The rates at which concentrations of oxygen and carbon dioxide change under the influence of the exchange with the atmosphere and biological production are derived from in situ measurements of O2 and CO2 in the mixed layer over a 10-day period in the Guinea Dome area. The rates of O2 production (10.3 mmol m–2 d–1) and CO2 consumption (13.0 mmol m–2 d–1) by photosynthetic activity exceed the evasion of O2 and CO2 into the atmosphere (respectively 2.4 and 1.2 mmol m–2 d–1). The net apparent production of O2 from day to day is only 11% of the diurnal O2 increase. The biological production of oxygen at a short distance from the center of the Guinea Dome appears to be higher than that in the exact center.