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

Abstract

An automated system capable of accurate determinations of 0,, CO,, and N, partial pressures (p) in surface waters is described. The system is composed of a programmable data logger-controller, an infrared CO, analyzer, a polarographic 0, 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 p0, measurements to atmospheric readings. pN, is calculated by subtracting p0, + pH,O + pC0, from total dissolved gas pressure. Complete (>99%) equilibration of internal gases with water pC0, occurs within 4 min. Equilibration half-times for p0, range from 7.1 min at 21°C to 9.4 min at 05°C. The equilibration half-time for pN, is 15.1 min at 15*C. Observations conducted in a small oligotrophic Shield Lake show that the epilimnion is usually supersaturated with both CO, (50-150 patm) and 0, (500-4,000 patm), with both gases exhibiting marked daily variations. The epilimnetic supersaturation (up to 2%) with respect to atmospheric 0, 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. Measurements of dissolved gas concentrations or partial pressures in surface waters are required in several fields of investigation in limnology and oceanography. Frequent needs for such measurements include the estimation of carbon fluxes across the air-water interface (Broecker and Peng 1984; Wanninkhof et al. 1987), the assessment of net ecosystem production (autotrophy) or respiration (heterotrophy) in lakes and rivers (Schurr and Ruchti 1977; Cole et al. 1994), and the monitoring of gas supersaturation and associated risks to aquatic life below reservoir spillways (Weitkamp and Katz 1980). These needs have stimulated a continuing interest in the design of automated systems for measuring dissolved gases in aquatic environments (Kelly et al. 1974; D’Aoust and Clarke 1980; DeGrandpre et al. 1995; Sellers et al. 1995). Determinations of the partial pressures, concentrations, and fluxes of CO, and 0, in surface waters can be used to document the production-respiration balance of lakes, rivers, and oceans at the ecosystem level. Gas fluxes (F) across the air-water interface can be estimated from differences in dissolved gas concentrations between surface water (C,) and air-saturated water (C,Y) using a relationship of the form F = K(C, - C,),