Abstract: An apparatus and method for introducing oxygen into a subsurface stratum of a thermally stratified body of water, while maintaining the thermal stratification thereof substantially undisturbed, comprising a closed flow path wherein water enters an inflow aperture, extends upwardly through an upflow chamber, through a horizontal chamber, downwardly through a downflow chamber, and through an outflow aperture. Both the inflow aperture and the outflow apertures are positioned within the subsurface stratum. Within the conduit is an aerator which introduces oxygen, an impeller which assists the flow of water through the conduit and which mixes water with oxygen, and a collector which collects undissolved gas within the conduit at the horizontal chamber. Outside the conduit, and above the outflow aperture, an interceptor means collects undissolved gas outside of the outflow aperture, and prevents vertical circulation of water discharged from the outflow aperture means.