Large-amplitude internal waves benefit corals during thermal stress

Abstract

Tropical scleractinian corals are particularly vulnerable to global warming as elevated sea surface temperatures (SSTs) disrupt the delicate balance between the coral host and their algal endosymbionts, leading to symbiont expulsion, mass bleaching and mortality. While satellite sensing of SST has proved a reliable predictor of coral bleaching at the regional scale, there are large deviations in bleaching severity and mortality on the local scale that are poorly understood. Here, we show that internal waves play a major role in explaining local coral bleaching and mortality patterns in the Andaman Sea. Despite a severe region-wide SST anomaly in May 2010, frequent upslope intrusions of cold sub-pycnocline waters due to breaking large-amplitude internal waves (LAIW) mitigated coral bleaching and mortality in shallow waters. In LAIW-sheltered waters, by contrast, bleaching-susceptible species suffered severe bleaching and total mortality. These findings suggest that LAIW benefit coral reefs during thermal stress and provide local refugia for bleaching-susceptible corals. LAIW are ubiquitous in tropical stratified waters and their swash zones may thus be important conservation areas for the maintenance of coral diversity in a warming climate. Taking LAIW into account can significantly improve coral bleaching predictions and provide a valuable tool for coral reef conservation and management.

Large-amplitude internal waves sustain coral health during thermal stress

“…Our study supports earlier suggestions that internal waves may alleviate the deleterious effects of prolonged periods of anomalously high sea surface temperatures (Sheppard 2009;Phongsuwan and Chansang 2012;Storlazzi et al 2013). It gives an explanation at an eco-physiological and organismic level for the spatial differences in coral community bleaching responses to LAIW (Wall et al 2014). …”

“…As a measure of the intensity and extent of heat stress and cooling intensity, temperature anomalies were calculated as cumulative degree heating weeks (DHW; cf. Wall et al 2014) as this is the most commonly used heat stress index for bleaching events. The higher resolution in days of heating and cooling (DD in°C d; Schmidt et al 2012) was used for a more precise description of the actual temperature conditions especially due to the short-term cold water intrusions.…”

“…The higher resolution in days of heating and cooling (DD in°C d; Schmidt et al 2012) was used for a more precise description of the actual temperature conditions especially due to the short-term cold water intrusions. Temperature values above the NOAA bleaching threshold of 30.62°C (Wall et al 2014) were considered as heating (DHW and degree days heating, DDH). Temperature values below the daily running mode before and after the daily mode exceeded the bleaching threshold were considered as cooling (degree days cooling: DDC).…”

“…In spite of the severity of the 2010 event, some reefs were less affected, allegedly due to stronger currents or the presence of cold water (Phongsuwan and Chansang 2012) as suggested by earlier observations of high temperature variability in Indian Ocean (Sheppard 2009) and Andaman Sea coral reefs (Roder et al 2010(Roder et al , 2011Schmidt et al 2012;Wall et al 2012). Wall et al (2014), linking coral community data on bleaching and mortality with heat stress and LAIW, suggested that LAIW benefit corals under thermal stress: they observed that LAIW-exposed coral communities across the Thai Andaman shelf showed reduced bleaching and mortality, while LAIW-sheltered communities suffered intense bleaching and total mortality. Although the study highlighted the importance of species composition, notably the proportion of bleaching-susceptible versus bleaching-resistant species, a mechanistic understanding of the bleaching and mortality response at the organism level is still lacking.…”

“…Monthly mean temperatures exceeded the NOAA coral bleaching threshold for the area (30.62°C) over a period of 3 months (April to June; Wall et al 2014). Severe bleaching and up to 80% mortality marked the 2010 event as the strongest so far on record (Brown and Phongsuwan 2012).…”