Thermal signature of Sargassum aggregations from Landsat 9 observations

Since 2011, Sargassum blooms in the Tropical Atlantic have led to massive strandings along coastlines, posing significant ecological, economic, and public health threats. A key research challenge is to better understand the environmental drivers of Sargassum growth and decay, particularly in the context of a warming climate. In previous studies, temperature has been identified as a major factor influencing growth rates and in situ measurements in the Sargasso Sea suggest that Sargassum aggregations could be warmer than the surrounding Sea Surface Temperature.In this study, we analyze three years (2022–2024) of Landsat 9 observations in the Caribbean, using both optical and thermal sensors. To estimate the temperature of the Sargassum aggregations, we combine high resolution Sargassum detections based on the optical sensor and surface temperature obtained from the thermal one. A state of the art algorithm provide a first guess for the surface temperature and higher accuracy is obtained through regional calibration using in situ SST observations. The dataset obtained allows to quantify the temperature difference between Sargassum aggregations and surrounding waters for 4,390 aggregations in three key regions: the Lesser Antilles, Haiti, and Mexico.Our results confirm a consistent warming of Sargassum aggregations compared to nearby waters, with temperature differences reaching 1.2°C in the Lesser Antilles, and up to 1.8°C near Haiti and Mexico. We show that the magnitude of warming is influenced by aggregation size, shape, and local environmental conditions, particularly net heat flux and wave height.These findings highlight the need to account for aggregation warming in Sargassum growth and decay models, as the actual temperature experienced by the algae may exceed bulk SST values. Furthermore, this work underscores the importance of expanded in situ measurements to better characterize the vertical thermal structure and diurnal variability of Sargassum aggregations features not resolved by satellite sensors such as Landsat.