Optimal resource allocation across gradients in environmental conditions can determine the distribution of aquatic organisms. This study addressed how water motion and herbivory can alter the distribution and ecological characteristics of a tropical macroalgal species. Maximum water flow was estimated in three reef habitats (reef crest, back reef, and fringing reef) using dynamometers in Moorea, French Polynesia. Sargassum mangarevense abundance, growth rates, herbivore abundances, and grazing intensity were estimated across habitats. Both the abundance and growth rates of Sargassum were highest on the reef crest where herbivorous sea urchin abundance also was highest and fish grazing intensity was reduced. In addition, Sargassum stipe strength, holdfast tenacity, chemical defenses, fertility, and carbon allocation to different components was estimated. Holdfast tenacity and stipe strength were greatest in the back reef despite a reduction in maximum water flow. An echinoid-feeding assay suggested that algal thalli from the reef crest were the most preferred despite the presence of highest phlorotannin concentrations in thalli from the reef crest and back reef. Fertility was highest on the fringing reef in January, and then switched to include the reef crest suggesting a seasonality effect. The amount of carbon allocated to stipe and holdfast structures was highest in back reef and reef crest habitats, while allocation of carbon to receptacles was highest in the fringing reef habitats following the higher fertility patterns estimated in January. There was an apparent trade-off between growth and allocation to biomechanical strength as estimated through stipe strength and holdfast tenacity. A second trade-off occurred between allocation to chemical defenses and investment in reproduction. These results suggest that this seaweed allocates resources differentially across gradients in herbivory and hydrodynamic exposure, and that trade-offs between life functions are habitat-specific across the reef.