The diel carbon cycle of the Biosphere 2 ocean

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 (B0.05 mmol l) and phosphate (0.00–0.04 mmol l) values, the Biosphere 2 ocean not only showed large daily TCO2 changes of 30.597.0 mmol kg but also a net production of organic carbon and CaCO3 during the period of this study. A model used to predict pCO2 values based on measured O2 concentrations suggested CaCO3 precipitation and organic carbon production were dominating the average diel ocean pCO2 signal while the gas exchange with the atmosphere was relatively insignificant to the pCO2 in the Biosphere 2 ocean. In the first 4 days of the experiment, the alkalinity decreased at a calculated rate of about 10 meq kg d 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 pCO2 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 pCO2 (255984 matm), changes in the photosynthetic quotient (PQ) were more significant. A PQ of 1.0 was used in the model to match calculated pCO2 with measured pCO2 of the Biosphere 2 ocean. © 1999 Elsevier Science B.V. All rights reserved.