Standardization of in situ coral bleaching measurements highlights the variability in responses across genera, morphologies, and regions

Abstract

Marine heatwaves and regional coral bleaching events have become more frequent and severe across the world’s oceans over the last several decades due to global climate change. Observational studies have documented spatiotemporal variation in the responses of reef-building corals to thermal stress within and among taxa across geographic scales. Although many tools exist for predicting, detecting, and quantifying coral bleaching, it remains difficult to compare bleaching severity (e.g., percent cover of bleached surface areas) among studies and across species or regions. For this review, we compiled over 2,100 in situ coral bleaching observations representing 87 reef-building coral genera and 250 species of common morphological groups from a total of 74 peer-reviewed scientific articles, encompassing three broad geographic regions (Atlantic, Indian, and Pacific Oceans). While bleaching severity was found to vary by region, genus, and morphology, we found that both genera and morphologies responded differently to thermal stress across regions. These patterns were complicated by (i) inconsistent methods and response metrics across studies; (ii) differing ecological scales of observations (i.e., individual colony-level vs. population or community-level); and (iii) temporal variability in surveys with respect to the onset of thermal stress and the chronology of bleaching episodes. To improve cross-study comparisons, we recommend that future surveys prioritize measuring bleaching in the same individual coral colonies over time and incorporate the severity and timing of warming into their analyses. By reevaluating and standardizing the ways in which coral bleaching is quantified, researchers will be able to track responses to marine heatwaves with increased rigor, precision, and accuracy.

Corals that survive repeated thermal stress show signs of selection and acclimatization

“…Massive Porites employs a stress tolerant life history strategy [ 25 , 61 , 90 , 91 ] and has a demonstrated ability to acclimatize to repeated heat stress [ 43 , 57 , 92 ]. Massive Porites taxa are generally among the most thermally tolerant corals on Indo-Pacific reefs [ 14 , 31 ], although thermal tolerance relative to other taxa has been shown to decline under severe thermal stress [ 54 ]. The heat tolerance of Porites contrasts with that of faster growing coral taxa such as Acropora and Pocillopora , which have been consistently found to be among the least resistant to thermal stress [ 14 , 45 , 54 , 76 , 93 ].…”

“…While mortality is not an inevitable outcome of coral bleaching [ 28 – 30 ], prolonged bleaching often results in partial or complete mortality of coral colonies. A variety of environmental stressors can cause coral bleaching, but bleaching is most commonly studied in relation to marine heatwaves [ 11 , 31 ]. In this context, bleaching onset is determined primarily by the intensity and duration of heat stress that corals experience [ 32 , 33 ].…”

“…Tracking the fate of individual corals through multiple marine heatwaves is one mechanism to better understand the drivers and prevalence of coral acclimatization to heat stress [ 25 , 59 – 61 ]. Many observational studies of coral bleaching use population-scale metrics to quantify bleaching trends (i.e., percent of colonies bleached [ 31 ]) rather than track the fate of individual coral colonies (but see [ 25 , 38 , 62 , 63 ]). Fate tracking can be hindered by the challenge of relocating and precisely delineating the boundaries of coral colonies in situ .…”