Automated determination of carbon dioxide, oxygen, and nitrogen partial pressures in surface waters

An automated system capable of accurate determinations of O-2, CO2, and N-2 partial pressures (p) in surface waters is described. The system is composed of a programmable data logger-controller, an infrared CO2 analyzer, a polarographic O-2 electrode, and a barometer linked to a submersed gas exchanger made of thin-walled (100 mu m) silicone elastomer tubing. Oxygen electrode calibration and drift problems are eliminated by continuously referencing pO(2) measurements to atmospheric readings. pN(2) is calculated by subtracting pO(2) + pH(2)O + pCO(2) from total dissolved gas pressure. Complete (>99%) equilibration of internal gases with water pCO(2) occurs within 4 min. Equilibration half-times for pO(2) range from 7.1 min at 21 degrees C to 9.4 min at 0.5 degrees C. The equilibration half-time for pN(2) is 15.1 min at 15 degrees C. Observations conducted in a small oligotrophic Shield Lake show that the epilimnion is usually supersaturated with both CO2 (50-150 mu atm) and O-2 (500-4,000 mu atm), with both gases exhibiting marked daily variations. The epilimnetic supersaturation (up to 2%) with respect to atmospheric O-2 indicates that primary production slightly exceeds community respiration in the epilimnion. The system appears adequate for metabolic and gas flux studies, even in the most oligotrophic waters.