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 improve our understanding of the environmental factors influencing Sargassum growth and decay, particularly in the context of climate change. Previous studies have identified temperature 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 (SST). In this study, we analyse 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 with surface temperatures obtained from the thermal sensor. A state-of-the-art algorithm provides an initial estimate of the surface temperature, and higher accuracy is obtained through regional calibration using in-situ SST observations. The dataset obtained allows us to quantify the temperature difference between Sargassum aggregations and the surrounding waters for 4390 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 1.5 °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 the size and shape of the aggregations, as well as by local environmental conditions, particularly net heat flux with potential modulation by sea state and wind velocity. These findings highlight the need to account for warming of Sargassum aggregations in growth and decay models, since the temperature experienced by the algae may exceed bulk SST values. Furthermore, this work underscores the importance of expanded in-situ measurements to better characterise the vertical thermal structure and diurnal variability of Sargassum aggregations - features that cannot be resolved by satellite sensors such as Landsat.