The photosynthetic responses of tropical seaweeds to irradiance, salinity, and desiccation were compared to determine if acclimation to these physical factors had occurred for populations of the same species found in subtidal and intertidal habitats. One green (Anadyomene stellata),
two brown (Dictyota divaricata, Padina gymnospora) and two red (Gelidiella acerosa, Laurencia poitei) macroalgae all had low compensation (Ic: 10 to 20 μE m−2 s−1) and saturation (Ik: 30 to 98 μE m−2
s−1) irradiances regardless of the habitat. Four of the five macroalgae had P vs. I initial slopes which indicated high photosynthetic efficiency under low irradiances, characteristic of shade tolerant plants. The subtidal (4 m) population of Padina gymnospora showed
significantly lower photosynthetic and respiratory responses to 10‰ salinity while the intertidal population did not show significantly different responses to 10, 30, or 50‰ salinity. The same species did not show any significantly different photosynthetic or respiratory responses
to 30 or 60 min desiccation. The study demonstrates that macroalgae with intertidal and subtidal populations do not have unique acclimations to irradiance, all being shade tolerant plants. Further, neither the subtidal or intertidal populations of P. gymnospora showed acclimation with
regard to salinity and desiccation, while Protomonostroma undulalum and Cysloseira myrica restricted to intertidal habitats, showed acclimation to desiccation and low salinity.
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