ABSTRACTPhotosynthesis/photon flux density (PFD) relationships were compared among some of the major components of sparse algal turfs (green endoliths, red crusts, red filaments,Crouania, Sphacelaria)/ fromTague Bay forereef St. Croix, U.S. Virgin Islands. Algal turfs grazed by the sea urchinDiadema antillarumwere 2–10 times more productive per unit chlathan when not grazed by sea urchins. The maximum rate of net photosynthesis (Pnetmax) and saturation PFDs, but not the slope of the light‐limited portion of the curve (α), differed significantly among the algal turf components examined. The hypothesis that increased biomass‐specific primary productivity results from shifts in algal community structure was not supported because the maximum difference in photosynthesis between algal components was only a factor of two, and the less productive components were relatively more abundant under grazing. In the understory, green endoliths exhibited higher α and lower Pnetmax, suggesting shade adaptation. Photon flux density measurements taken with a fiber optic microprobe within the algal turfs indicated that photosynthesis of basal portions of algal filaments and red crusts are light‐limited in ungrazed algal turfs. As self‐shading changes with grazer‐mediated canopy removal, algal sublayers will contribute differentially to whole turf primary