Breaking waves result in very different populations of bubbles in fresh water and seawater. The differences in sizes and numbers of bubbles in the two media cause significant differences in many natural processes including gas transfer and generation of aerosols as well as affect sound and light propagation and ambient sound production. Observed differences in bubble populations in seawater and fresh water have in the past been attributed to coalescence inhibition in seawater, but bubble-shattering studies described here suggest that other factors may be important. When air bubbles are expanded through an orifice at constant pressure drop, they can be made to shatter into clouds of smaller bubbles. We report here measurements showing that the number and size distribution of resulting populations depend on the physical and chemical characteristics of the water samples. In particular, bubbles were found to break up into 4--5 times more small bubbles in seawater than in fresh water. The number of bubbles produced in shattering was found to be a function of salt concentration but was especially sensitive to the types of ions present. In addition, medium from the marine diatom Phaeodactylum tricornutum was found to further double production numbers, and a decrease in temperature from 20 ¿C to 3 ¿C was found to increase bubble production in seawater by nearly 50%. These effects appear to be separate from coalescence inhibition, the factor usually cited for differences in bubble populations in fresh water and seawater. ¿ 1999 American Geophysical Union