Marine heatwaves lead to bleaching and mass mortality in a key zoantharian.

Abstract

The increased frequency of severe marine heatwaves (MHWs) is a major contributing factor contributing to mass bleaching and large-scale mortality in reef-building corals. However, the effects of MHWs on other major reef organisms (e.g., zoantharians) are poorly studied. In this study, we demonstrate the vulnerability of the key habitat-forming zoantharian Zoanthus sociatus to severe MHWs. Temporal image analysis from 2019 to 2022 on intertidal South Atlantic reefs shows extensive healthy colonies (2019), bleaching (2020), and mass mortality of Z. sociatus with the occupation of the entire space by filamentous algae (2022). We calculated a loss of 99.6% of total area of the colonies due to mass mortality after the bleaching event. Thermal stress was identified during mass bleaching in Z. sociatus in 2020 by indicators such as MHWs and degree heating weeks (DHWs). In the first 6 months of 2020, this low-latitude region underwent three MHWs at 1.5, 2.0, and 1.3°C above the mean temperature. Moreover, the most intense (2°C) and longest (76 days) MHW occurred near the detected mass bleaching. Furthermore, DHWs (14.5 to 17.6) showed that 2020 had the highest and longest heat stress recorded in this century (since 2001) in this extreme reef. Despite its adaptation to this harsh intertidal habitat, our results showed the vulnerability of Z. sociatusto high heat-light stress on these reefs when exposed to high desiccation, elevated temperatures (>30°C), higher salinities, and solar irradiation at low tides. We suggest that Z. sociatus is most vulnerable to severe and repetitive bleaching due to its higher dependence on autotrophy. In this regard, these marginal reef specialists are near the bleaching threshold limit that increases their vulnerability to environmental changes, such as MHWs, especially in intertidal reefs. Our study highlights the vulnerability of symbiotic zoantharians to heatlight stress and the need to include these organisms in long-term monitoring to assess their recovery potential and resilience to climate change in reefs worldwide.