NOAA scientists quantify coral reef growth to monitor the effects of ocean acidification

by Bernardo Vargas-Ángel
Assemblage of branching and foliose corals at Swains Island, American Samoa (NOAA Photo by James Morioka).

Assemblage of branching and foliose corals at Swains Island, American Samoa (NOAA Photo by James Morioka).

Often referred to as the “rainforests of the sea,” coral reefs are some of the most biologically rich and economically valuable ecosystems on Earth. Most coral reefs occur in warm, shallow, clear waters and are built by stony corals together with other organisms that form hard, calcium carbonate skeletons over decades and centuries.

Underwater photo of coral assemblages at Fagatele Bay, American Samoa (NOAA Photo by Louise Giuseffi).

Underwater photo of coral assemblages at Fagatele Bay, American Samoa (NOAA Photo by Louise Giuseffi).

Scientists at the Coral Reef Ecosystem Program of NOAA’s Pacific Islands Fisheries Science Center are conducting long-term research to monitor the rates at which reef organisms build their calcium carbonate skeletons and how changes in ocean chemistry, particularly ocean acidification, might impact their growth.

Ocean acidification is a global phenomenon in which increasing carbon dioxide in the atmosphere is absorbed into the ocean making the seawater more acidic. The lower pH and higher acidity of the ocean makes it harder for marine creatures, such as shellfish and corals, to build their calcium carbonate shells or skeletons.

CAU assembly unit: a. oblique view, b. side view, and c. in-situ image of deployed CAU unit (NOAA Drawing by Daniel Merritt).

CAU assembly unit: a. oblique view, b. side view, and c. in-situ image of deployed CAU unit (NOAA Drawing by Daniel Merritt).

Throughout the coral reefs of the U.S. Pacific Islands, we are monitoring the production of calcium carbonate using calcification accretion units (CAUs). These underwater units are made of two PVC plates that are placed at specific locations on coral reefs to allow for the recruitment and colonization of crustose coralline algae and hard corals onto the plates. By measuring net accretion, we can determine how much calcium carbonate is produced over a given period of time. Total net accretion on coral reefs can be calculated by measuring the change in weight of CAUs deployed on the reefs for periods of two to three years.

Newly deployed CAU assembly installed at Swains Island, American Samoa.

Newly deployed CAU assembly installed at Swains Island, American Samoa.

CAU plates encrusted with crustose coralline algae after two years of deployment.

CAU plates encrusted with crustose coralline algae after two years of deployment.

We hypothesize that net accretion will vary based on island, region, and habitat—and will change over time. By monitoring net accretion on coral reefs, we will be able to detect changes in calcification rates over time and therefore, assess the effects of ocean acidification.

Spatial distribution and mean carbonate accretion rates derived from CAU deployments by study site (left panel) and island-wide (right panel).

Spatial distribution and mean carbonate accretion rates derived from CAU deployments by study site (left panel) and island-wide (right panel).

A recently published article in the journal PLoS ONE, Baseline Assessment of Net Calcium Carbonate Accretion Rates on U.S. Pacific Reefs, presents a comprehensive baseline of carbonate accretion rates primarily by crustose coralline algae (CCA) on CAU plates deployed on coral reefs at dozens of sites across 11 islands in the central and south Pacific Ocean. The study underscores the pivotal role CCA play as a key reef calcifier and offers a unique perspective to better understand the potential effects of ocean acidification at different scenarios of future ocean chemistry.

CAU plates prepared for processing in the lab show a diverse collection of organisms (a. corralline algae, b. shellfish, c. coral, d. encrusting algae).

CAU plates prepared for processing in the lab show a diverse collection of organisms (a. corralline algae, b. shellfish, c. coral, d. encrusting algae).

Five main conclusions can be gleaned from this study:

Reef at Swains Island, American Samoa (NOAA Photo by Louise Giuseffi)

Reef at Swains Island, American Samoa (NOAA Photo by Louise Giuseffi)

  • Due to the highly variable nature of the carbonate accretion rates, it is expected that coral community responses to ocean acidification will likely vary widely between reef ecosystems, as well as between sites within islands.
  • Crustose coralline algae deposit a highly soluble form of calcium carbonate (CaCO3) known as high-Mg-calcite. Increases in the acidity of ocean water will likely result in lower CCA accretion rates.
  • Under acidified conditions CCA may lose their competitive advantage as the dominant calcifying group of the early reef colonizers, which in turn may have adverse implications for the settlement and development of other important reef calcifying organisms such as corals themselves.
  • Under the projected changes in marine seawater carbonate chemistry (e.g. ocean acidification), the ability of marine calcifying organisms to cope with such changes, and continue offering the ecosystem services they currently provide, will likely be determined by both the magnitude and rate of seawater pH decrease.
  • The combined effects of chronic human impacts, together with decreased pH from ocean acidification, will likely affect reef community structure and therefore carbonate accretion on coral reefs worldwide.

To read the full article go to: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0142196

Vargas-Ángel B, Richards CL, Vroom PS, Price NN, Schils T, Young CW, Smith J, Johnson MD, Brainard RE (2015) PLoS ONE 10(12): e0142196. doi:10.1371/journal.pone.0142196

The final count: cruise for monitoring of effects of ocean and climate change in the Northwestern Hawaiian Islands completed

By Chip Young

Scientists from the PIFSC Coral Reef Ecosystem Division (CRED) recently completed a 17-day expedition to the Northwestern Hawaiian Islands, where they conducted coral reef monitoring surveys at Pearl and Hermes Atoll, Lisianski Island, and French Frigate Shoals. These 3 locations are part of the Papahānaumokuākea Marine National Monument and World Heritage Site, the third largest marine protected area on Earth and the largest conservation area in the United States.

This PIFSC research cruise (HA-13-05) aboard the NOAA Ship Hi`ialakai implemented a standardized set of methods for the measurement of fluctuations in the region’s coral reef ecosystems caused by global climate change. NOAA’s National Coral Reef Monitoring Plan (NCRMP) outlines the importance of monitoring changes in temperature and the chemical composition of ocean waters within which the coral reef ecosystems of the United States are found. Coral reefs are fragile biological systems that have been observed to live best in specific ranges of water temperatures and composition parameters. Changes in either of these ranges can cause a coral reef system to malfunction, through problematic processes that are familiar to much of the general public. Such processes, including coral bleaching (a result of increased ocean temperatures) and ocean acidification (a result of a drop in the ocean’s pH), affect the ability of corals and other reef organisms to calcify or “build their houses.” Other potential effects can occur, as well, such as shifts in biogeochemical cycles, shifts in species diversity, and changes in the ocean’s food web.

Jamison Gove and Chip Young of the PIFSC Coral Reef Ecosystem Division deploy oceanographic instrumentation on Sept. 13 at Lisianski Island as part of the recent research cruise to the Northwestern Hawaiian Islands. NOAA photo by Oliver Vetter

Jamison Gove and Chip Young of the PIFSC Coral Reef Ecosystem Division deploy oceanographic instrumentation on Sept. 13 at Lisianski Island as part of the recent research cruise to the Northwestern Hawaiian Islands. NOAA photo by Oliver Vetter

As part of the implementation of the NCRMP, CRED scientists on Sept. 3–19 deployed 16 arrays of temperature sensors along various reef systems, installing a total of 64 instruments at depths of 1–25 m. At its specific location on a reef, each sensor records the seawater temperature at the same time as other sensors, every 5 min, over a period of 3 years. The resulting product is a high-resolution picture of temperature variability of 16 different reef systems across space (across the archipelago and to a depth of 25 m) and time (3-year deployment of each sensor).

During the monitoring cruise earlier this month, 100 calcification accretion units (CAUs), like the one shown above, were installed in the Northwestern Hawaiian Islands by staff of the PIFSC Coral Reef Ecosystem Division. CAUs are used to measure not only net reef calcification rates but also species-specific recruitment rates and the percent cover of corals, crustose coralline algae, and fleshy algae. NOAA photo

During the monitoring cruise earlier this month, 100 calcification accretion units (CAUs), like the one shown above, were installed in the Northwestern Hawaiian Islands by staff of the PIFSC Coral Reef Ecosystem Division. CAUs are used to measure not only net reef calcification rates but also species-specific recruitment rates and the percent cover of corals, crustose coralline algae, and fleshy algae. NOAA photo

CRED scientists and partners also collected samples of seawater for chemical analysis, conducted hydrocasts with a conductivity-temperature-depth (CTD) instrument, and deployed installations designed to measure specific biological activities that can be affected by changes in the pH of a reef’s waters. Settling plates, known as calcification accretion units (CAUs), are used to measure net reef calcification rates, species-specific recruitment rates, and the percent cover of corals, crustose coralline algae, and fleshy algae. Bioerosion monitoring units (BMUs) are made up of precisely measured pieces of calcium carbonate, the material that makes up the skeletal structure of corals, and will provide a value for how much biological removal of reef structure is naturally present along the reef. Autonomous reef monitoring structures (ARMS) essentially act as “hotels” for cryptic biota living within the matrix of a reef ecosystem and provide a standard method for evaluation of the existing community of sessile and mobile organisms found on a reef.

Including work conducted during this cruise and the earlier PIFSC cruise SE-13-05 to Kure Atoll in July, CRED scientists have installed 100 CAUs, 50 BMUs, and 24 ARMS throughout the Northwestern Hawaiian Islands this year. Because monitoring activities associated with NCRMP are conducted on a triennial basis, CRED will return to these islands in 2016. At that time, researchers will retrieve and replace all instruments. NCRMP is a long-term project, and the goal of this work is to measure change over time. The results from this ongoing project will be available to help the managers of these remote islands monitor, evaluate, and predict the ecological effects of global climate change on the reefs of the Papahānaumokuākea Marine National Monument.