Corn stover and respiration experiments
October 26, 2013
We had two primary goals for today’s ROV dive. First, to deploy the benthic respiration system (BRS), then set it up to measure the oxygen consumption of some deep-sea crabs. Second, we planned to inspect a bale of “corn-stover” (like a large hay bale, except made of corn stalks) that we sank to the seafloor five years ago, to evaluate its rate of decay and effects on seafloor animals.
We started before sunrise, positioning the benthic elevator with the respiration system on the well deck, completing a final check of the system, then launching it over the stern to sink 3,206 meters to the bottom. The launch went smoothly, thanks to good teamwork and calm seas.
Next we planned to launch the ROV Doc Ricketts and dive down to the respiration system to capture squat lobsters near the corn bale and place them in the BRS chambers.
Although the ROV was ready to go, we had problems with a key piece of research equipment—a combination oxygen probe and water sampler—which we had planned to carry down with the ROV to make measurements in and around the corn bale. The probe worked well during testing yesterday evening, but failed this morning, delaying the ROV launch. We tried to repair the probe, but after 90 minutes, we removed it from the ROV and continued with the launch.
The day went pretty smoothly after the oxygen probe hiccup. We descended to the seabed and found our way to the benthic respirometer. It had drifted about 75 meters to the southwest as it sank. The BRS is light enough that the ROV could pick it up by a rope, so we moved it closer to the corn bale. This made it easier to capture animals and place them in the respiration chambers.
This large bale of “corn stover,” four feet by four feet by eight feet in size, was placed on the seafloor almost five years ago. Over the years, we have been studying the decomposition of this bale by microbes and seafloor scavengers. The little white dots are galitheid crabs, which may be living off of the corn stover.
Before placing animals in the BRS chambers, we decided to use the high definition video camera on the ROV to capture detailed video recordings of the entire exposed surface of the bale. At four feet by four feet by eight feet, the bale is quite large, and is wrapped in coarse mesh.
We discovered that the bale is largely intact. Because most deep-sea animals cannot digest the cellulose in the corn stover, it degrades quite slowly. Nevertheless, there were several species of “squat lobsters”—small crabs in the family Galatheidae—living on the bale. Some of these galatheid crabs are known to be capable of digesting cellulose, relying on bacteria in their guts to provide the proper digestive enzymes. We also noticed that white colonies of bacteria had become more abundant since our last visit two years ago.
Unlike the corn stover itself, the pine lumber used to make a frame holding the bale for deployment was highly degraded. We saw evidence of a number of scavenger species in the wood. The wood-boring clam, Xylophaga, in the bivalve family Pholadidae, was particularly obvious due to the characteristically round, large holes it bores in the wood. Roughly 75 to 85 percent of the lumber was degraded, while only about 10 to 20 percent of the corn stover appeared to have been lost.
In addition to being consumed by crabs and other small scavengers such as snails and polychaete worms, the bale is probably being consumed by anaerobic bacteria in the interior of the bale, where there is little or no oxygen. We’d hoped to use our oxygen probe to collect water samples and measure the variation in oxygen concentrations from the outside to the inside of the bale. But that will have to wait until the probe is repaired.
After we’d inspected the corn bale and the sediment around its base, we collected about five push cores of sediment from around the bale. We will examine these cores to measure the abundance of infaunal animals—the macrofauna—mostly small crustaceans, worms, and molluscs. In addition, we’ll measure the sediment’s grain size distribution, concentration of stable isotopes of carbon and nitrogen, and ATP (energy molecule). The characteristics of cores near the bale will be compared with cores that we collected later in the day farther from the corn bale.
Next we collected galatheid crabs for the respiration chambers, as well as other types of animals for stable-isotope analyses, using the ROV’s suction sampler. This is a tricky process because the crabs can swim away when threatened. After collecting a crab in the sampler, the pilots would fly the ROV over to the respiration system on the benthic elevator and release the animal into the chamber, then try to close the chamber door quickly to prevent the crab’s escape. We placed crabs in seven of the BRS’s eight chambers. We left one chamber empty to measure the background respiration rate of the microbial community in the ambient seawater.
The BRS will now perform experiments automatically for the next six weeks, measuring the changes in oxygen concentrations in each sealed chamber as the crabs (and microbes) consume the oxygen. Every two hours, fresh seawater is pumped into each chamber so that the crabs don’t suffocate.
For the first day or so, the system will measure the natural oxygen consumption rate of each animal. After that, pumps will automatically inject a small amount of seawater containing extra carbon dioxide. Adding the carbon dioxide will make the seawater a bit more acidic. We will use these experiments to understand how changes in the acidity of the ocean affect the metabolic rate of the crabs.
Thus, this experiment simulates the effects of “ocean acidification”—an ongoing increase in the acidity of the world’s oceans. Ocean acidification is caused by the massive emissions of carbon dioxide to the atmosphere from fossil fuel burning by humans. The oceans absorb about one quarter of this carbon dioxide each year. This is causing seawater throughout the world ocean to become more acidic, just like the water in our test chambers.
In some chambers in the BRS, we will also inject water with lower than normal oxygen concentrations. This will simulate a decrease in oxygen in deep ocean waters that is occurring along with global warming. Thus, our BRS experiments will help us understand how marine life may respond to a variety of changes in ocean conditions caused by fossil fuel emissions.
After setting up the BRS, we flew the ROV back to the area where we were conducting our sunken log experiments. Then we gathered a few more logs to bring back to the surface for detailed study. For details on that research, see yesterday’s log. All in all, a pretty full day!
— Jim Barry