Carbon Counting

I like to think of carbon as the currency of coral reefs. The bank account of (inorganic) carbon in seawater is adjusted by a number of different processes - respiration and CaCO₃ dissolution make deposits, photosynthesis and CaCO₃ precipitation make withdrawals. And the carbon in seawater is diversified, a little bit is in the form of CO₂, most is bicarbonate (HCO₃^-), and some is carbonate (CO₃^2-).

While conducting our experiments in Palau, we are accountants of sorts. We are keeping track of the carbon in seawater: adding up all the deposits and withdrawals and tallying all its different forms. How can we do this on a coral reef? Essentially, it boils down to (1) tracing changes in the carbon system parameters and (2) keeping track of time, or how long it takes for these changes to occur.

The RATS keeps track of the different forms of carbon in
seawater. Photo credit: Tom DeCarlo

Step 1: Tracking carbon. The heavy-lifting for this step is accomplished by our robots out on the reef. Every 90 minutes, the RATS (see January 13 post) measures the concentration of dissolved inorganic carbon (DIC) and pH. The DIC tells us how much carbon is in the seawater, and the pH tells us in which forms that carbon exists. While the RATS is positioned about half a kilometer onto the reef, every day we take a small boat just offshore to collect water samples that we can use to analyze the carbon system of the seawater that is flowing onto the reef.

One of our instrument arrays on the barrier reef. The
instrument on the right is one of our current profilers. Photo
credit: Tom DeCarlo

Step 2: Tracking seawater. From Step 1, we know what's in our carbon bank account offshore, and we know the total deposits and withdrawals made by the time the water reaches RATS. But what we really need is the rate of these changes, and to get that we need to know how long that seawater was on the reef while the carbon system was modified. We track the seawater using acoustic doppler current profilers, which measure how quickly, and in which direction, water is moving. Because we know the geometry of the reef (the depth and horizontal distance), we can calculate how much time elapsed between when a parcel of water flowed on the reef and that same water reached RATS.

Putting it all together, we can determine the rates of photosynthesis/respiration and calcification/dissolution on a certain stretch of the Palauan barrier reef. This is really key information. Our experiments are in the same location as where these measurements were made over a decade ago, so we can begin tracking long-term changes in these rates. We can also test whether these rates are influenced by the temperature or chemistry of water flowing onto the reef. And we can compare these rates to other reef systems around the world (for example, where we conducted similar experiments in Taiwan).

- Tom DeCarlo