MBARI’s Benthic Rover

The Monterey Bay Aquarium Research Institute's (MBARI) Benthic Rover is a subsea lab which methodically charts deep-sea carbon cycles. To measure metabolic activity in the sediment, twin respirometry chambers isolate samples of seafloor for 3 days at a time. Agitators stir up the sediment while sensors record oxygen levels. Diagram: Ken Smith, MBARI. The rover also has two optical/acoustic scanners that detect active chlorophyll and animals like worms buried up to 4 inches in the sediment. These two pieces of information help to fine-tune the respirometry measurements and to determine how quickly sediment arrives.

At the MARS site 891 m (nearly 3,000 feet) below the surface, it’s always dark and there are no plants making food. The food found on this dark seafloor is composed of small flakes of dead organic matter suspended in the water, these form clouds, which drifts down from the productive waters above. Small benthic creatures eat the dead organic matter, subsequently becoming food for larger invertebrates and fish. This organic matter in suspension is the bottom of the food chain on the seafloor. Scientists have a good idea of how carbon travels around most of the world, including how living things build their bodies with it and how carbon returns to the earth, air, and ocean. Yet the dynamics of seafloor feeding is one of the last remaining mysteries of the carbon cycle. Beyond about 500 m (1,600 feet) deep in the ocean, scientists have only a fuzzy understanding of supply and demand for carbon.

The benthic rover was designed to shed light to the mysteries of the seafloor. The rover is a mobile physiology lab. In a series of 3-day experiments, the rover measures how much oxygen seafloor animals are using. It lowers two 30-cm-wide (12-inch) sample chambers into the sediment, where probes record oxygen levels. Two acoustic scanners use ultrasound (in 4-MHz pulses) to look 10 cm (4 inches) deep into the sediment for large animals like worms. Measuring oxygen consumption helps scientists calculate the seafloor’s demand for carbon. To measure the supply, the rover uses fluorescent scanners. The scanners detect still-active chlorophyll in plant cells, a sign that the cells have only recently settled out from sunlit surface waters. For each experiment, the rover camps out at a single location for 3 days, then moves about 5 m (15 feet) to begin another sample. Before moving, the rover consults a current meter and waits until the current shifts so that its next predetermined study location lies upstream. This patient approach allows the rover to keep from stirring up sediment into the area it is about to measure. By the end of 50 measurements, it will have stayed underwater for six to nine months and moved a total distance of about one and a half football fields.