Stunted canopy: Marine forests under the thermal effluent of a nuclear power plant.

It has been well established that warming beyond certain thresholds can negatively affect the growth of canopy-forming macroalgae. However, most studies evaluating these effects have been conducted under controlled laboratory conditions. Observational studies investigating the impact of extreme temperatures on growth rates have been rare and typically limited to events such as marine heatwaves or areas affected by thermal pollution. The decline in vertical growth could be detrimental to the development and recovery of macroalgal canopies, significantly impacting habitat complexity. This study examined spatiotemporal variations in thallus height and vertical growth rates of benthic Sargassum species near the Brazilian Nuclear Power Station (BNPS). Samples were collected from sites exposed and unexposed to the thermal effluent of the BNPS, across different periods. Throughout the sampling periods, sites exposed to the thermal effluent consistently exhibited lower vertical growth rates than unexposed sites. Reduced thallus height was also observed at impacted sites during the first sampling period, whereas in the second period, this stunted canopy was observed only at the site closest to the thermal effluent outfall. This decline in vertical growth could reduce habitat complexity, potentially altering the structure of shallow rocky macroalgal communities. Even acknowledging the limitations in establishing cause-effect relationships in in situ studies, these results can provide important insights into the potential effects of warming on shallow, rocky-bottom communities and may offer valuable guidance for managing and monitoring Sargassum populations in the face of thermal pollution and global climate change.