Abstract
Annual coral bleaching events, which are predicted to occur as early as the next decade in the Florida Keys, are expected to cause catastrophic coral mortality. Despite this, there is little field data on how Caribbean coral communities respond to annual thermal stress events. At Cheeca Rocks, an inshore patch reef near Islamorada, FL, the condition of 4234 coral colonies was followed over 2 yr of subsequent bleaching in 2014 and 2015, the two hottest summers on record for the Florida Keys. In 2014, this site experienced 7.7 degree heating weeks (DHW) and as a result 38.0% of corals bleached and an additional 36.6% were pale or partially bleached. In situ temperatures in summer of 2015 were even warmer, with the site experiencing 9.5 DHW. Despite the increased thermal stress in 2015, only 12.1% of corals were bleached in 2015, which was 3.1 times less than 2014. Partial mortality dropped from 17.6% of surveyed corals to 4.3% between 2014 and 2015, and total colony mortality declined from 3.4 to 1.9% between years. Total colony mortality was low over both years of coral bleaching with 94.7% of colonies surviving from 2014 to 2016. The reduction in bleaching severity and coral mortality associated with a second stronger thermal anomaly provides evidence that the response of Caribbean coral communities to annual bleaching is not strictly temperature dose dependent and that acclimatization responses may be possible even with short recovery periods. Whether the results from Cheeca Rocks represent an aberration or a true resilience potential is the subject of ongoing research.
Highlights
Annual coral bleaching is the greatest future threat to global coral reef health
At Cheeca Rocks, an inshore patch reef near Islamorada, FL, the condition of 4234 coral colonies was followed over 2 yr of subsequent bleaching in 2014 and 2015, the two hottest summers on record for the Florida Keys
Temperatures at Cheeca Rocks generally track the trends observed at Molasses Reef, suggesting that 2014 and 2015 were likely extreme events at Cheeca Rocks (Fig. 3b)
Summary
Annual coral bleaching is the greatest future threat to global coral reef health. Coral bleaching events have become more common and more severe as ocean temperatures have increased due to global climate change and are predicted to intensify over time Annual bleaching is generally predicted to occur by 2050 for all reefs globally and could occur as early as 2020 for the Florida Keys (Manzello 2015). Despite these concerns, coral bleaching events can have a range of potential outcomes. Coral growth and reproductive output can be suppressed for years after bleaching (Baird and Marshall 2002; Cantin and Lough 2014; Levitan et al 2014)
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