Measuring Coral Growth to Help Restore Reefs

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It is critical to start measuring calcification rates in a systematic way now, particularly at subtropical latitudes where conditions fluctuate seasonally, so that we can understand how dynamic ocean conditions affect calcifying organisms today and predict possible changes in the future. We established a calcification monitoring network in the Florida Keys and have been measuring calcification rates since 2009.

Calcification monitoring station with a colony of the massive starlet coral, Siderastrea siderea, fastened in place.

Figure 1. Calcification monitoring station with a colony of the massive starlet coral, Siderastrea siderea, fastened in place. The white plastic "cow tags" are used as settlement tiles for measuring calcification by the crustose coralline algae (CCA) community, and the black temperature logger records ocean temperature every 15 minutes (temperature data are availableat (Public domain.)

Coral reefs around the world have suffered ecosystem decline over the past fifty years, particularly in the western Atlantic (Kuffner and Toth, 2016). Scientific consensus is that the most impactful stressors directly causing coral mortality are (in order of acreage of coral killed): coral bleaching caused by high ocean temperatures, coral diseases, and coral predators. Stressors that can prevent coral populations from recovering after a mortality event are more varied, since their influence is often indirect and complicated by multiple stressors acting at once. Among them are disturbances to food webs (e.g., overfishing of herbivorous fish), eutrophication (e.g., fertilizers applied to land arriving in the ocean), and changes in water quality induced by land-use change (e.g., sedimentation). Another stressor relevant to reefs is "ocean acidification." This term refers to the chemical changes that occur in the ocean when it absorbs carbon dioxide derived from humans burning fossil fuels. The average ocean pH at the surface has already declined by about 0.1 pH units. While ocean acidification does not directly cause corals to die, some species show slower growth rates at lower pH (Jokiel and others, 2008). Another expected impact from ocean acidification to reefs is that erosion (the natural processes of breaking down reef structure) will increase; thereby further compromising the important role of reefs in shoreline protection.

Florida Keys showing sites where calcification monitoring

Figure 2. Map of the Florida Keys showing sites (red stars) where calcification monitoring stations are located. Our work occurs within the boundaries of Biscayne National Park, the Florida Keys National Marine Sanctuary, and Dry Tortugas National Park. Stations are on the outer-reef tract in approximately 12 to 15 feet (4 to 5 m) of water depth in relic spur-and-groove or hard-bottom habitat. (Public domain.)

As coral populations fluctuate in response to changing ocean conditions, reef managers need new metrics to track the status of coral reefs. Traditional reef monitoring programs are usually limited to measuring the area of reef covered by live corals and other organisms. The USGS is developing new tools and approaches (fig. 1) to directly measure reef processes, including calcification—the process by which organisms produce their calcium-carbonate skeletons. We are directly measuring calcification rates of corals and calcifying algae as they grow in their natural habitat on the outer reef tract of the Florida Keys (fig.2). This new approach to reef-process monitoring has already revealed that different coral species and populations respond differently to various environmental conditions (Kuffner and others, 2013), highlighting the complexity of the response and providing hope that some species may be ideal candidates for conservation and restoration efforts. Measuring calcification rates in a systematic way also provides key baseline data that can be used to quantify impacts to corals in the event of unforeseen events, such as oil spills or other water-quality crises.

live and dead elkhorn coral

Figure 3. The top panel shows the threatened elkhorn coral (Acropora palmata) alive and performing the critical ecosystem service of building the reef crest that protects shorelines during storms. The lower panel shows a dead and quickly eroding skeleton of elkhorn coral. Dead corals are reduced to sand by the actions of waves, grazing fishes, and internally boring sponges. The potential for restoration of Elkhorn populations to reestablish shoreline protection is a current research focus of the USGS. (Public domain.)

Our present work focuses on a very important and threatened species, the Elkhorn Coral (Acropora palmata). Until the 1970s, this coral was a huge contributor to building reef structure throughout Florida and the Caribbean, particularly the reef-crest habitat that attenuates waves and protects coastlines. We are presently testing five genetic strains of Elkhorn from our collaborators at the Coral Reef Foundation for their growth capacity at our calcification monitoring sites. Our work showing which genetic strains grow best in what environments will directly benefit stakeholder efforts in replenishing this important species to western Atlantic reefs, and thereby help restore the critical ecosystem service of coastline protection from storms.

This research is part of the Coral Reef Ecosystem Studies (CREST) project.