source of CO2 to the atmosphere5). On the other hand, Kayanne et al. proposed that some coral reefs might act as sinks for atmospheric C02, based upon direct PCO2 measurements of reef water on Shiraho Reef, the Ryukyu Islands6). Better understanding of the carbon cycle in the coral reef ecosystem is required to answer the question of whether coral reef ecosystems work overall as sinks or sources for atmospheric CO2. In this study, we present data from Majuro Atoll in the central Pacific (Fig-1). We examined the dissolved CO2 system in the reef and offshore waters and the composition of suspended matter and reef sediments, in order to understand the overall carbon cycle in these coral reefs.
Fig-1. Location of Majuro Atoll in the Marshall Islands, the northern equatorial Pacific.
2. RESEARCH SITE AND METHODS
2-1. Study site
Majuro Atoll (7°06'N, 171°10'E) is located in the Marshall Islands, the central Pacific. It is a ring-shaped reef with a semi-enclosed lagoon (Fig-2), and covers an area of 393 km2, with a
