By Amanda N. Netburn, NOAA Cooperative Institute for Ocean Exploration, Research, and Technology (Florida Atlantic University)
May 9, 2017
The water column is one of the most underexplored environments on the planet. As a follower of NOAA Ship Okeanos Explorer expeditions, you may have seen streaks of particles, as well as transparent and small creatures, that we pass on the way down and back up from every dive. The majority of our dives are focused on surveying the seafloor to understand the habitat and life there. However, if we slow down and take a closer look, we find that there is abundant life throughout the full water column.
The distribution of fauna in the water column can feel sparse compared to the seafloor. Yet, if you took all that water away, the biomass you would be left with would be many times greater than that of the seafloor fauna. From the surface to the seafloor, the water column is the largest habitat on the planet for multicellular animals. This is because seafloor and land-based animals are limited in space to mostly two-dimensional habitats; but midwater animals have the full volume of the water column available to them and can move freely about in all three dimensions.
And move about, they do! If you take a peek at any single spot in the water in one moment in time, minutes later you will find something different in that same space. This is because even weak currents will move poor swimmers like zooplankton ("drifting animals") around in the ocean. For the swimmers (e.g., fish, sharks, whales, etc.), it is because they are almost always moving.
Migrations are common in the open ocean. Tunas, sharks, and some marine mammals can migrate thousands of miles, moving freely between countries' boundaries. Many smaller animals migrate every single night in the greatest migration on the planet. These animals live in deeper layers of the water, typically 200-1,000 meterse (or 655-3,280 feet), hiding in the darkness of the deep open ocean and swimming up to the surface at night to feed.
This daily migration can be observed using active acoustics. By regularly transmitting sound beams through the water column, we can see where animals are distributed because they reflect some of the sound back to the ship. Through this migration, these animals move massive quantities of energy from the surface to the deep ocean in what is known as the "carbon pump." Algae near the surface incorporate carbon dioxide into organic matter through photosynthesis and are eaten by small zooplankton, which in turn are eaten by larger animals. When they swim down in the morning, they move all that organic matter to the deep sea, where eventually it sinks as marine snow to the seafloor.
The water column is one of the most underexplored environments on the planet. You are quite unlikely to have seen many of the creatures that live there. Many of the animals that live in the water column have ways to produce light; this is called bioluminescence. This capability exists for three different reasons: (1) lures to attract prey (e.g., anglerfish); (2) decoys to throw off predators (e.g., a squid releasing glowing ink); and (3) for intra-species identification (to find a mate).
Scientists are still developing best methods for detecting bioluminescence and understanding its role in the ocean. Many midwater animals are gelatinous – they have high water content and are transparent. Gelatinous animals fall apart when collected in nets, the traditional way of surveying for water column animals. Because of this, current knowledge is sparse about behavior, feeding, reproduction, and physiology for many of the most abundant fauna in the ocean.
In summary, we know very little about the water column, despite the huge biomass that lives there and its importance to the global carbon and other biogeochemical cycles. The exploration we conduct on NOAA Ship Okeanos Explorer during this expedition is providing a first glimpse into the water column at some of the most remote and unexplored places on the planet.