Intraspecific phenotypic variation in two estuarine rhodophytes across their intertidal zonation

The intraspecific variability in the physiology and biochemical composition of the rhodophytes Bostrychia scorpioides (Hudson) and Catenella caespitosa (Withering) was investigated in relation to the environmental gradients across their 30-cm intertidal vertical zonation in the Palmones estuary (Southern Spain). Both species are spatially segregated but overlap at intermediate tidal levels, with B. scorpioides dominating at intermediate to upper parts and C. caespitosa at lower to intermediate ones. Photosynthesis-irradiance (PE) curves, nutrient uptake rates, pigment content, elemental composition and organic osmolytes were compared between specimens collected from their upper and lower zonation limits. Photosynthetic maximum capacity (P-max) and efficiency (alpha), and the content of liposoluble pigments increased from upper to lower individuals of B. scorpioides, reflecting its photoacclimation to a broad range of irradiance. Osmotic acclimation via increased concentrations of organic osmolytes (D-dulcitol and D-sorbitol) was only detected in upper specimens of B. scorpioides. In C. caespitosa, ammonium and phosphate uptake, total P content and phycobiliprotein content were lower in upper specimens. Decreasing turnover rates of internal nutrients towards upper tidal positions support the ability of B. scorpioides to cope with reduced nutrient availability due to longer emersion. This study underlines the occurrence of phenotypic variations in a small-scale intertidal zonation. How such changes may influence the outcome of interspecific interactions and shape this intertidal community is discussed.