Reef‐Scale‐Dependent Response of Massive Porites Corals From the Central Indian Ocean to Prolonged Thermal Stress: Evidence From Coral Sr/Ca Measurements

Abstract

Here we present coral Sr/Ca data of biweekly resolution from three modern coral cores drilled from living Porites corals from two different reef settings at Chagos (tropical Indian Ocean). Chagos lies at the eastern margin of the Seychelles‐Chagos thermocline ridge and features open ocean upwelling. In situ temperatures have been recorded by temperature loggers since 2006. High‐resolution satellite temperatures closely track the logger data. Two cores were collected from a patch reef in the lagoon of Peros Banhos, a site characterized by high mean temperatures and low‐temperature variability. An open ocean core was collected at the outer reef slope of Diego Garcia, which experiences larger temperature fluctuations related to open ocean upwelling. The open ocean core shows clear seasonal cycles in Sr/Ca that closely track the satellite temperatures. The Sr/Ca records from the two lagoon corals show good reproducibility. Between 2007 and 2010, Sr/Ca tracks satellite temperatures. However, between 2003 and 2006 the Sr/Ca curves are almost flat and annual coral growth rates are low. During these years, warm sea surface temperatures and coral bleaching have been observed at Chagos. It is likely that the observed reduction in coral growth rates is a response to prolonged warming events, adding to the growing evidence that seemingly healthy and surviving corals are affected by thermal stress. Further, the high correlation between the Sr/Ca records of the two lagoon cores suggests that this effect is not limited to single coral colonies but may affect the entire reef setting.

ENSO and internal sea surface temperature variability in the tropical Indian Ocean since the Maunder Minimum

“…Coral SST proxy of the central Indian Ocean record similar, but higher anomalies during El Niño events compared to the satellite composites ( Fig. 7), which may reflect the greater sensitivity of the corals to reef-scale temperatures (Leupold et al, 2019) or the different time periods covered by these records (only two El Niño events in the AVHRR record overlap with the coral data). The coral composite records of the 17-18th century show higher anomalies than the coral composites of the 19-20th century (Fig.…”

“…Observations indicate that some upwelling events in the central Indian Ocean are not forced by large-scale ENSO or IOD variability but associated with cyclonic wind stress curls in the southern tropical Indian Ocean (Dilmahamod et al, 2016;Hermes & Reason, 2009). Such an upwelling event occurred in August 2002 and was found in both the coral and satellite SST records at Chagos (see Leupold et al, 2019). To investigate the effect of these negative anomaly events on the La Niña composites, the 19-20th century composites were split up into composites of La Niña events and other negative anomaly events, which are not related to La Niña.…”

“…High-resolution SST data of the AVHRR satellite product (Casey et al, 2010) reveal different mean SST and seasonality at Chagos depending on the reef setting (Leupold et al, 2019;Fig. 3).…”

“…All coral Sr/Ca records were centered, i.e. normalized with respect to their mean values (Pfeiffer et al, 2009) and translated into SST using a temperature dependence of -0.06 mmol/mol per 1°C for Porites corals at Chagos (Leupold et al, 2019;Pfeiffer et al, 2009). The values are shown in Figure 4.…”

“…We used the AVHRR satellite SST (Casey et al, 2010) averaged over entire Chagos (4-8° S; 70-74° W). All SST anomalies were generated by subtracting the long-term monthly mean from each monthly mean SST value and converting coral Sr/Ca into SST using a temperature dependence of -0.06 mmol/mol per 1°C (see Leupold et al, 2019).…”