When the British Admiralty charted the Florida Keys between 1773 and 1775, their surveyor, George Gauld, noted coral locations as navigational hazards. Now researchers are using these 18th-century maps to study coral reef loss, with results indicating a staggering disappearance of reefs, especially along the shore.
“Ghost reefs: Nautical charts document large spatial scale of coral reef loss over 240 years” was published Wednesday in Science Advances, the publication of the American Association for the Advancement of Science (AAAS). Its team was led by Loren McClenachan, assistant professor of environmental studies at Colby College, with contributions from researchers at Columbia University, Bigelow Laboratory for Ocean Sciences, University of Queensland, Smithsonian Institution, and University of California, San Diego. They examined high-resolution images of the 18th-century charts from the Admiralty Library and Archive of the UK Hydrographic Office. The mapped coral was surveyed visually and with lead lines that retrieved samples.
Two charts with 143 coral observations ranging from Key Largo to the Marquesas Keys were compared to contemporary data from the Millennium Coral Reef Mapping Project, the Benthic Habitats South Florida Map, and the Unified Florida Coral Reef Tract Map. “When we compared historical observations to benthic habitat data, we estimate a 52% (SE, 6.4%) loss in the occurrence of corals in the Florida Keys over 240 years,” the researchers write. “That is, just more than half of the historical coral observations are in locations where coral habitat does not exist today.”
Near the shore, the loss was most dire, with declines of 87.5% and 68.8%. Unlike the offshore reefs, where the effect of acidification is more evident, these nearshore losses could be attributed to the draining of the Everglades and other water use changes. The researchers note that nautical charts for “other heavily trafficked colonial regions — including Jamaica’s Kingston Harbor and Hong Kong Harbor” could also measure the “decline of coral reef systems since European contact.” The approach is similar to scientists now utilizing historic photographs to study glacial ice, and 19th-century maps to monitor forest trends in the United States.
As the Washington Post reported, the 18th-century maps are recognized as the oldest known records of these reefs. They are fallible, and there’s no way to know what amount of their data was witnessed firsthand. Yet they are an important record through which to understand the impact of climate change and human development on the Florida coast and its Keys. Particularly with Hurricane Irma barreling towards the state, it is worth noting how essential reefs are as a storm buffer, in addition to their necessity for a biodiverse ocean.
“Ghost reefs: Nautical charts document large spatial scale of coral reef loss over 240 years” is readable online at Science Advances.