Mass coral mortality under local amplification of 2 °C ocean warming

Abstract

A 2 °C increase in global temperature above pre-industrial levels is considered a reasonable target for avoiding the most devastating impacts of anthropogenic climate change. In June 2015, sea surface temperature (SST) of the South China Sea (SCS) increased by 2 °C in response to the developing Pacific El Niño. On its own, this moderate, short-lived warming was unlikely to cause widespread damage to coral reefs in the region, and the coral reef “Bleaching Alert” alarm was not raised. However, on Dongsha Atoll, in the northern SCS, unusually weak winds created low-flow conditions that amplified the 2 °C basin-scale anomaly. Water temperatures on the reef flat, normally indistinguishable from open-ocean SST, exceeded 6 °C above normal summertime levels. Mass coral bleaching quickly ensued, killing 40% of the resident coral community in an event unprecedented in at least the past 40 years. Our findings highlight the risks of 2 °C ocean warming to coral reef ecosystems when global and local processes align to drive intense heating, with devastating consequences.

The perfect calm: Reoccurring mass die-offs on a remote coral atoll

“…Neap tides occurred on May 22, 2014, and June 10, 2015, each preceding a die-off event by 1-2 days (figure 1b). Although the mortality events observed on June 1, 2014, and June 16, 2015 did not occur during neap tides, current speeds on the adjacent reef flat at both of these times were relatively slow (<5 cm s -1 ) due to weak winds and small waves [7] [8], whereas bleaching in the later half of June through July 2015 caused 40% coral mortality [7]. The coral bleaching events were separated from the die-offs that we report here by 1-2 km, and in 2015 the coral mortality occurred 1-2 months after our observations.…”

“…Dongsha Atoll is a ring-shaped barrier reef encircling a lagoon that is flushed by tidal-and wind-driven currents [5] [6]. During May-June of 2014 and 2015, calm conditions stalled water exchange with the surrounding sea [7], affecting the animals living in the reef system. …”

Validation of the remotely sensed nighttime sea surface temperature in the shallow waters at the Dongsha Atoll

“…Visual surveys of the coral ecosystem at various locations in the lagoon of the DSA, including Station D3 (Fig. 1), indicated that, while there was no evidence of bleaching of the corals in prior to early June, extensive bleaching was found by late July (DeCarlo et al 2017). These observations were consistent with the elevated SST bulk in the lagoon in June that was derived from SST sat after the operation calibration algorithm had been applied.…”

“…1, Table 1) by using Onset Corporation HOBO U22 temperature loggers, which had been calibrated in a constant temperature bath to an accuracy of ±0.1°C (DeCarlo et al 2015(DeCarlo et al , 2017. The locations, nominal moored depths relative to the sea surface and the time-periods of the deployment of these loggers are shown in Table 1 and Fig.…”

“…However, the observed warming in the northern South China Sea is based primarily on satellite SST measurements, which may not accurately characterize the thermal stress to which corals on reefs in this region are exposed. Coral bleaching, a physiological response to unusually high water temperature, was evident on the DSA in 1983, 1998, and 2007(Li et al 2000DeCarlo et al 2017), but in situ temperature measurements are not available from that time. In an effort to use SST sat to evaluate the temperature condition and the related thermal stress that the corals at the DSA are actually facing, this study attempts: (1) to evaluate the discrepancy, if any, between SST sat and SST bulk in the shallow waters at the DSA, and (2) if a discrepancy exists, to devise an operational calibration algorithm for minimizing the discrepancy.…”