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In vivo transmittance and reflectance spectra from 280 to 700 nm were obtained from 14 species for macroalgae to quantitatively compare to what extent species from different divisions, of dissimilar functional forms and with various cell-wall constituents, absorb incident irradiance. Absorptance of photosynthetically active radiation (PAR) ranged from 64 to 92%. Ultraviolet radiation (UV-R) absorptances were found to be higher than absorptances of PAR for all species in both ultraviolet-B radiation (UV-BR) and ultraviolet-A radiation (UV-AR) portions of the spectrum. Transmittance of UV-BR through macroalgal thalli was markedly low compared with other wavelength ranges and is likely due to photoprotective UV-absorbing compounds within macroalgal cells. Reflectance of irradiance off algal thalli was higher than transmittance for all calcified species and for 57% of uncalcified species. Irradiance reflected from the surface of macroalgae accounted for as little as 7% and as much as 23% of incident PAR. Irradiance reflected off of calcified species exposed their chloroplasts to a diminished irradiance environment than species with higher absorptances. Reflectance needs to be quantified for accurate, comparable estimates of quantum yield via oxygen evolution and electron transport rates via chlorophyll fluorescence. Three sympatric members of the order Dictyotales (class Phaeophyceae) differed significantly in their photosynthetic performance. Sun to shade type P vs I curve parameters were strongly correlated with decreased absorptances in Dictyota acutiloba vs Padina japonica and Stypopodium hawaiiensis. Elevated thallus absorptance in D. acutiloba coupled with postabsorbance mechanisms of dissipating photoinhibitory irradiance may contribute to elevated productivity for this species.