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.
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.
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.
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.
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.
Five main conclusions can be gleaned from this study:
- 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.
NOAA’s Pacific Islands Fisheries Science Center, 22 December 2015. Article.
