Coinciding with the news on Friday that the waters surrounding Cayman are the hottest they have been in over a decade, the Department of Environment confirmed significant amounts of coral bleaching on local reefs.
The National Oceanic and Atmospheric Administration’s Coral Reef Watch bleaching alert levels for Cayman reached Alert level 1 at the beginning of September following a steady rise in water temperatures beyond 30 degrees Celsius.
Fed by information from the recently installed ICON monitoring station in Little Cayman, the data from the station are reported and integrated with data collected via satellite.
‘[Sea surface temperatures] at the Caymans are presently running higher than anything we’ve ever seen [in this decade] at our Virtual Station there,’ said NOAA Coral Reef Watch Senior Consultant Alan E. Strong in an email.
Commenting that it was ‘a bit of a worry,’ he also noted: ‘The good news is the fall downturn in [sea surface temperatures] is close at hand.’
Also on Friday, the Department of Environment announced that following up on reports from the diving community as well as the ICON station’s ‘bleaching potential’ alert, DoE staff conducted a rapid assessment of reefs on the north, west and south coasts of Grand Cayman.
The scientific diving team found that nearly all corals in the shallow reefs to about 30 feet in depth showed signs of moderate to severe bleaching, while approximately 80 per cent of corals in the deeper reefs to 120 feet exhibited the early signs of coral bleaching. Bleaching appeared more intense on the north coast although the reasons for this are not fully understood at this stage.
Coral bleaching is a stress related reaction whereby the coral colonies lose their colour and turn white either due to the loss of pigments by microscopic algae living in symbiosis with their coral hosts, or because the algae have been totally expelled. Bleaching is closely associated with sustained elevated water temperatures and UV light and has been linked to global climate change as the world’s oceans heat up.
‘Coral is the basic building block for reefs,’ said DoE Deputy Director for Research and Assessment Tim Austin.
He said reefs are already under many local stresses like pollution, anchor damage and cumulative diving pressure, as well as chemical stresses such as eutrophication, and ocean acidification.
With the hotter water, the threshold of 29.5 degrees Celsius has been reached for the reefs’ temperature tolerance.
‘It is rather distressing news,’ said Mr. Austin.
‘It seems that climate change is the straw that broke the camel’s back.’
When bleaching happens, the corals stop growing, and they become much more susceptible to disease and they may be smothered by algae.
When the hard and soft corals making up the reefs collapse, the marine creatures that live on the reefs lose their habitat.
Corals can recover from less severe bleaching episodes although recovery is variable and in some instances entire reefs have been lost to single bleaching events. The last major bout of bleaching to impact the Cayman’s reefs occurred in 1998 with significant mortality following. Minor bleaching events have been recorded in the warmer summer months with increasing frequency during the last decade.
Mr. Austin explained the importance of coral bleaching events cannot be overstated, as everything that lives on or near a reef depends on coral.
‘Basically we don’t know at this stage how bad it is going to be but if is anything like the 1998 bleaching episode, the potential for mass mortality of corals is very high,’ he said.
‘Of course there would be a lot of knock on effects such as loss of fisheries, loss of sand production, and reductions of the reefs’ ability to protect our coastline,’ he said.
The DoE has a Long Term Coral Reef Monitoring Programme, in place since 1997, to the track the health of Cayman’s reefs. It recently completed an extensive video survey at 55 reef sites around all three islands. Monitoring efforts will be increased over the next few weeks to better quantify and assess the impact of this bleaching event and to determine levels of recovery.
The DoE said it expects the current bleaching episode to increase significantly in severity in the following weeks as water temperatures remain above the threshold.
However, past water temperature data collected since 1996 suggest that local waters start cooling down from mid-October, which hopefully may bring some relief for the heat stressed corals.