Shallow waters

December 16, 2013

We moved to shallow waters last night to prepare for our last cruise day, working on finishing our sampling of the urchin caging experiment. The 200 meter site is the shallowest of three depths used for the experiment (200, 600, and 1,000 meters). Here, the water is warmer, more oxygen-rich, higher in pH, and is generally more productive. Since all of the benthic animals we study are cold-blooded, the warmer conditions raise their metabolic rates above the levels found in deeper waters. Although they will need more energy to live in this warmer water, they will also be able to grow and reproduce faster—at least that is what we expect. Part of our experiment is to measure differences in growth between depths.

An urchin exclusion cage on the seafloor at 200 meters. This has several sea stars and sea cucumbers on top.

An urchin exclusion cage on the seafloor at 200 meters. This has several sea stars and sea cucumbers on top.

The major task for the day was to complete all activities at the 200 meter urchin growth study site. This included finding all six urchin cages, moving them aside, collecting all the megafauna (large animals) inside, collecting sediment samples using our sediment core devices, sampling three nearby sites without cages in a similar manner, then gathering all of our cages in a bundle and pulling them to the surface under the remotely operated vehicle (ROV). This required three ROV dives and the entire day.

Collecting sediment cores from an urchin cage location (after the cage was removed). The sediment samples will be sieved to separate the small animals (macrofauna) to be later identified and counted in the lab.

Collecting sediment cores from an urchin cage location (after the cage was removed). The sediment samples will be sieved to separate the small animals (macrofauna) to be later identified and counted in the lab.

Everything went quite well. When we arrived at the bottom (only about 10 minutes after the ROV was launched from the Western Flyer), we immediately notice lots of prawns and juvenile octopi near our cages. They turned out to be quite curious, and even gathered around the ROV in front of the camera as we worked on each cage. A few days ago, the sablefish were the troublemakers. Today, it was octopus and prawns.

The caging work required all day. For each cage, we observed the megafauna in and around the cage, then moved the cage and collected 10 sediment cores. One core will be used to measure sediment grain size, carbon content, and ATP (a proxy for bacterial abundance). Nine cores will be sieved to remove the sediment and separate the small animals, that can then be identified and counted in the laboratory under a microscope. Ultimately, we will compare the differences in macrofauna within cages with urchins to cages without urchins to get an idea of the effects the urchins have on these smaller animals.

Once the caging activities were complete, we collected about 20 adult urchins in the area for studies of reproductive conditions and size, as well as about 20 juvenile octopus for laboratory studies concerning their tolerance to ocean acidification—for more on ocean acidification, see the cruise report from December 14th.

Collecting a young octopus using the ROV suction sampler. We will use the 20 octopi collected to study their tolerance to ocean acidification in laboratory experiments. Several spot prawns can be seen watching the action.

Collecting a young octopus using the ROV suction sampler. We will use the 20 octopi collected to study their tolerance to ocean acidification in laboratory experiments. Several spot prawns can be seen watching the action.

We finished up just after dinner and are now heading back to Moss Landing where we’ll wait for the tide to rise to enter the harbor at about 9:30 pm. This was a great cruise. We completed all we set out to do, had great weather, and fantastic support from the crews of the ship and ROV.

— Jim Barry