Written by Tim Bateman
Increased sea surface temperature is the primary threat to coral reefs worldwide. It is not enough, however, to acknowledge this statement and just move on. Anomalously high-temperature events are becoming more prevalent as anthropogenic climate change progresses, but there are different types of thermal events that produce distinct responses in reef corals and must be characterized separately. Traditionally, bleaching events are measured by degree heating weeks (DHWs) calculated as the amount of time above the mean monthly maximum temperature over a twelve-week period. Above 4°C heating weeks bleaching is expected with the classical response of corals losing algal symbionts from their tissues and the eventual recovery or death of the coral over weeks to months, and above 8°C weeks extensive bleaching is expected. Degree heating weeks have not always proven accurate implying differences in thermal stress events and how corals respond to them. Indeed, there have been recorded bleaching events with fewer than 4°C weeks, while there have also been reefs with little to no bleaching recorded after > 15°C weeks. Specifically, DHWs fail to characterize extreme acute thermal events that can still cause mass coral bleaching, especially on small spatial scales.
Marine heatwaves (MHWs) are thermal events that can occur on much shorter timescales and compound the effects of elevated sea surface temperatures causing rapid tissue necrosis and total colony mortality in a matter of hours to days. Marine heatwaves represent the most extreme type of thermal stress event that reef ecosystems experience and are currently defined as a period in which the water temperature is above the 90th percentile for five or more days. Immediately, this allows thermal anomalies to be characterized on shorter timescales than degree heating weeks, which span a twelve-week characterization period. For example, extreme heating of Dongsha Atoll in 2015 resulted in the mortality of 40% of the reef corals, and temperatures soared to 6°C above the climatological mean, but it did not register on the DHW scale because of the short-lived nature of the event. Marine heatwaves do not always translate into a meaningful value on the DHW scale but must be considered in order to accurately compare bleaching events.
Marine heatwaves are often caused by regional warming in combination with local weather patterns that result in rapid heat accumulation in the marine environment. On coral reefs, the abiotic factors that contribute to the formation of MHWs are tidal cycles, calm winds, reduced cloud cover, and low water flow. Because these factors are the result of local drivers, hot spots can form where MHWs are more commonly subjecting reefs to repeated extreme thermal stress. Additionally, the smaller spatial scale of these drivers can make the prediction and detection of MHWs by satellites near impossible, furthering the difficulty of quantifying their impacts.
Reefs within these hotspots experience ecological effects distinct from reefs that experience traditionally elevated temperature events. For example, massive corals that are typically considered thermally tolerant experience severe bleaching and mortality during MHWs, but during milder events of 4-8°C heating weeks, these corals show no signs of bleaching. This is likely because MHWs overwhelm the thermal coping mechanisms of tolerant corals that usually maintain function during milder temperature anomalies. The altered response of tolerant corals changes the outcome for organisms that are expected to be winners and losers when exposed to more traditional thermal stress events. On a reef-wide scale, this can change the entire benthic community with significant ramifications for the ecosystem functions these corals provide, causing negative impacts long after the MHW has passed.
With these events becoming more prevalent as climate change progresses, it is necessary not only to continue to study how they impact reefs mechanistically but also to factor these events into climate forecasts and projections of coral reef systems moving forward.
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