A Special Net for Special Organisms

At midnight on a warm night off Bermuda in July, research technician Joe Cope and a small team of crew members prepared to deploy a net system stretching nearly the length of a city bus from the stern of the research vessel Atlantic Explorer. Though it’s not unusual for oceanographers to work around the clock during a research cruise, the timing of this particular cast was important. Every night, under cover of darkness, the marine animals they hoped to capture—some a few inches in length, others the size of a sand grain—come to the surface to feed on phytoplankton, after spending the daylight hours far below the surface, hiding from predators.

The net system, acquired this summer by BIOS scientists Amy Maas and Leocadio Blanco-Bercial with a grant from Simons Foundation International, is distinct in its ability to sample zooplankton at discrete depths—from well-lit surface waters down through the ocean’s “twilight zone,” where light steadily decreases into darkness. Known as the Multiple Opening/Closing Net and Environmental Sensing System, or MOCNESS, it has nine nets, each 30 feet (9 meters) in length. Each net, typically composed of a very fine, filtering mesh material, can be opened and closed, one by one, as they are towed through the water across different depths.

Sensors mounted on the instrument’s frame record a suite of environmental characteristics, including temperature, depth, salinity, and oxygen levels, as well as volume of water filtered through each net, providing vital context about the habitat of captured zooplankton.

Cope, from the Virginia Institute of Marine Science, came to BIOS for four days in July to assist Maas and Blanco-Bercial on a research cruise with two deployments, each lasting four hours from launch to recovery. Cope has 15 years experience assisting scientists with MOCNESS deployments and remains excited by what the system can accomplish.

“It’s exciting to see what we bring up in the nets,” he said, noting that he’s most fascinated by the anglerfish, squid, and krill that may come from more than a half mile (1,000 meters) below the surface.

As the nets fill with water from depths closer to the surface, the excitement builds, especially for zooplankton experts like Maas and Blanco-Bercial. In the warmer, sunlit layers, “suddenly the water is just packed with life, probably like triple the biomass, because we hit the place with all the food,” Maas said. Bringing them on deck with the MOCNESS “is like Christmas morning,” she said. Scientists then quickly hustle the sample and water-filled buckets into shipboard labs for preservation, identification, counting, and further study.

Maas calls the new MOCNESS tow system “a completely critical tool” for research questions related to the diversity of zooplankton and their role at the base of the marine ecosystem. Zooplankton are primarily drifting animals whose diminutive sizes and soft bodies often restrict their ability to swim against the currents. But remarkably, a subset of these drifters, most smaller than a coin, and many even smaller than the edge of a coin, swim to depths of one-quarter to one-half-mile (400 to 800 meters) every day to evade fish and other hungry predators.

These zooplankton spend daylight hours resting in deep, dark waters, but when the sun sets they swim back up to feast on freshly-grown, single-celled algae near the surface. That’s why Cope and his team performed two deployments during the July cruise, one at midnight, and the second at noon a day later, to compare the differences in organisms as they moved up and down during each 24-hour period.

This daily migration was first discovered by U.S. Navy sonar surveys during World War II. Deep aggregations of zooplankton were sometimes so dense that they were mistakenly identified as shallow seafloors. However, when that “seafloor” disappeared, scientists and sonar operators realized they were witnessing a biological phenomenon.

Now, as owners of the MOCNESS system, BIOS scientists and visiting researchers can rigorously investigate the consequences of this phenomenon on nutrient cycling in the Sargasso Sea, teasing apart how the daily and seasonal patterns of zooplankton abundance intersect with patterns in ocean chemistry and microbial abundance. In mid-April, Blanco-Bercial tested the MOCNESS calibration and deployment protocols with the BIOS marine operations team. This summer, with assistance from Cope and BIOS technician Joe Jones, the system was deployed in conjunction with a full suite of physical, chemical, and microbial sampling methods during a research cruise as part of the multi-year BIOS-SCOPE investigation to study the ecosystem’s summer functions.

The researchers emphasize that the MOCNESS is a resource for anyone working in the oceanographic community and they hope that other scientists using the Atlantic Explorer will take advantage of its availability. They aim to use the system again this fall with an oceanic flux program cruise operated by fellow BIOS scientist Maureen Conte, as well as in February on a Bermuda Atlantic Time-series Study (BATS) cruise.

“The net gives scientists a chance to fill in some blanks about what is happening at depth, biologically,” Maas said. “We’re grateful for a chance to see that picture filled out and expanded.”