Coral bleaching and recovery from 2016 to 2017 at East and West Flower Garden Banks, Gulf of Mexico

Michelle A Johnston,Marissa F Nuttall,Ryan J Eckert,Raven D Blakeway,George P Schmahl,Emma L Hickerson,Travis K Sterne

doi:10.1007/s00338-019-01788-7

Abstract

East Flower Garden Bank (EFGB) and West Flower Garden Bank (WFGB), part of Flower Garden Banks National Marine Sanctuary (FGBNMS) in the northwestern Gulf of Mexico, support tropical coral reefs that exhibit over 50% living coral cover. These reefs have been monitored annually since 1989, and in 2016 were exposed to higher than normal seawater temperatures leading to a severe bleaching event. Corals at EFGB and WFGB showed no signs of bleaching until September 2016, occurring later in the year compared to other reefs in the Caribbean region. Coral bleaching and subsequent recovery at each bank were documented through a time series of repetitive photographs within previously established long-term monitoring stations. Preceding the event, mean live coral cover within monitoring stations was collectively 64 ± 2%. Prior to signs of bleaching from July to September 2016, seawater temperatures on the reef were above 30 °C for a total of 36 d at EFGB and 21 d at WFGB. By October 2016, 67 ± 5% of the coral cover within EFGB monitoring stations and 25 ± 3% within WFGB monitoring stations exhibited signs of bleaching or paling stress, with dissimilarities in the amount of bleaching most likely due to significant differences in thermal profiles between banks. Significantly increasing long-term trends for daily mean seawater temperature indicate that temperatures on the banks have become warmer over time, and calculated bleaching threshold curves suggest that more than 50 d above 29.5 °C would initiate a bleaching year at EFGB and WFGB. Even though recovery within monitoring stations at both banks was documented with no significant declines in mean coral cover from 2016 to 2017 (64% and 62%, respectively), it is likely FGBNMS will be subject to additional and more frequent bleaching events in the future as ocean temperatures continue to rise.

Highlights

East Flower Garden Bank (EFGB) and West Flower Garden Bank (WFGB), positioned approximately 20 km apart, are two of the three banks containing well-developed coral reefs that presently comprise Flower Garden Banks National Marine Sanctuary (FGBNMS) located in the northwestern Gulf of Mexico (Fig. 1)
While coral bleaching was observed at both banks in 2016, significant dissimilarities in the amount of bleaching at EFGB compared to WFGB were likely due to the significant differences in seawater temperatures at depth surrounding the banks (Fig. 2)
Bleaching was less in the deep photostations, as seawater temperatures at these deeper depths are usually cooler along the sloping shelf than the central portion of the reef where the shallow photostations are located; the 40 m HOBO at WFGB was anomalous in that it logged more days above 30 °C than the WFGB 30 m HOBO

Summary

Introduction

East Flower Garden Bank (EFGB) and West Flower Garden Bank (WFGB), positioned approximately 20 km apart, are two of the three banks containing well-developed coral reefs that presently comprise Flower Garden Banks National Marine Sanctuary (FGBNMS) located in the northwestern Gulf of Mexico (Fig. 1). Bathymetric map insets display 10,000 m2 long-term monitoring (LTM) study sites, deep photostation sites, and water quality datasonde (SBE 37) locations at both banks several diverse habitats, with the shallow coral reef cap (16–46 m) harboring tropical hermatypic coral species (Bright et al 1985; Schmahl et al 2008; Johnston et al 2016). Shallow water octocorals are absent and scleractinian corals of the genus Acropora are exceedingly rare, likely due to the banks being at the northernmost latitudinal limit of the coral distribution range in the Gulf of Mexico (Bright et al 1985; Aronson et al 2005)

Methods

Results

Conclusion