Depth‐Dependent Thermal Stress Around Corals in the Tropical Pacific Ocean

Abstract

Thermally driven bleaching events are a growing concern for reef ecosystems across the tropics.To assess and predict thermal stress impacts on reefs, remotely observed sea surface temperature (SST) commonly is used; however, reef communities typically extend to depths where SST alone may not be an accurate measure of in situ variability. Here nearly two decades of temperature observations (2-to 90-m depth) at three stations around Palau are used to develop an empirical model of temperature variability versus depth based on SST and sea level anomaly (SLA). The technique yields depth-averaged R 2 values >0.88, with SLA predicting fore reef temperatures near the thermocline and SST capturing upper mixed layer temperatures. SLA complements SST by providing a proxy for vertical isotherm displacements driven by local and remote winds on intraseasonal to interannual time scales. Utilizing this concept, thermal stress on corals can be predicted from the surface through the mesophotic zone.Plain Language Summary Coral reefs are often bleached, leading to their death, due to exceedingly warm ocean temperatures. The temperature of the ocean's surface, measured globally by satellites, is often used as an indicator of the temperature and stress that corals experience, but it can only tell us what is happening near the surface. We present nearly two decades of temperature records from the reefs of Palau, an island nation in the tropical Pacific. This array of instruments was maintained by skilled divers routinely going deeper than 90 m. The observations allow us to show that the height of the ocean surface is a strong indicator of how ocean temperatures are changing tens of meters below. This can be coupled with observed sea surface temperature to predict the temperatures experienced by coral reefs living near the surface as well as those living deeper, down through the mesophotic zone, an area between 30 and 150 m deep. The research suggests that significant improvements can be made to how temperature stress on corals is assessed. We also find that thermal stress events can penetrate into the realm of deep mesophotic coral reefs, meaning that this zone might not be refugia for corals living in a warming ocean.

Relative Diazotroph Abundance in Symbiotic Red Sea Corals Decreases With Water Depth

“…While previously investigated in a comparative taxonomic framework (Pogoreutz et al, 2017b), we here provide a spatial context. Both temperature and PAR decreased with depth at rates that are comparable to measurements in other coral reef systems (Frade et al, 2008;Mass et al, 2010;Schramek et al, 2018). Out of the measured parameters, temperature was the environmental variable that best explained the observed patterns in the present study.…”

Nearshore coral growth declining on the Mesoamerican Barrier Reef System

“…Collectively, these results indicate differential growth responses to bleaching between reef environments and years and that the magnitude of acute thermal stress alone is likely not a reliable predictor of coral growth rate. This is likely due to spatial variation in warming across the MBRS, site-and depth-specific thermal variability that can reduce bleaching (Safaie et al, 2018;Schramek, Colin, Merrifield, & Terrill, 2018), as well as the ability of coral populations to modify their bleaching thresholds (Coles & Brown, 2003;Palumbi, Barshis, Traylor-Knowles, & Bay, 2014).…”

Ecological insights from environmental disturbances in mesophotic coral ecosystems

“…Lower temperatures found on MCEs seem to control the lower depth limits of coral communities (Bongaerts et al 2015). However, recent research suggests that the reef zone between 50 and 130 m depth exhibits a large variability in temperature (Smith et al 2016b, Baldwin et al 2018, Schramek et al 2018. In Eilat, our current observations show that ongoing summer temperature variation is causing mass bleaching events in coral species specialized to the mesophotic zone .…”

“…, Schramek et al. ). In Eilat, our current observations show that ongoing summer temperature variation is causing mass bleaching events in coral species specialized to the mesophotic zone (Eyal et al.…”