Dissolved O2 concentration and delta18O-O2 diel curves can be combined to assess aquatic photosynthesis, respiration, and metabolic balance, and to disentangle some of the confounding factors associated with interpretation of traditional O2 concentration curves. A dynamic model is used to illustrate how six key environmental and biological parameters interact to affect diel O2 saturation and delta18O-O2 curves, thereby providing a fundamental framework for the use of delta18O-O2 in ecosystem productivity studies. delta18O-O2 provides information unavailable from concentration alone because delta18O-O2 and saturation curves are not symmetrical and can be used to constrain gas exchange and isotopic fractionation by eliminating many common assumptions. Changes in key parameters affect diel O2 saturation and delta18O-O2 curves as follows: (1) an increase in primary production and respiration rates increases the diel range of O2 saturation and delta18O-O2 and decreases the mean delta18O-O2 value; (2) a decrease in the primary production to respiration ratio (P:R) decreases the level of O2 saturation and increases the delta18O-O2 values; (3) an increase in the gas exchange rate decreases the diel range of O2 saturation and delta18O-O2 values and moves the mean O2 saturation and delta18O-O2 values toward atmospheric equilibrium; (4) a decrease in strength of the respiratory isotopic fractionation (alphaR closer to 1) has no effect on O2 saturation and decreases the delta18O-O2 values; (5) an increase in the delta18O of water has no effect on O2 saturation and increases the minimum (daytime) delta18O-O2 value; and (6) an increase in temperature reduces O2 solubility and thus increases the diel range of O2 saturation and delta18O-O2 values. Understanding the interplay between these key parameters makes it easier to decipher the controls on O2 and delta18O-O2, compare aquatic ecosystems, and make quantitative estimates of ecosystem metabolism. The photosynthesis to respiration to gas exchange ratio (P:R:G) is better than the P:R ratio at describing and assessing the vulnerability of aquatic ecosystems under various environmental stressors by providing better constrained estimates of ecosystem metabolism and gas exchange.