Why Do Scientists Care About the Deep Sea?

When scientists study tide pools at the ocean’s edge, they gather their equipment, drive down to the rocky shoreline, and put on their “science boots.” Using only their eyes, they can observe squishy anemones, colorful starfish, and thousands of barnacles clinging to the rocks. To study what’s in the sea, however, scientists need different tools and techniques. The invention of SCUBA diving was important because it allowed scientists to study deeper underwater ecosystems like coral reefs for the first time.¹ Beyond the coral reefs, the deep sea is as full of life as a tropical rainforest, but it is too deep for humans to visit. To uncover the mysteries of the deep sea, scientists first must build specialized equipment to go deep!

The deep sea includes everything in the ocean below 200 m. In some places, like California, deep-sea ecosystems can be found close to shore (less than 2 km from the beach). The deep waters are dark, cold, and mysterious. Since there is a limit to how deep the human body can go without special equipment, scientists build technologies to take people deeper [1]. Some scientists have used a one-person submarine to explore places like the Mariana Trench, which is almost 7 miles (11,265 m) below sea level [2]. Others build robots to regularly scan, collect, and record information about the seawater.

Accessing the deep sea is the key to learning about the animals living there. On the land, we have learned how worms need moist, airy soil, and tortoises prefer the dry desert heat. Ocean environments and their animals are just as unique and selective. Certain ocean animals prefer warmer, Caribbean waters, while others like colder, Arctic waters. Like us, marine animals need oxygen to breathe. Rather than coming up to the surface to breathe, most marine animals use oxygen dissolved in the seawater. We have learned that there is generally more oxygen at the surface of the ocean than in the deeper waters. Some animals can tolerate areas with less oxygen, while others need more oxygen to breathe comfortably.

Within the surface layers of the ocean, oxygen varies a lot. The constantly moving water is one reason for this: the oxygen levels change a little bit as the water moves up, down, and side-to-side. This is called environmental variability because these changes in oxygen happen naturally. Oxygen availability can also change with the seasons, or because of storm systems. In addition, oxygen in the ocean’s surface layers is decreasing due to climate change caused by humans. As humans continue to burn fossil fuels and pollute the environment, they release chemicals into the air that lead to warmer oceans with less oxygen. This change happens slowly but can cause permanent damage.

Animals that live in constantly changing environments may have a better chance of adapting to oxygen decreases caused by climate change. However, if oxygen becomes too low, many animals, like fish, will need to find new homes with more oxygen. Our goal was to figure out how vulnerable the animals along California’s coast are to changes in ocean oxygen, by watching their reactions to varying conditions. We hoped that our deep-sea observations would tell us which deep-sea animals and environments will be threatened by future decreases in oxygen, so that we can better protect them.

Building a Deep-Sea Spy to Explore

To study animals in the deep sea, we built a deep-water lander, called Deep Ocean Vehicle (DOV) BEEBE, which we call our “deep-sea spy” (Figure 1). A lander is a technology that “lands” in new environments that humans cannot easily get to, like the Mars Lander that studied Mars. Landers can be customized based on the goal of the mission. BEEBE landed on the seafloor and its mission was to observe different deep seafloor communities for up to 3 weeks. We focused on the nearshore deep-sea ecosystems off San Diego, California. This is an upwelling area, where cold, deep water, low in oxygen, is brought up to shallower depths in the spring and summer.

Landers like BEEBE are extremely helpful when studying the deep sea because they are small enough for one person to deploy from a small boat! BEEBE stands five feet tall, about the height of an average 12-year old. At the start of each mission, BEEBE traveled to the seafloor with weights and began attracting animals with attached bait (Figures 1B, F, G). With a special camera and lights to brighten the seafloor, BEEBE recorded short videos every 20 min, to capture who was there and what they were doing. With special sensors, BEEBE also collected information on the temperature, saltiness, pressure, and oxygen in the water. After a few weeks of sampling, a signal was sent through the water to tell BEEBE to release the weights, so it could float back up (Figure 1C).

What Did We See Through Beebe’s Cameras?

BEEBE conducted seven spy missions for us, visiting seafloor communities from 100 to 400 m deep. From each spy mission, BEEBE brought back fascinating video footage and information about the ocean waters that we could upload to our computer and learn from!²

During each mission, BEEBE observed seafloor communities at different depths. The videos BEEBE recorded revealed that, closer to the surface of the ocean at 100 m, there are mostly fish! We called this environment fish-dominant (Figure 2A). Rockfish loved this environment, and many other fish gathered when oxygen levels increased. At deeper depths, like 200, 300, and 400 m, there were fewer fish and more crustaceans and sea urchins. We called this a transition to an invertebrate-dominant seafloor (Figures 2B–D). Invertebrates are animals without backbones, like crabs or urchins. Pink urchins and tuna crabs covered the seafloor. They seemed to like the colder, lower-oxygen environments. The few fish we observed in the invertebrate-dominant area were much less active, sitting still along the seafloor, compared to those seen at 100 m, which swam around frequently. Being less active could be a behavioral adaptation to preserve energy while living in a cold, low-oxygen environment.

What Did Beebe’s Sensors Teach Us?

Our sampling equipment measured the temperature and oxygen level of the seawater every 5 min! This helped us compare ocean environments at various depths. It also helped us see how one environment changes from day to day. The 100 m environment had the most oxygen and highest variability of temperature, meaning the temperature at 100 m changed the most from day to day. At 200 m, the oxygen and temperature were lower than at 100 m. We were surprised to find high oxygen variability here, meaning the oxygen levels changed the most from day to day at 200 m (Figure 3). Conditions at 300 and 400 m had extremely low oxygen levels that did not change much throughout the entire mission. We called these regions hypoxic because they are extremely low in oxygen and can be stressful for fish and other organisms.

How Do Changes in Oxygen Affect Ocean Animals?

Our sensors taught us that water at 200 m experiences the most oxygen variability. By comparing which animals were present in the video footage to the oxygen conditions at the time, we noticed a pattern! We found that some animals prefer high-oxygen waters, while others like low-oxygen waters. For example, during high-oxygen periods, spot prawns, crabs, and lizardfish were more common. During low-oxygen periods, tuna crabs and Dover soles were more common (Figure 3). This shows that certain animals living at 200 m are sensitive to changes in oxygen. Overall, most animals did not seem bothered by these natural and temporary oxygen changes. However, as oxygen loss worsens due to climate change, we still do not know how each animal will respond to permanent decreases in available oxygen.

Spies Like Beebe Can Help Scientists Understand Climate Change Impacts

As climate change causes the ocean to warm, the water loses oxygen. This is a crisis called ocean deoxygenation. Exploring with our seafloor lander BEEBE gave us day-to-day insight into which animals and depths may be more sensitive to permanent climate change impacts [3]. What will happen to the animals that prefer high-oxygen conditions, like rockfish, spot prawns, crabs, and lizardfish? These animals may be forced to find new homes in shallower, better oxygenated waters. When animals shift habitats, they may experience more stress or become more vulnerable to predators. It is also possible that, as some animals move away from low-oxygen areas, other animals that are not stressed by low oxygen conditions, such as tuna crabs and Dover soles, may expand into these areas.

Maintaining biodiverse ecosystems with many types of animals is key to supporting a healthy ocean. Oceans around the world are facing similar concerns stemming from warming and oxygen loss. Ocean landers can capture unique footage of seafloor communities in deep-sea ecosystems that are close to shore and could help scientists in other parts of the world explore their understudied seafloor habitats, too. Someday this type of information may help marine managers or young scientists like you to understand which deep-sea ecosystems and species are most vulnerable to warming and oxygen loss. This knowledge will help us to make better decisions about how to manage deep-sea ecosystems and preserve biodiversity in a changing world.

Funding

This research was made possible by generous funding from several sources: the Mullin Fellowship, a Mildred E. Mathias Research Grant, the Edna B. Sussman Fellowship, the Mia J. Tegner Fellowship, Friends of the International Center Scholarship, the DEEPSEA CHALLENGE Expedition, the National Science Foundation Graduate Research Fellowship, and the Switzer Environmental Leadership Fellowship to NG.

Glossary

Environmental Variability: ↑ The changes and fluctuations that occur in an environment over a short period of time.

Climate Change: ↑ Climate change is the process of the Earth heating up due to human activity.

Upwelling: ↑ The process of deep, cold, nutrient-rich water rising to the surface.

Invertebrates: ↑ An animal without a backbone. More than 90% of all living animal species are invertebrates.

Hypoxic: ↑ Having an extremely low oxygen concentration, making it difficult for many animals to survive.

Ocean Deoxygenation: ↑ The loss of oxygen in the ocean due to human-caused climate change.

Conflict of Interest

KH was employed by Global Ocean Design LLC.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

Global Ocean Design LLC provided internal R&D resources. This work would not have been possible without the support of a tremendous number of people who helped with Nanolander deployments and recoveries, especially Phil Zerofski, Brett Pickering, Rich Walsh, Jack Butler, Mo Sedaret, Lilly McCormick, Andrew Mehring, Ana Sirovic, Rebecca Cohen, Ashleigh Palinkas, and Jen McWhorter. We are forever grateful to Javier Vivanco and others at Baja Aqua Farms for recovering and returning DOV BEEBE after it drifted into Mexican waters following an unsuccessful recovery.

Footnotes

1. ↑https://www.nationalgeographic.com/news/2010/6/100611-jacques-cousteau-100th-anniversary-birthday-legacy-google/

2. ↑Check out footage from BEEBE’s deployments here