The purpose of this study was to find the dominant processes affecting the diel carbon cycle of the Biosphere 2 ocean from July 27, 1995 to August 14, 1995. Despite excessively low nitrate (< 0.05 mu mol l(-1)) and phosphate (0.00-0.04 mu mol l(-1)) values, the Biosphere 2 ocean not only showed large daily TCO2 changes of 30.5 +/- 7.0 mu mol kg(-1) but also a net production of organic carbon and CaCO3 during the period of this study. A model used to predict pCO(2) values based on measured O-2 concentrations suggested CaCO3 precipitation and organic carbon production were dominating the average diel ocean pCO(2) signal while the gas exchange with the atmosphere was relatively insignificant to the pCO(2) in the Biosphere 2 ocean. In the first 4 days of the experiment, the alkalinity decreased at a calculated rate of about 10 mu eq kg(-1) d(-1) while in the following 10 days the model indicated that the alkalinity was nearly constant. The model also confirmed that the positive net production of organic carbon effectively held the pCO(2) in the Biosphere 2 ocean below equilibrium with the atmosphere. Although changes in the gas exchange had a small effect on the diel amplitude of pCO(2) (255 +/- 84 mu atm), changes in the photosynthetic quotient (PQ) were more significant. A PQ of 1.0 was used in the model to match calculated pCO(2) with measured pCO(2) of the Biosphere 2 ocean. (C) 1999 Elsevier Science B.V. All rights reserved.
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