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The exchange of oxygen between lakes and the atmosphere is assumed to be driven by diffusion. Here we show that lakes can emit significant amounts of O-2 by emerging gas bubbles-a process called ebullition. We found very high proportions of 17 +/- 10% O-2 (maximum 34%) in emerging gas bubbles in two shallow eutrophic reservoirs. In the studied reservoirs, O-2 emission by ebullition was of similar magnitude as diffusive O-2 fluxes. By reanalyzing previous studies, we show that the process is ubiquitous and probably quantitatively relevant in many places. We present evidence that O-2 in bubbles originates both from photosynthetic oxygen production and hence bubble formation in the oxic water and from stripping by emerging methane bubbles. Ebullition can turn lakes undersaturated in respect to the atmosphere into a net O-2 source. Neglecting O-2 ebullition leads to an overestimation of lake internal respiration.

Plain Language Summary Oxygen is probably the most fundamental parameter of lakes, aside from water itself. We investigated a new escape path of oxygen from lakes: gas bubbles. Oxygen bubbles may be produced by algae as well as by uptake of oxygen into bubbles of other gases. For the first time this process was quantified and we show that it is relevant compared to other processes affecting the oxygen concentration in lakes. Ignoring the new process results in an overestimation of respiration in lakes.