The past century of coral bleaching in the Saudi Arabian central Red Sea

Abstract

Accurate knowledge of the spatial and temporal patterns of coral bleaching is essential both for understanding how coral reef ecosystems are changing today and forecasting their future states. Yet, in many regions of the world, the history of bleaching is poorly known, especially prior to the late 20th century. Here, I use the information preserved within skeleton cores of long-lived Porites corals to reconstruct the past century of bleaching events in the Saudi Arabian central Red Sea. In these cores, skeletal “stress bands”—indicative of past bleaching—captured known bleaching events that occurred in 1998 and 2010, but also revealed evidence of previously unknown bleaching events in 1931, 1978, and 1982. However, these earlier events affected a significantly lesser proportion of corals than 1998 and 2010. Therefore, coral bleaching may have occurred in the central Red Sea earlier than previously recognized, but the frequency and severity of bleaching events since 1998 on nearshore reefs is unprecedented over the past century. Conversely, corals living on mid- to outer-shelf reefs have not been equally susceptible to bleaching as their nearshore counterparts, which was evident in that stress bands were five times more prevalent nearshore. Whether this pattern of susceptible nearshore reefs and resistant outer-shelf reefs continues in the future remains a key question in forecasting coral reef futures in this region.

Forecast Coral Bleaching by Machine Learnings of Remotely Sensed Geospatial Data

“…The existing works can be categorized by factors being considered into two main groups. The former consists of those that take solely SST [10][11][12][13][14][15][16] into account, while those in the latter combine it with other factors (e.g., wind speed and water turbidity, etc.) in their analyses [30,31].…”

“…in their analyses [30,31]. However, based on the approach taken, these groups can be further divided into those detecting and monitoring corals bleaching by using satellite image processing [32,33], by statistical analyses of SST, recorded in the photos archived in the National Oceanic and Atmospheric Administration (NOAA) repository [12][13][14][15][16], and by different ML algorithms, namely, random forest [30] and Bayesian [16]. Critical discussion and detailed insights into these methods are given as follow.…”

“…Conventionally, DeCarlo presented the result of coral bleaching correlated to only higher SST. Therefore, a framework based on advanced statistical approach was proposed to improve the accuracy of forecast [15]. However, detailed quantitative evaluation on its accuracy was not presented.…”

“…It is defined as a phenomenon that corals become white or paler due to the loss of Zooxanthellae, which in turn is resulted from too unsuitable states for seaweed to survive. According to the related studies, it was revealed that, on one hand, the factors causing coral bleaching include exceedingly high sea temperatures [10][11][12][13][14][15][16], salinity [17][18][19], turbidity [20][21][22], and human doings, e.g., releasing wastes into seas, littering on beaches, and discharging wastewater. On the other hand, the factor that reduces sea temperatures, and hence helping corals to recover from bleaching and then to survive is wind speed [23][24][25].…”

Assessment of temperature optimum signatures of corals at both latitudinal extremes of the Red Sea

“…The central and southern Red Sea have experienced sporadic bleaching events (e.g. 1998, 2010 and 2015 in Saudi Arabia; Monroe et al ., 2018 ; Decarlo, 2020 ), with summer sea surface temperatures (SSTs) reaching up to 33–34°C ( Sawall et al ., 2014 ). Yet, bleaching has not been observed in the northern Red Sea and Gulf of Aqaba (GoA), despite a 0.4–0.5°C increase in summer SSTs per decade over the past 30 years ( Osman et al ., 2018 ) and multiple thermal anomalies.…”