In the present study we analyzed the phosynthetic responses of C. racemosa in order to assess the pat- tern of algal productivity along a depth gradient. To this end a carbon balance approach was taken, based on the numerical integration of photosynthesis vs ir- radiance (P vs E) models throughout continuous meas- urement of instantaneous irradiance recorded at the sea floor. This mechanistic approach has been previously demonstrated to provide reliable estimates of primary productivity in marine macrophytes (Matta and Chap- man, 1991; Zimmerman et al., 1994). Photosynthesis and respiration rates of C. racemosa fronds, together with continuous irradiance field data, were measured at three different locations of contrasting depth and light on the coast of the Murcia Region of SE Spain, a part of the Spanish Mediterranean coast invaded by the alga since 2005 (Ruiz et al., 2011).