Photosynthesis by phytoplankton is a key source of oxygen (O₂) in aquatic systems. We have developed a mathematical technique to calculate the rate of photosynthesis from time series of O₂. Additionally, the approach leads to a better understanding of the influence on O₂ measurements of the tides in coasts and estuaries. The results are important for correctly interpreting the data that are gathered by a growing set of continuous O₂ sensors that are deployed around the world.