Wednesday, May 10, 2017

Hawaii's reefs are eroding, and eroding faster than elsewhere

The threat to Hawai`i’s reefs and coastlines from a changing climate keep growing.

The latest data point is actual evidence that reefs are eroding off Hawaiian shores, and eroding faster than other reefs studied. 

That’s on top of coral bleaching from superwarm water, a huricane season that now starts in May instead of mid-summer, and retreating shorelines due to rising sea levels.

A team of researchers went to five coastal locations in Hawai`i, Florida and the Caribbean, and found that the seafloor was eroding in all five locations. The Hawaiian location was off Maui.

They found the reefs eroding so severely that the water depth increased—potentially meaning that coastal protection that reefs provide was at risk.

“Regional-scale mean elevation and volume losses were observed at all five study sites and in 77% of the 60 individual habitats that we examined across all study sites,” the authors said. The researchers included Kimberly K. Yates, David G. Zawada and Nathan A. Smiley from the U.S. Geological Survey and and Ginger Tiling-Range from Cherokee Nation Technologies.

As reefs eroded, the sea floor got effectively lower—sometimes by as much as 2 to 3 feet in Hawai`i.over a few decades. The reefs off Maui were losing more than 20 millimeters per year, compared to two to five millimeters per year in Florida and the Caribbean. For Maui, that's nearly an inch a year.

“Current water depths have increased to levels not predicted until near the year 2100, placing these ecosystems and nearby communities at elevated and accelerating risk to coastal hazards,” they wrote.

“These greater losses may be caused by higher sediment export rates due to a combination of higher wave energy, physical erosion, and a narrow shallow shelf surrounding the island allowing sediment to be more easily transported offshore into deep water, as has been observed in other high-energy reef environments,” the paper says.

The team studied bottoms of many kinds from 7 to 50 feet in depth, including coral and hard bottom, sandy bottom, patch reefs, rubble and mud, coral pavement and more.

The only places off Maui that showed increases were areas with mud and rubble bottoms—perhaps due to sediment runoff from the island. Or, as the scientists phrased it: “Elevation and volume gains occurred in mud and rubble habitats and may be associated with terrigenous sediment transport from the island.”

Exactly why is this seafloor erosion happening? I’ll use the paper’s language, and then try to translate it.

“Our results include elevation and volume changes caused by chronic erosion and accretion processes that occur slowly over time frames of months to decades such as changes in carbonate production rates, bioerosion, chemical erosion from carbonate dissolution, degradation of large framework building coral colonies, and physical movement of reef sediments due to persistent oceanographic conditions such as waves and currents.”

Essentially, the authors say there are lots of factors at play. Among them are reductions in coral and coralline algae growth, damage by biological factors like coral-eating creatures, ocean acidification that dissolves corals and shells, and other things like changing weather and current patterns.

But while those are all possibilities, the authors say the focus of their paper was actually measuring the changes in ocean depth, and not so much figuring out why it happens: “Detailed analysis of the processes causing elevation change in these systems is beyond the scope of this paper and should be undertaken in future studies.”

But they make a key observation about Hawaiian reefs. On Maui, material is being transported off the reefs, and the reefs right now are not able to replace it. That’s bad news for the future.

“Our results indicate net export of sediments from the coastal seafloor of Maui and support… previous observations that reef and terrigenous sediment is transported offshore and no new sediment from the reef is contributed to coastal beaches.”

Hawai`i beaches on balance are eroding, and this study seems to indicate that the factors causing that erosion are continuing.

This is the news release from the European Geosciences Union that accompanied the publication of the study.

Here is a Miami Herald piece on the study. 

And here’s a Washington Post piece.

© Jan TenBruggencate 2017

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