Exploring the mismatch between the theory and application of photosynthetic quotients in aquatic ecosystems

Estimates of primary productivity in aquatic ecosystems are commonly based on variation in O2, rather than CO2. The photosynthetic quotient (PQ) is used to convert primary production estimates from units of O2 to C. However, there is a mismatch between the theory and application of the PQ. Aquatic ecologists use PQ=1-1.4. Meanwhile, PQ estimates from the literature support PQ=0.1-4.2. Here, we describe the theory on why PQ may vary in aquatic ecosystems. We synthesize the current understanding of how processes such as NO3- assimilation and photorespiration can affect the PQ. We test these ideas with a case study of the Clark Fork River, Montana, where theory predicts that PQ could vary in space and time due to variation in environmental conditions. Finally, we highlight research needs to improve our understanding of the PQ. We suggest departing from fixed PQ values and instead use literature-based sensitivity analyses to infer C dynamics from primary production estimated using O2. Scientific Significance StatementAccurate measures of primary production in aquatic ecosystems are necessary to quantify energy availability to higher trophic levels and biological effects on global CO2 concentrations, among other reasons. However, we commonly measure primary production using O2 because it is easier, despite our motivation to measure the rate of fixed C, and then use the photosynthetic quotient (the ratio of O2 release to CO2 fixed, PQ) to convert O2 based metabolism to CO2. This study provides a summary of the current mismatch between our current knowledge and the application of PQ, highlights our current knowledge gaps, and emphasizes the need to use literature-based sensitivity analysis rather than uninformed fixed PQ values.