A delicate balance

Severe bioerosion of a living coral colony in

Panama. In this example, the living coral 

(which is green) is infested by bioeroding

bivalves (the key-shaped holes in the 

skeleton). Photo credit: Hannah Barkley

Coral reefs exist in a delicate balance where rates of calcium carbonate production - primarily by corals and coralline algae - are nearly matched by rates of calcium carbonate removal.

An important component of this removal is bioerosion, the biologically mediated breakdown and dissolution of calcium carbonate. Our lab published a new paper, released just this week, which shows that rates of bioerosion of coral skeleton - by worms, sponges, and bivalves - are accelerated by ocean acidification and nutrients. As CO2 levels increase in the atmosphere, some of the CO2 enters the ocean, and through a series of well-known chemical reactions, seawater pH decreases (hence "ocean acidification"). Using cores drilled from living coral colonies across natural gradients of CO2 and nutrients in the Pacific Ocean, we found that bioerosion rates are greatest where pH is low and nutrients are high. In Palau, where there is a strong natural gradient in pH under persistently low nutrients, bioerosion rates clearly tracked the pH variability around the archipelago.

Computerized tomography (CT) scans of

cores of coral skeleton. The light gray to white

colors indicate the coral skeleton. Boreholes

are visible within these skeletons. Photo

credit: Tom DeCarlo

Our current expedition in Palau is an excellent complement to our bioerosion findings. By tracking changes in seawater chemistry (specifically, the alkalinity of seawater), we measure the integrated signal of calcification and dissolution of calcium carbonate as water flows across the reef. Corals and coralline algae build their skeletons using ions dissolved in seawater, a process which decreases alkalinity. On the other hand, dissolution of calcium carbonate reverses this process and alkalinity increases. The instruments that we are deploying in Palau will paint us a picture of the delicate balance between calcium carbonate production and removal, and how this balance tips one way or the other depending on the chemistry of the source water to the reef - for example, what is the sensitivity to seawater pH?

Our experiments begin here shortly! The 4-story scaffolding tower is built out on the reef, and some of our instruments are in the water collecting data. Today and tomorrow, we are deploying the rest of our instruments in time for our experiments to begin this weekend.

- Tom DeCarlo