Coastal power plants generally use seawater as cooling fluid, discharging heated waters into the sea after the cooling process. The ensuing increase in seawater temperature could affect the marine biota in the nearby areas, causing alterations at different level of biological organization, from individuals to populations and assemblages. In the Mediterranean Sea, few attempts have been made to assess the effects of this point sources of thermal pollution, especially on rocky habitats. Here, we investigated the putative impact of a thermal effluent from one of the largest European coal-fired coastal power plants on sessile assemblages of subtidal rocky reefs. Sessile assemblages on rocky substrates were photographically sampled at one location near the effluent (I), and at two control locations (Cs) virtually unaffected by thermal discharge. An asymmetrical after-control impact experimental design was employed to test the hypothesis that the thermal discharge significantly modified sessile assemblages at I if compared to Cs. We detected significant differences in assemblages at I versus Cs, indicating a clear effect of the effluent on assemblage structure. Such differences were mostly due to shift in dominance among macroalgae between I and Cs, which likely depended on different tolerance limits of species to increased seawater temperature and other sources of disturbance associated to the effluent, such as increased sedimentation rates and water turbidity. Our findings stressed the need for further investigations of the impact of thermal effluents on marine communities, considering the potential synergistic effects of climate change especially in the Mediterranean Sea.
