Automated measurement of carbon dioxide, oxygen, and nitrogen partial pressures in surface waters
Limnol. Oceanogr., 43(5), 1998, 969-975 | DOI: 10.4319/lo.1998.43.5.0969
ABSTRACT: An automated system capable of accurate determinations of O2, C2, and N, partial pressures (p) in surface waters is described. The system is composed of a programmable data logger-controller, an infrared C2 analyzer, a polarographic 2 electrode, and a barometer linked to a submersed gas exchanger made of thin-walled (100 pm) silicone elastomer tubing. Oxygen electrode calibration and drift problems are eliminated by continuously referencing p2 measurements to atmospheric readings. pN2 is calculated by subtracting pO2 + pH2O + pC2 from total dissolved gas pressure. Complete (>99%) equilibration of internal gases with water pC2 occurs within 4 min. Equilibration half-times for p2 range from 7.1 min at 21°C to 9.4 min at 05°C. The equilibration half-time for pN2 is 15.1 min at 15°C. Observations conducted in a small oligotrophic Shield Lake show that the epilimnion is usually supersaturated with both C2 (50-150 µatm) and 2 (500-4,000 µatm), with both gases exhibiting marked daily variations. The epilimnetic supersaturation (up to 2%) with respect to atmospheric 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.