Beneath the sunlit surface layer of the ocean lies a half-mile thick layer in which light, temperature, and dissolved oxygen decrease rapidly with depth. This dimly lit layer—the mesopelagic zone or ocean twilight zone—ranges in depth from 200–650 feet to 3,300 feet, with pressure increasing to 1,500 pounds per square inch. Conditions for life at these depths are harsh, but there is a tremendous amount of life in the twilight zone—a greater amount by weight than exists in the sunlit zone and the deeper “midnight” zone combined.
The twilight zone gained attention in the 1940s when sonar revealed the deep scattering layer (DSL), a dense layer of organisms that reflects sound waves back to the sonar receiver and, at times, has caused confusion about bottom depths. The animals comprising the DSL were observed to migrate vertically on a daily cycle, rising to feed near the surface at sunset and descending to darker, safer depths at sunrise. In terms of biomass, this daily, global excursion is the greatest animal migration on Earth and plays a major role in transporting carbon from the sunlit zone to the midnight zone.
Twilight zone species feed on and produce “marine snow”—organic detritus that is either recycled in the twilight zone food web or transported by gravity to the seafloor. It is estimated that 2 to 6 billion metric tons of carbon (an amount comparable to the total annual carbon emissions of the United States) is transported annually by marine snow from the twilight zone into the deep ocean, where it may be sequestered for millennia.
I have been fortunate to observe extraordinary views of twilight zone creatures during deep-sea expeditions on board the exploration vessel Nautilus. Most of these views were fleeting, as our remotely operated vehicles (ROVs) transited through the midwater on their way to and from the seafloor. An expedition near the Channel Islands in 2016 provided sustained views, as we used the ROVs to conduct horizontal transects at multiple depths, gaining information on the diversity and vertical distribution of these species. During these transects, I watched large screen displays in amazement as gelatinous siphonophores and salps, iridescent ctenophores, jellyfish, shrimp, squid, and small fish streamed through the field of view of ROV Hercules’s high-definition video camera.
The need to better understand the biodiversity, behavior, and distribution of twilight zone ecosystems is driven by the current threats of ocean warming, ocean acidification, and marine pollution. Potential threats include the establishment of mesopelagic fisheries to meet the growing demand for fishmeal and fish oil, the spread of waste plumes from seabed mining, and processes being considered to increase the rate at which CO2 is removed from the atmosphere, such as iron fertilization of surface waters and kelp farming.
Over the past decade, new technologies have enabled persistent and detailed observations over broad areas, providing a more complete picture of life in the twilight zone. For example, in the northwest Atlantic, Woods Hole Oceanographic Institution has deployed an autonomous underwater vehicle designed for quietly hovering or maintaining station on an animal of interest; profiling floats and sediment traps to image and collect marine snow; new fish tags for tracking large predators such as tuna and swordfish; multifrequency bioacoustic moorings; and a towed system with broadband sonar designed to detect a variety of organisms. These innovations have revealed the significant temporal and spatial variability of twilight zone species and the strong dependence of commercially important fish on these species for food.
Expanding these improved observations would help quantify more precisely the role of twilight zone food webs in carbon cycling, inform deliberations on establishing marine protected areas in waters beyond national jurisdiction, and shed light on the potential effects of mesopelagic fisheries and carbon dioxide removal activities on the rarely seen but vitally important ecosystems of the ocean twilight zone.
