Annual Thermal Stress Increases a Soft Coral's Susceptibility to Bleaching

Abstract

Like scleractinian corals, soft corals contain photosymbionts (Family Symbiodiniaceae) that provide energy for the host. Recent thermal events have resulted in soft coral bleaching in four of five years on Guam, where they dominated back-reef communities. Soft coral bleaching was examined in Sinularia maxima, S. polydactyla, and their hybrid S. maxima x polydactyla. Results from annual field surveys indicated that S. maxima and the hybrid were more susceptible to bleaching than S. polydactyla, and this was related to differences in their Symbiodiniaceae communities in 2016 and 2017. The photosymbionts of S. polydactyla were apparently more stress tolerant and maintained higher photosynthetic potential through three years of bleaching, in contrast to the other species that exhibited a decline in photosynthetic potential after the first year of bleaching. Nonetheless, by the 2017 bleaching event all soft coral populations exhibited significant bleaching-mediated declines and loss of photosynthetic efficiency suggesting a declining resiliency to annual thermal stress events. While S. polydactyla initially looked to succeed the other species as the dominant space occupying soft coral on Guam back-reefs, cumulative bleaching events ultimately turned this "winner" into a "loser", suggesting the trajectory for coral reefs is towards continued loss of structure and function.

Figure 1

Sea surface temperatures for Guam. Plotted are the mean monthly SSTs (°C) from March 2013 to December 2017 using NOAA Coral Reef Watch products. The night-time Advanced Very High Resolution Radiometer (AVHRR) SST data provided near real-time 5-km thermal profiles for the study site. The dashed line equates to the regional bleaching threshold value, defined by NOAA CRW as 1 °C above the mean temperature of the warmest month in the regional seasonal cycle.

Figure 2

Representative bleaching in Sinularia spp. Presented are: (A) “complete” bleaching in S. maxima, and (B) “partial” bleaching in S. polydactyla. The bleached surface area of the S. maxima colonies (photo area = 1 m²) are >70%, while the bleached surface area of S. polydactyla colonies (photo area = 10 cm²) are <1%. Completely bleached soft corals almost always resulted in colony mortality, while partially bleached soft corals rapidly recovered. Note: the hybrid soft coral S. maxima x polydactyla (not shown) exhibits a bleaching pattern similar to S. maxima.

Figure 3

Soft coral populations in Guam. Histograms represent the mean ± 1SE (A) percent of each soft coral population that was bleached between April 2013 and December 2017, and (B) percent cover of each soft coral population at PBH Guam during that period. Different letters above histograms indicate significant differences by ANOVA. Arrows (↓) correspond to approximate periods when sea surface temperature exceeded the bleaching threshold. See Fig. 1 for approximate dates of the bleaching events each year (note: bleaching threshold line), and Table S1 for results of post hoc analyses on significant two-way ANOVAs. N/A indicates no bleached colonies in April 2013.

Figure 4

Soft coral populations in Guam. Histograms represent the mean ± 1SE (A) percent of each soft coral colony [n = 15] that was bleached between April 2013 and December 2017, and (B) average quantum yield of each soft coral colony [n = 15] at PBH Guam during that period. Different letters above histograms indicate significant differences by ANOVA. Arrows (↓) correspond to approximate periods when sea surface temperature exceeded the bleaching threshold. See Fig. 1 for approximate dates of the bleaching events each year (note: bleaching threshold line), and Table S2 for results of post hoc analyses on significant two-way ANOVAs. N/A indicates no bleached colonies in April 2013, and no PAM data in April 2016.

Figure 5

Soft coral photosymbiont communities. Nonmetric MDS plot of the photosymbiont composition across the soft corals Sinularia maxima (Sm), S. polydactyla (Sp), and their hybrid (SH) in each of two bleaching years, 2016 (white symbols) and 2017 (grey symbols), based on normalized read abundances of each Symbiodiniaceae variant recovered from MiSeq (Table S4). Data indicate that symbiont communities are more similar in S. maxima and the hybrid, than in S. polydactyla (Table S5), and that this soft coral’s photosymbiont community change coincides with reduced resilience.