Cold water effects on corals
Our observations suggest that an ENSO-related cold-water anomaly led to bleaching and death of finger corals. The bleaching period coincided with the arrival of a plume of cold water (15-20oC minimum SST) flowing up the west coast of South America, arriving in the central Galápagos during October 2007. During this time period, corals were exposed to temperatures at or below published cold water thresholds for coral bleaching, including 18oC for up to 34 days and 16oC or less for 2.3 days at Guy Fawkes (15 m depth). Temperatures at Baltra (15 m) were at or below the 18oC threshold for 18.8 days. As expected, temperatures decreased with depth from 6 to 15 m. Longer periods of unusually cold water occurred at the reference site Cuatro Hermanos (where corals are rare) likely related to the presence of stronger upwelling currents relative to those at Guy Fawkes or Rocas Beagle (Witman et al. 2010).
In general, cold-water coral bleaching has received less attention than warm water bleaching, although it can cause extensive coral mortality. For example, (González-Espinosa & Donner 2020) documented cold-water coral bleaching at 14 sites in the Eastern Tropical Pacific alone from 1998 -2017. One of these events was reported from the northern islands of Darwin and Wolf in the Galápagos where bleaching of three coral species (Porites lobata, Pocillopora spp. and Pavona clavus ) occurred in 2007 when SST were 16oC (Glynn 2009, Glynn et al. 2017), which was likely caused by the same La Niña event reported in this study. Given that climate models predict that the frequency of extreme La Niña events and associated cold-water anomalies will increase with climate warming (Cai et al. 2015), our study suggests that more attention should be paid to the consequences of coral bleaching caused by low temperatures.