ABSTRACT: The present paper elaborates the development of a mass transfer model to determine rates of photosynthetic oxygen flux from the giant kelp Macrocystis pyrifera in a moving fluid. The model can be used to calculate local dissolved oxygen flux, blade-averaged oxygen flux and daily average net primary productivity for a kelp forest. Predicted rates of photosynthetic oxygen flux agree well with previously reported direct measurements. This model was derived from the fundamental principles of conservation of mass and momentum over a flat plate using an integral method approach and is applicable in laminar, transitional and turbulent flows. It predicts oxygen flux as an explicit function of fluid flow conditions above the kelp blade. Experimental velocity profiles and dissolved oxygen concentration profiles, measured under a range of fluid flow conditions, followed power-law scaling and demonstrated local similarity between oxygen and velocity distribution within the boundary layers above the kelp blade. The proposed model provides a useful tool for estimating rates of photosynthesis with minimal data collection.