Historical nautical maps show coral loss more extensive than previously believed

Researchers used nautical charts produced in the 1770s to help quantify changes in the coral reefs of the Florida Keys over the past 240 years.

Article by Mike Gaworecki.

A team of researchers based in Australia and the United States have used historical nautical maps to determine that coral reef loss in the Florida Keys is much more extensive than previously understood.

The British empire began mapping its overseas territories in the 18th century, and coral reefs in particular were quite thoroughly documented given the danger they posed to wooden-hulled ships. In the process, these imperial cartographers unwittingly provided a source of high-resolution spatial data on coastal areas that, as it turns out, can still be useful today in establishing historical baselines for the extent of coral reefs and assessing changes to those reef systems over the ensuing centuries.

“The degree of biologically relevant information recorded varied by cartographer, but the best of these British maps describes the depth, shape, and color of shallow-water corals and distinguishes them from other hard structures such as rocks,” the authors of a study published in the journal Science Advances earlier this month wrote.

The researchers used nautical charts dating from the 1770s to help quantify changes in the coral reefs of the Florida Keys over the past 240 years.

“The maps were essential to expansion of the British Empire, and luckily for us, they also included a lot of useful ecological information,” according to Benjamin Neal, a postdoctoral researcher at the Bigelow Laboratory for Ocean Sciences in East Boothbay, Maine and a co-author of the study.

Loren McClenachan, a professor at Colby College in Waterville, Maine who led the study, said that, after comparing the historical charts to modern coral maps produced using satellite data, she and her team discovered that, overall, 52 percent of the coral reef habitat mapped by British cartographers in the 18th century no longer exists — and that in some areas, especially nearer to the coastline, coral loss was even more severe: 87.5 percent in Florida Bay, for instance, and 68.8 percent in nearshore patch reefs.

“We found near the shore, entire sections of reef are gone, but in contrast, most coral mapped further from land is still coral reef habitat today,” McClenachan said in a statement.

Then and now: Key West, Florida. Image Credit: Courtesy of the authors; McClenachan et al. (2017). doi:10.1126/sciadv.1603155

The estimates produced by McClenachan and team complement more recent observations of the decline of living coral cover off the coast of the Florida Keys in some important ways. The researchers were able to look at coral reef habitats across a larger geographic area than most modern studies, which tend to focus on the loss of living coral in smaller sections of reef.

The broader timespan and area of study led to some key insights — including the fact that we have greatly underestimated the extent of coral loss in the Florida Keys, which scientists already believed to have been considerable. The researchers point out, however, that their findings do not contradict more recent research into the Florida Keys’ declining coral reef systems.

“[O]ur estimate of 52% loss are in addition to, not contradictory of, previous estimates of decline in coral cover estimated over the scale of decades, such as the 50% loss across the Caribbean basin or the 75 to 80% decline in the Keys,” the researchers write. “These estimates are based on field measurements of live coral since the 1970s on areas of known reef habitat, such as those at Key Largo Dry Rocks, where stony coral declined from 57 to 14% between 1974 and 2000, and at Carysfort Reef, where living coral cover declined by 92% between 1974 and 1999.”

They go on to explain that “modern surveys are typically designed to assess change only within the species’ known, extant range. For species ranging from corals to sea turtles, this approach may overlook spatial loss over longer time frames, resulting in both overly optimistic views of their current conservation status and underestimates of their restoration potential.”

John Pandolfi, a professor at Australia’s University of Queensland and co-author of the study, said that reefs once existed in areas of the Florida Keys that are no longer even classified as reef habitat. “When you add this to the 75 percent loss of living coral in the Keys at that finer scale,” he noted, “the magnitude of change is much greater than anyone thought.”

“We tend to focus on known areas where we can measure change,” McClenachan added. “That makes sense. Why would you look for coral where you never knew it was?”

McClenachan said her team’s findings hold crucial implications for the conservation of what’s left of coral reef systems in the Florida Keys, as they improve our estimates of historical abundance and the full extent of subsequent coral loss and therefore must also alter our aspirations for their recovery.

“In addition, our analysis demonstrates the untapped value of early nautical charts for describing long-term ecological changes for biogenic habitats, such as coral, that otherwise lack written historical records over the scale of centuries,” McClenachan and team write in the study. “In particular, nautical charts produced for other heavily trafficked colonial regions—including Jamaica’s Kingston Harbor and Hong Kong Harbor—have the potential to provide a more complete measure of decline of coral reef systems since European contact.”

Coral reef at the NOAA Florida Keys National Marine Sanctuary. Photo Credit: NOAA.

CITATION

McClenachan, L., O’Connor, G., Neal, B. P., Pandolfi, J. M., & Jackson, J. B. (2017). Ghost reefs: Nautical charts document large spatial scale of coral reef loss over 240 years. Science Advances, 3(9), e1603155. doi:10.1126/sciadv.1603155

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getessy
getessy

ohhh well this is something new

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