Rapid Recent Warming of Coral Reefs in the Florida Keys

Abstract

Coral reef decline in the Florida Keys has been well-publicized, controversial, and polarizing owing to debate over the causative agent being climate change versus overfishing. The recurrence of mass bleaching in 2014, the sixth event since 1987, prompted a reanalysis of temperature data. The summer and winter of 2014 were the warmest on record. The oldest known in-situ temperature record of any coral reef is from Hens and Chickens Reef (H&C) in the Florida Keys, which showed significant warming from 1975-2014. The average number of days ≥31.5 and 32(o)C per year increased 2670% and 2560%, respectively, from the mid-1990 s to present relative to the previous 20 years. In every year after 1992 and 1994, maximum daily average temperatures exceeded 30.5 and 31°C, respectively. From 1975-1994, temperatures were <31 °C in 61% of years, and in 44% of the years prior to 1992 temperatures were <30.5 °C. The measured rate of warming predicts the start of annual bleaching between 2020 and 2034, sooner than expected from climate models and satellite-based sea temperatures. These data show that thermal stress is increasing and occurring on a near-annual basis on Florida Keys reefs due to ocean warming from climate change.

Figure 1. Coral bleaching in the Florida Keys during the summer of 2014.

(a) Large colony of Orbicella (formerly Montastraea) faveolata completely bleached. Colony pictured is several meters in height and diameter. (b) Bleached reef landscape showing multiple species of affected coral. (c) Black band disease on colony of pale Montastraea cavernosa. The incidence of coral disease increases during coral bleaching events. Photographs taken by author at Cheeca Rocks, Florida Keys in September 2014.

Figure 2. Sites in Florida Keys where temperature measurements were obtained.

Open circles are offshore sites, closed circles are inshore sites, and asterisk is Hens and Chickens. Abbreviations, MLRF, Molasses Reef (25.012°N, 80.376°W); UKO, Upper Keys Offshore (24.947^oN, 80.502^oW); H&C, Hens and Chickens Reef (24.933°N, 80.549°W); UKI, Upper Keys Inshore (24.939^oN, 80.563^oW); MKO, Middle Keys Offshore (24.767°N, 80.753°W); MKI, Middle Keys Inshore (24.812^oN, 80.761^oW); LKO, Lower Keys Offshore (24.551^oN, 81.402^oW); LKI, Lower Keys Inshore (24.597^oN, 81.455^oW). The map was taken from ArcGIS base layers and modified in Adobe Illustrator.

Figure 3. Time-series of running 30-day mean seawater temperature from Molasses Reef since 1988 plotted against climatology.

Periods of known thermal stress are indicated. The MLRF threshold (red dashed line) is the coral bleaching threshold determined previously based on mass bleaching events in 1997, 1998, and 2005 (ref. 15). The 1990 threshold (blue dashed line) corresponds to temperatures that occurred when Milleporid hydrozoan corals bleached throughout the Florida Keys. Milleporid corals are known to be thermally sensitive, therefore this value may represent a threshold for these heat-sensitive corals.

Figure 4

Number of days ≥30.5 (a), 31 (b), and 31.5 °C (c) at Hens and Chickens Reef plotted by year from 1975-2014. Solid circles are Hens and Chickens data (H&C), open circles are the points predicted by the regression with Snake Creek data (SC predicted). Poisson linear regression lines shown for H&C (solid line), H&C plus SC predicted (dashed line) and H&C with ENSO years removed (orange line).