Palmaria palmata, which is able to use HCO3 as a carbon source for photosynthesis, and Lomentaria articulata, which is dependent on diffusive uptake of dissolved C02, were grown under constant light and light with sunflecks designed to model wave-induced fluctuations of near-shore underwater light . Both species exhibited significantly increased stable carbon isotope discrimination (more negative values of 8 13 C relative to PDB) when grown with sunflecks . More negative 8 133 C values were associated with decreased growth rate of P palmata but not of L. articulata . The contrasting effects of sunflecks on the carbon-use characteristics of the two species are discussed in terms of the energetic cost of HCO3 use and the susceptibility of CO2 diffusion-dependent species to photoinhibition .