Methods
Subtidal temperatures were recorded at 10-minute intervals during the La
Niña period (NOAA/NESDIS,
https://origin.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/
ONI_v5.php) by Onset TidbitTM loggers placed at 6 and
15 m depth at two of the three study sites: Baltra (00 24.705 S, 090
16.482 W) and Guy Fawkes (00 29.964 S, 090 30.773 W). No temperature
data is available for the third study site Rocas Beagle (00 24. 797 S,
90 37.685 W), due to logger malfunctioning at this site. However,
temperatures were also recorded at 15 m depth at an additional reference
site, Cuatro Hermanos (00 84.787 S, 090 74.908 W), located 40 km
southwest of Guy Fawkes to provide insight into the spatial scale of the
cold-water event. Bleaching of Pocillopora corals during cold
water events has been recorded at temperature thresholds of
19oC in the Gulf of California (Paz-García et al.
2012), while temperatures of 18.5-19oC stressedP. verrucosa in lab experiments (Rodríguez-Troncoso et al. 2013).
Sustained cold water temperatures well-below reported thresholds of
16oC (11 and 12oC, respectively)
resulted in widespread bleaching and mortality of many scleractinian
coral species along the Florida reef tract (Lirman et al. 2011, Kemp et
al. 2011). We thus considered the 2007-2008 Galápagos temperature
records for the amount of time (i.e., hours and days) that temperatures
were less than or equal to one or both of two cold-water thresholds
identified in the literature (16oC and
18oC), which serve as conservative indicators of the
low temperature stress to Pocillopora colonies (Table 1). In
addition to our temperature loggers, we also obtained information
regarding the cold-water plume from the NOAA Coral Reef Watch (Fig. S2,
extracted from NOAA/NESDIS, https://coralreefwatch.noaa.gov).
As part of our bi-annual biodiversity monitoring surveys we observed
substantial bleaching of finger (Pocilloporidae) and massive (Poritidae,
Pavonidae) corals in the central Galápagos Archipelago. This coral
bleaching occurred during a 5-7-month window, between June 2007 (when
bleaching was not present) and January 2008 (when it was first observed)
(Figure S1). To address our research questions, we first counted using
SCUBA the number of finger coral colonies that were alive, bleached or
dead in January 2008 in 2-3 large plots (30 x 5 m) at 6-12 m depth at
each of the three aforementioned study sites. In July 2008, we marked
dead and live (recovered) finger coral colonies with numbered plastic
tags at 4-15 m depth across larger areas (approximately 600
m2) than the initial survey plots at these sites.
Tagged corals (potentially including P. verrucosa , P.
capitata , P. damicornis , P. eydouxi , P. meandrina)were aggregated at the genus level (Pocillopora ) due to the
difficulty of identifying them to species, as the coral calcium
carbonate structure of many dead corals had degraded. A total of 96
discrete Pocillopora coral heads were tagged and successfully
tracked across all time-series surveys at the three sites (Baltra, n=29,
Guy Fawkes, n=31, and Rocas Beagle, n=36).
We surveyed the fish and mobile macroinvertebrates (1.0 cm total body
length or larger) associated with the tagged corals in July 2008,
January 2009, July 2009, and February 2010. A diver slowly approached
each tagged coral (at a short distance) and first identified and counted
all visible fishes sheltering within, swimming in and out of, or
hovering over the coral head. We only counted fishes within 10 cm of
coral branches, with the exception of juvenile damselfish, which were
included in fish counts but were often slightly further from coral
heads. The diver then approached the coral more closely and identified
all cryptic fishes (e.g., blennies and gobies) and mobile
macroinvertebrates (e.g., crustaceans, echinoderms, and gastropods). One
observer (O. K. Rhoades) conducted all the in situ counts to
maintain the consistency of methodology and species identifications.
Gastropods were identified with the assistance of local experts at the
Charles Darwin Research Station. We did not census mobile invertebrates
smaller than 1 cm total body length, nor did we survey sessile
invertebrates or algae that recruited onto the dead corals.
The size of each tagged coral head was measured as the 2D surface area
from an overhead photograph of the colony taken with a quadrapod camera
framer (Witman 1985) designed to hold a Nikonos camera, a 15 mm lens
corrected for barrel distortion and two strobes. Colony photographs were
taken perpendicular to the substrate to obtain a standard overhead view
in a 0.25 m2 photo quadrat. We also surveyed the
maximum branch length per coral head. Surveys of the communities
associated with each finger coral head were conducted at a separate time
and during a different dive from photographing of coral area (to avoid
potentially disturbing and displacing fishes from corals with
photography equipment and additional SCUBA divers), and at midday (to
avoid confounding diurnal differences in associated community
composition with variation in associated composition over time), on a
single day at each time point to avoid recounting fish moving among
adjacent coral habitats.
During the initial bleaching survey in January 2008, and on the four
subsequent sampling periods (July 2008 - February 2010), we recorded the
status of tagged corals as live if they had at least 50% cover of live
tissue. For dead corals, we recorded if they had eroded, broken apart
into rubble, or completely disappeared. A final survey was conducted in
July 2011 at all three sites by J. Witman to only determine the
persistence of the tagged, dead corals.