Historical Nautical Maps Show Loss of Coral Reefs

Scientists have used detailed nautical maps created by British sailors in the 1700s to study more than two centuries of coral loss in the Florida Keys. They found that over the past 240 years, the region has lost more than half of its coral structures, with some areas, particularly closer to shore, either gone completely or having lost up to 90 percent of their extent.

The researchers published their findings this week in the journal Science Advances.

“We found that reef used to exist in areas that today are not even classified as reef habitat anymore,” co-author John Pandolfi, a palaeoecologist at the University of Queensland, said in a statement.

Cartographers for the British Royal Navy created the detailed historical maps of the Florida Keys in the late 18th century for navigational purposes. They contained “substantial amounts of ecological information,” the study says, “with coral of particular interest as a navigational hazard.”

CLICK IMAGE TO ENLARGE. A nautical map shows the locations of coral reefs in Key West in 1774 compared with today.

CLICK IMAGE TO ENLARGE. A nautical map shows the locations of coral reefs in Key West in 1774 compared with today. McClenachan et al 2017

The team of American and Australian researchers identified 143 coral reefs in charts that spanned from Key Largo to the Marquesas Keys, 20 miles west of Key West. They then compared those reefs to modern benthic habitat data, created largely using satellite images. They found that “just more than half of the historical coral observations are in locations where coral habitat does not exist today,” the scientists wrote. They also found that areas where coral disappeared are now dominated by seagrass and bare ocean floor. The losses were greatest closer inland, particularly in Florida Bay, the area of shallow ocean between the Keys and the Everglades, where nearly 90 percent of the coral is gone.

The scientists said using navigational charts could bolster researchers’ understanding of long-term coral loss in other parts of the ocean. “We tend to focus on known areas where we can measure change. That makes sense. Why would you look for coral where you never knew it was?” said Loren McClenachan, a palaeoecologist at Colby College and lead author of the new study.