Spatial-temporal reliability of satellite-derived temperature and thermal stress in the abrolhos bank, Brazil

Accurate estimates of ocean temperatures are needed to assess the consequences and improve predictions of thermal stress over coral reefs, which are declining globally due to mass coral bleaching and mortality episodes caused by maritime heatwaves. Here, we contrasted satellite-derived sea surface temperatures (SSTs) recovered from NOAA Coral Reef Watch (CoralTemp) with in situ temperatures measured between 2012 and 2018 in nine locations within the Southwestern Atlantic's largest coral reefs (Abrolhos Bank, Brazil), at different depths. Our results revealed a strong correlation (over 0.95) between the two datasets for measurements carried out to 10 m depths or within the mixed layer. In the dry period (April to September), when the water column is homogenous and the net heat flux through the air-sea interface is negative, temperature data collected below the mixing layer or at the bottom were consistent with CoralTemp. However, in situ measurements were much lower (up to 4 °C at some sites) during the rainy period (from October to March) due to cold water occurrence in the bottom half of the water column. During the Third Global Bleaching Event (2016–2017) bleaching levels in shallow reefs were more intense than the thermal stress estimated by CoralTemp. Still, they were lower in the deeper reefs that remained with lower in situ temperatures throughout the summer and autumn. While we confirm that CoralTemp data provide a reliable source of sea surface temperatures (SSTs), caution is advised when inferring thermal stress at greater depths, particularly in mesophotic sites deeper than 20 m.