Lithophytic algal diversity in a coastal supralittoral zone of Argentina: effects of rock orientation, roughness, and light exposure

Upper intertidal coastal regions are high-stress environments where desiccation, temperature, and solar radiation limit biological diversity. The Bahía Blanca Estuary (BBE), located in southern Buenos Aires Province, is a temperate coastal system with a semidiurnal mesotidal regime and a windy climate dominated by NNW, NW, and N components. On its northern margin lies Puerto Rosales (38°55′09″S, 62°04′06″W), featuring large granite boulders emplaced for coastal erosion mitigation. This study evaluated algal diversity, composition, and abundance in relation to rock orientation (north vs. south), surface roughness (Joint Roughness Coefficient, JRC), and solar radiation. During autumn and winter of 2025, 36 upper-intertidal rocks were randomly sampled. JRC was recorded using Barton’s comb, light exposure was measured, and macroalgae were collected within 10 × 10 cm quadrats, and taxonomically identified and their fresh weight estimated. The lithophytic community comprised three Cyanophyceae, six Bacillariophyceae, three Ulvophyceae, and two Bangiophyceae taxa. JRC ranged from 8 to 20 (slightly rough to very rough). Algal communities were exclusively restricted to south-facing surfaces, which had lower light exposure, whereas north-facing surfaces remained entirely uncolonized. Principal Component Analysis explained 65.5% of total variance (PC1 = 43.2%; PC2 = 22.3%). Bangia sp. was the most abundant taxon across all roughness categories, whereas Ulothrix flacca associated with higher JRC, being more abundant on rough to very rough surfaces. Light exposure did not differentially affect abundance on south-facing surfaces, suggesting the tolerance threshold to solar exposure is exceeded on north-facing orientations. These results demonstrate that rock orientation (modulating solar radiation) is the primary factor determining algal presence in the Puerto Rosales upper littoral. Under conditions where this factor ceases to be limiting, substrate roughness defines species composition, highlighting the key role of physical microhabitats in structuring temperate estuarine algal communities. Furthermore, biological interactions such as spatial competition and herbivory may regulate species abundance.