Midwater Mysteries: A Deep-Sea Expedition to Uncover the Secrets of the Water Column

I am a deep-sea benthic ecologist, so I study life on the bottom of the ocean. You can often hear me talking about how little we know about the deep seafloor, which covers 66% of the Earth's surface. We know so little about this part of the world that we are just starting to be able to answer questions about these ecosystems that our terrestrial counterparts were answering about habitats on land 100 years ago.

Despite how little we know about the deep seafloor, we know even less about an even bigger biome, the midwater. The pelagic organisms living in the deep sea's water column are so beautiful and mysterious that we are barely scratching the surface (pun intended) about the animals and ecosystems found here. That is why we dedicated an entire dive to exploring the midwater during the current Beyond the Blue expedition to Papahānaumokuākea Marine National Monument onboard NOAA ship Okeanos Explorer. During these dives, we used our ROV, Deep Discoverer (D2), a 9,000-pound minivan-sized robot, to try to image small gelatinous organisms that weigh less than a gram. Needless to say, using such a large vehicle effectively was a tricky process when trying to study and collect such small, delicate creatures. We sometimes refer to the operation as "using D2 as a microscope." As tricky as this operation can be, the data it yields is invaluable.

During our seven transects at 1200m, 900m, 700m, 600m, 500m, 400m, and 300m, we spent about 45 minutes at each depth imaging whatever we could see with the ROV. We were rewarded with spectacular beauty and some fascinating scientific observations. Many of the organisms that live in the water column at these depths are daily vertical migrators, meaning they partake in the world's largest migration every day.

An unfathomable number of organisms rise from the depths to the surface at night to feed in the cover of darkness, then descend again to hide during the day. We generally refer to this layer of organisms as the deep scattering layer because of the way it shows up on sonar. During our transects, we saw stark transitions between the depths, helping us understand which groups of organisms live in what part of the deep scatter layer. Additionally, we collected six inhabitants of these depths, all of which we expect are either species new to science or undescribed species.

In addition to simply observing where the organisms live in the water column, we also observed behavior. This type of documentation is where visual observations with ROVs shine. While you can collect specimens and learn what depth they live at with net tows, you can't tell what these creatures are doing and how they live with anything other than a camera. Capturing video of how these creatures interact with others they encounter provides a deeper understanding of these midwater communities in a unique way.

We made some groundbreaking discoveries in our short six hours in the midwater. I can't wait to see what we see next.

Credit: NOAA Ocean Exploration

Brian Kennedy is ODL's Chief Scientist and was recently Expedition Science Co-Lead along with Sara Kahanamoku-Meyer, Assistant Researcher at the University of Hawaiʻi at Mānoa, during the recent Beyond the Blue expedition. The expedition explored the Papahānaumokuākea Marine National Monument aboard NOAA ship Okeanos Explorer.