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Cover Story

Polluted Groundwater Threatens Hawaiian Coral Reefs



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Coral reefs along densely populated shorelines are especially vulnerable to the effects of ocean acidification amplified by local pollution, according to a study by USGS scientists reported in the Journal of Geophysical Research: Oceans.

Photo shows degraded coral reefs at Kahekili Beach Park, west Maui, Hawai'i.
Above: Degraded coral reefs at Kahekili Beach Park, west Maui, Hawai‘i. The study’s researchers found little evidence of live coral near groundwater seeps. Photo credit: Peter Swarzenski, USGS. [larger version]

Corals around the world are already stressed by ocean acidification—the gradual decline in seawater pH as the ocean absorbs increasing amounts of CO2 from the atmosphere. Lower pH (greater acidity) slows calcification, the process by which corals and other marine species make their skeletons out of the mineral calcium carbonate. It also increases rates of coral bioerosion—the breakdown of coral by other organisms—and it can cause carbonate skeletons and sand to dissolve.

The authors show that polluted, low-pH groundwater discharging onto a shallow coral reef off Kahekili Beach Park in west Maui, Hawai‘i, further increases seawater acidity and exposes corals to nitrate concentrations 50 times higher than normal. Such land-based pollution could contribute to the collapse of coastal coral reef ecosystems sooner than predicted based on ocean acidification alone. 

 “What we’ve shown here is that local stressors work in concert with global changes in ocean conditions to accelerate coral reef decline through nutrient driven-bioerosion,” said Nancy Prouty, lead author of the study.

Photo showing scientists measuring  acidity, nutrient levels, and other characteristics of water sampled near the coral reef off Kahekili Beach Park, Maui, Hawai'i, in March 2016
Above: Kim Yates (left) and Nate Smiley of the USGS St. Petersburg Coastal and Marine Science Center measure acidity, nutrient levels, and other characteristics of water sampled near the coral reef off Kahekili Beach Park, Maui, Hawai‘i, in March 2016 (https://soundwaves.usgs.gov/2016/05/fieldwork.html). Photo credit: Nancy Prouty, USGS. [larger version]

The recent report synthesizes field and laboratory work done over the past several years (for example, see “Examining the Chemistry of Seawater and Coral to Promote the Health of West Maui’s Coral Reefs”). To carry out the multifaceted study, research oceanographers from the USGS Pacific Coastal and Marine Science Center in Santa Cruz, California (Prouty, Curt Storlazzi, Peter Swarzenski), and the USGS St. Petersburg Coastal and Marine Science Center in St. Petersburg, Florida (Kim Yates) joined forces with scientists from the Woods Hole Oceanographic Institution (Anne Cohen) and Hawai‘i’s Division of Aquatic Resources (Darla White). The team analyzed the chemistry of groundwater flowing into the ocean through seafloor springs near the reef, and measured rates of calcification and bioerosion in coral cores collected near and far from the springs.

Maps of the island of Maui, Hawai'i, and the study area off Kahekili Beach State Park
Above: Maps of the island of Maui, Hawai‘i, and the study area off Kahekili Beach State Park. Red dot is the primary seep (seafloor spring) site. Blue triangles are sites of coral cores and seawater samples. [larger version]

Groundwater flowing from seafloor springs at Kahekili consists primarily of wastewater effluent from injection wells at the nearby Lahaina Wastewater Reclamation Facility. A previous study using tracer dyes (see “Lahaina Groundwater Tracer Study”) demonstrated a direct connection between the springs and the sewage-treatment plant. The USGS study yielded further evidence of this connection, finding that coral skeletons sampled near the springs contain higher levels of 15N, a nitrogen isotope considered diagnostic for sewage and wastewater-treatment processes.

Cartoon diagram showing how wastewater effluent (yellow arrows) moves from injection wells to underwater springs off Kahekili Beach Park
Above: Cartoon diagram showing how wastewater effluent (yellow arrows) moves from injection wells to underwater springs off Kahekili Beach Park in west Maui, Hawai‘i. Blue arrows show generalized direction of groundwater flow. Green shades: Waikulu Basalt; tan shades: sedimentary deposits. Corals are healthier (darker brown) farther from seafloor springs. Not to scale. Highly modified from figure 7 in USGS Scientific Investigations Report 2012–5010. [larger version]

The lower pH of the polluted groundwater slows calcification of new coral skeleton, and the excess nitrates accelerate the growth of organisms, such as algae, bacteria, and sponges, that break down the coral. Bioerosion is a natural process, but if it proceeds more quickly than calcification, the reef will decline. “In the case of Kahekili Beach Park,” said Prouty, “excess nutrients in the effluent appear to be accelerating bioerosion.”

To measure rates of calcification and bioerosion, the scientists collected cores of corals near the seep site and almost half a mile to the north. They obtained computerized tomography (CT) scans (series of X-ray images) from the Woods Hole Oceanographic Institution’s Computerized Scanning and Imaging Facility (<http://csi.whoi.edu/>). Analysis of the scans showed that bioerosion of coral off Kahekili Beach Park is proceeding at rates up to 8 times greater than expected for corals growing away from pollution. This bioerosion has drastically weakened local coral reefs for more than 10 years and could lead to the collapse of the underlying reef structure. Losing the reef would mean losing the benefits it provides, such as supporting fish populations and protecting the shoreline from coastal hazards like storm waves.

Computerized tomography images and photographs of coral cores collected near the seep site (left) and almost half a mile to the north (right) demonstrate greater bioerosion of coral near seep site
Above: Computerized tomography (CT) images (dark gray) and photographs of coral cores collected near the seep site (left) and almost half a mile to the north (right) demonstrate greater bioerosion of coral near seep site. [larger version]

Polluted groundwater is just one of many local stressors that affect corals around the world; others include overfishing, sediment-laden runoff, vessel groundings, and other recreational impacts. The USGS study shows how these pressures can exacerbate global threats to coral health.

“As the ocean changes, reducing stressors at a local scale could be vital for preserving coral reefs,” said Prouty.

The USGS study was published in the December 2017 issue of the American Geophysical Union (AGU) Journal of Geophysical Research: Oceans (see “Vulnerability of Coral Reefs to Bioerosion From Land‐Based Sources of Pollution”). It was also featured on AGU’s GeoSpace blog see “(Polluted groundwater eroding Hawaiian coral reefs, study shows”) and reported by the Honolulu Star-Advertiser (see “Discharge from West Maui sewage treatment facility found to be eroding reef) and the Associated Press (see “Scientists say sewage injected into ocean damaging coral off of Kahekili shoreline”). 

Related Sound Waves Stories
Examining the Chemistry of Seawater and Coral to Promote the Health of West Maui’s Coral Reefs
April / May 2016
Recent USGS Field Studies of Nearshore Hydrogeologic Exchange and Submarine Groundwater Discharge on U.S. West Coast and Hawai‘i
November 2009
Telling the Public About USGS Research on the Kona Coast, Hawai'i
Dec. 2005 / Jan. 2006

Related Websites
Scientific Investigations Report 2012–5010
USGS
Lahaina Groundwater Tracer Study
U.S. Environmental Protection Agency
Vulnerability of Coral Reefs to Bioerosion From Land‐Based Sources of Pollution
Journal of Geophysical Research: Oceans
Polluted groundwater eroding Hawaiian coral reefs, study shows
GeoSpace Blog
Discharge from West Maui sewage treatment facility found to be eroding reef
Honolulu Star-Advertiser
Scientists say sewage injected into ocean damaging coral off of Kahekili shoreline
The Maui News
CREST
USGS
Coral Reef Project
USGS

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Cover Story Polluted Groundwater Threatens Hawaiian Coral Reefs

News Brief
News Briefs

Field Work
Dredging Up Clues to the History and Resources of the Rio Grande Rise

Recent Fieldwork

Staff amd Center News
Fulbright Scholar Joins Coral Reef Project at PCMSC

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Regional Acidification Trends in Florida Shellfish Estuaries

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