*3.1. Bleaching and Mortality*

During the 2016 El Niño the Maldives experienced a severe bleaching event with the overall mean percentage of bleached corals (across 71 sites and depths ranging from 0–13 m) reaching 73%, with some sites exhibiting close to 100% bleaching [12]. Although the peak of the bleaching event occurred between mid-April and mid-June 2016 and some mortality was seen at this time, many colonies remained bleached for several months. Live mean coral cover was reduced to approximately 6–20% across the central and southern Maldives [33–35], with both lagoonal and oceanic reefs impacted [35]. Higher mortality was observed at reef sites in proximity to higher local population sizes, suggesting that these sites were more strongly impacted by human pressures (e.g., North and South Malé Atolls [35]). Coral reef sites investigated in the present study were exposed to comparatively lower human pressures (Baa and Alifu Alifu Atolls), but still exhibited extensive mortality (Figures 3 and 4), with mean live coral cover reduced to 3–27%. The observed bleachinginduced mortality followed similar patterns as previously recorded bleaching severity in the area (see [7,12,34]); mortality was highest in shallow depths (~30% mortality) and among branching corals (~40% mortality), especially *Acropora* spp.

Bleaching severity is considered to vary spatially (e.g., [36,37]) and to be influenced by microhabitat features [38]. Additionally, coral morphology is a factor and branching corals are more susceptible to bleaching and mortality during heat stress in comparison to massive corals [15,39,40]. Branching structures can help optimize light dispersion and reflection that may be important for maintaining colony health at depth or in normal light conditions. However, for colonies in shallow habitats where irradiance is naturally higher, this structure may cause supra-optimal irradiance conditions during heatwaves, accelerating or intensifying bleaching [15,39,40]. In contrast, massive corals have less structural complexity and have been found to have a higher resistance to bleaching, taking longer to bleach and experiencing less mortality [41]. Although branching corals did experience higher bleaching-induced mortality than massive corals in the present study, these mortality levels (approximately 40%) were comparably lower than branching coral mortality in other areas of the Maldives in the aftermath of the 2016 El Niño event. North Malé (Kaafu) Atoll experienced 80% mortality of *Acropora* spp., the dominant branching coral genus in the Maldives [12], and southern sheltered reefs exhibited 75% mortality in branching corals [33]. In a similar pattern, the Chagos Archipelago, situated south of the Maldives, lost 86% of *Acropora* spp. cover (from 2–14%) during the 2015–2016 thermal anomalies, shifting to a community dominated by *Porites* spp. post bleaching [9].

Interestingly, although branching corals are generally considered more susceptible than massive corals on mildly bleaching reefs, this pattern breaks down when reefs are exposed to severe bleaching: Some corals that are normally classified as thermotolerant (e.g., many massive species) can nevertheless become bleached under severe heat stress (e.g., [1]). This may suggest that the Maldives was not hit as hard by this El Niño event as some reefs in other areas around the globe. In support of this, although DHWs in the Maldives reached 11, which exceeds the proposed eight DHW mass bleaching threshold, other reefs around the globe reached much higher DHWs: up to 16 on the GBR and over 20 in the central Pacific [1,5]. Mortality in some of these areas was nearly 100% [42]. The mortality recorded here on Maldivian reef sites still exceeded 50%, and although this is less than what some of the hardest hit reefs globally experienced during the 2015–2016 El Niño, this remains a mass mortality event that can have major consequences for reef recovery.
