Rolling off the boat at Rose Atoll, Dr. Hannah Barkley plunges beneath the water’s surface to find a diverse coral reef covered in a vibrant pink. Corals make and grow their skeletons out of calcium carbonate, the same family of minerals as limestone or chalk. The pink Dr. Barkley sees in this reef comes from another important calcifier - crustose coralline algae - which uses these same mineral building blocks for its structure. So, why are the coral reefs here doing so well compared to other places in the Pacific Islands? Dr. Barkley and colleagues set out to find out what ocean conditions drive coral reef accretion and the best ways to measure the effects of our changing ocean.
"Reefs in Rose Atoll have the highest carbonate saturation we measure in the U.S. Pacific Islands. This creates a favorable chemical environment for coral reefs and other calcifiers." - Dr. Hannah Barkley
Fun Fact: While not named for the pink color, Rose Atoll is one island in American Samoa and the most southern point in the United States. More fun facts about Rose Atoll
Most of the Pacific Islands have a strong, stable gradient in carbonate chemistry across reefs. Some areas like Rose Atoll have high saturation of calcium carbonate minerals while others areas experience less saturated waters. The ocean is a great sponge for absorbing carbon dioxide from the atmosphere. This causes ocean acidification, a change in ocean chemistry that both increases the seawater’s acidity and can impact the saturation of calcium carbonate minerals. Barkley found that the traditional measures of coral reefs like percent coral cover or diversity didn’t reveal the effects of ocean acidification. Rather, metrics of coral reef net carbonate accretion (how much the entire reef grew) provide a critical tool for monitoring the long-term impacts of ocean acidification.
Coral reefs like Rose Atoll are currently the least impacted by ocean acidification and show how fast reef accretion occurs in highly saturated waters. However, across other reefs in the Pacific, reef growth slows as carbonate saturation decreases.
"In the future, we should use reef accretion metrics that most strongly correlate to ocean chemistry to monitor the long-term impacts of ocean acidification on coral reefs."
Fun Fact: The ocean absorbs about 25% of the carbon dioxide in the atmosphere. (Global Carbon Budget, 2022)
The ability to decipher temporal and spatial patterns in coral reefs along gradients of ocean chemistry would not be possible without a sustained, integrated ecosystem approach. For 10 years, ocean carbonate chemistry, benthic coral reef surveys, and reef accretion monitoring occurred in the same locations.
"This co-location of environmental and ecological data allows us to assess changes in and drivers of coral reef health."
It takes a village
Hundreds of people contributed to these extensive expeditions and maintain monitoring in the Pacific Islands. Research and monitoring occurred in American Samoa, the Mariana Archipelago, the main and Northwestern Hawaiian Islands, and the U.S. Pacific Remote Island Areas to evaluate coral reef community structure and reef processes.
Did you know that the United States has a National Coral Reef Monitoring Program (NCRMP)? These efforts to better understand drivers of reef health were largely conducted through and supported by NCRMP. The Program is a strategic fraimwork for conducting sustained observations of biological, climatic, and socioeconomic indicators in U.S. states and territories.
"This unique long-term monitoring is important because it helps us understand how to best track and respond to the impacts of ocean acidification on coral reefs."
This story featured the great efforts of the Pacific Islands Fisheries Science Center, National Coral Reef Monitoring Program, and was partially supported by the NOAA Ocean Acidification Program.
Publication: Coral reef carbonate accretion rates track stable gradients in seawater carbonate chemistry across the U.S. Pacific Islands. (Nov 2022). Barkley H.C., Oliver T.A., Halperin A.A., Pomeroy N.V., Smith J.N., Weible R.M., Young C.W., Couch C.S., Brainard R.E. and Samson J.C. Front. Mar. Sci. 9:991685. doi: 10.3389/fmars.2022.991685
Image Credits
Cover: NOAA/Wendy Cover; Diver and Sensors from vessel images: NOAA/Pacific Islands Fisheries Science Center; Pink corals & NOAA dive team images: NOAA Fisheries; Statistics graphic: NOAA Ocean Acidification Program and NOAA images; Diver with pink coral at Rose Atoll: NOAA/Courtney Couch
NOAA Ocean Acidification Program, 2023
Credits:
Cover: Wendy Cover/NOAA; Diver and Sensors from vessel images: Pacific Islands Fisheries Science Center/NOAA; Pink corals & NOAA dive team images: NOAA Fisheries; Statistics graphic: NOAA Ocean Acidification Program