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This study investigated the influence of light on the distributional limits of Bostrychia scorpioides and Catenella caespitosa within a low amplitude intertidal zonation in the Palmones estuary, Spain. Changes in photosynthesis-and growth-irradiance curves and pigment content were examined ex situ at irradiances found in their natural habitat (10-230 µmol photons m −2 s −1). The highest maximum photosynthesis rate (P m ) and photosynthetic efficiency (α) were found between 20-40 µmol photons m −2 s −1 in both species. Bostrychia scorpioides, the uppermost intertidal species, had a higher P m , α, dark respiration rate and light compensation point (E c ) than C. caespitosa at all acclimation irradiances. Net photosynthetic rates measured at their respective acclimation irradiances showed photosynthetic responses of B. scorpioides to be maximized at high irradiances, while in C. caespitosa they did not change. Higher growth rates were obtained in C. caespitosa than in B. scorpioides, which may be related to its lower thallus specific carbon content. When irradiance decreased, the chlorophyll a content of B. scorpioides increased whereas in C. caespitosa R-phycoerythrin increased. E c for growth of B. scorpioides coincided with the irradiance at its lower distributional limit in the estuary, below which this species showed losses in biomass. However, in C. caespitosa the sustained growth ex situ at saturating irradiances contrasts with its absence from the upper intertidal zone, where similar light regimes occur. We demonstrated that while light clearly restricts the growth of B. scorpioides to the uppermost intertidal zone, this environmental factor would not prevent C. caespitosa from growing at higher levels.

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