Daily variations of primary production and dependence of net photosynthetic rate (Ejm) on Írradiance (/) in seaweeds and seagrass Thalassodendron ciliatum inhabiting 0-52 m depths off sbore of the Seychelles Islands were studied by oxygen measurements in a flow systém under conditions close to the natural ones. The obtained curves displayed one maximum coiresponding to the daily maximum I. Saturation of P-^ by 1 was not observed even with intertidal macrophyte species. All the macrophytes studied exhibited daily variations in the shape of the P-1 curves. The I saturated photosynthetic rate {P) (4), maximum photosynthetic rate {P^, and the initial slope of the P-1 curve (a) increased from moming to midday and decreased towards evening. P in the macroalgae and seagrass was inhibited considerably by more than 280 pmol m"2 s'* photon flux densities (PFD) of photosynthetically active radiation (PAR) only in the evening. Thus in tropical waters of the Seychelles Islands radiant energy is the main factor limiting production of macrophyte communities in nátuře.