Thermal variance affects algal assemblage in rocky pools

Abstract

The intertidal rocky pools are habitats that, when disconnected from the sea, become highly stressful for sessile organisms such as algae. Algae experience wide changes of water temperature on short temporal scale (e. g. hour or lower) and their coverage and diversity are expected to depend on thermal variance of small water masses trapped at low tide. Here, we want to investigate the relationship between daily thermal variance and the composition, coverage and diversity of rocky pool’s algal community in Western Sicily (Southern MED). We chose 15 intertidal rocky pools and we deployed, in August 2012, thermo-loggers for two days to record temperature changes every 5-min (12 recordings per hour), whilst the algal assemblage was investigated by scraping three quadrats (10 x 10 cm) per pool; samples were brought back to the lab and classified to level of species. Our results show that the larger the thermal daily variance, the lower the number of pool’s species and the reduced the coverage. Green algae dominated under the most extreme thermal conditions, while the evenness was higher in those pools closer to the sea. Such an outcome corroborates the hypothesis that extreme thermal variance may affect the structure and composition of ecological communities and pushes towards the identification of approaches to rapidly assess the degree of stress experienced by organisms living on the edge of their physiological tolerance limits.