“Day and night vertical migration” of marine animals

Vertical migration to foraging
When it comes to animal migration, we usually think of the roaring wildebeest migration on the Serengeti Prairie, or the epic migration journey of the 4th-generation monarch butterfly that took 6 months to fly 4000 kilometers across North America. Or the hundreds of thousands of penguins who crossed Antarctica overcoming difficulties and obstacles. In fact, the migration of marine animals is also spectacular. In the global seas, every night, countless small marine creatures migrate hundreds of meters from the depths of the ocean under the cover of night, and swim to the sea surface to feed on phytoplankton, including crustaceans smaller than rice grains and transparent jellyfish. , Blood-red squids and huge schools of lantern fish glowing in the dark. Before dawn, they returned to the mid-ocean area at a depth of 200 to 1,000 meters, hiding in dark waters. In order to avoid predators that roam the sea during the day, marine animals move up and down at night, which is called “day-night vertical migration (DVM)”. This phenomenon was first observed when people used ship nets for transportation about 200 years ago, and is now usually recorded by shipborne acoustic systems (such as acoustic Doppler current analyzers).

The day and night vertical migration of marine animals varies with species and locations. Otters and octopuses can slide in the water with relatively little resistance. The gelatinous sea squirt is more powerful and may move nearly 1 km. But for zooplankton with a particle size, it is difficult to overcome the natural viscosity of water. For example, copepods may only move a few meters. Their starting point is to find food, and the purpose is quite simple and clear. Climate change will also affect the day and night vertical migration of marine animals, and the exact way of impact still needs further study. Zooplankton not only migrate up and down in relatively safe areas, but also migrate laterally to areas with relatively abundant food. The day and night vertical migration of marine animals is a common and quite complex ecological phenomenon. What happens in a thin layer of the sea has an impact on the upper and lower marine world. There are many uncertain factors, and many mysteries have not been solved yet, and further research is needed.

The tiny algae on the surface of the ocean get energy from the sun during the day and thrive. The maximum depth of photosynthesis for phytoplankton is determined by the clarity of seawater and other factors, and the clarity of seawater depends on a series of other variables, such as water temperature, wind speed, tides, ocean currents, salinity and light. The ocean is a three-dimensional space, not only changing horizontally and vertically, but also constantly changing over time. Phytoplankton is small, but numerous, and the food chain that constitutes nourishes countless marine life. In order to carry out life-sustaining photosynthesis, they must stay on the surface of the ocean at a depth of more than 200 meters, because that is the farthest distance that sunlight can penetrate. Only when marine animals migrate here vertically can they enjoy the “delicious” and feast on them. This migration is not only related to the survival of filter-feeding animals such as manta rays, but also important to various animals in the food chain. Big fish eat small fish, small fish eat dried shrimps, dried shrimps eat zooplankton. The larger the animal is, the larger the predator who preys on it will be relatively larger.

The most common way of day and night vertical migration of marine animals is “night migration”, that is, they begin to migrate upward at sunset, and migrate downward before sunrise, and they rise and fall once a day and night. The sickle-shaped shark seems to stay on the bottom of the sea during the day and go out for food at night. Another way is “morning and twilight migration”, that is, it rises at sunset and sinks at midnight; it rises again before sunrise, and falls to the water layer during the day after sunrise, which rises and falls twice a day and night. There is also a method called “reverse migration”, which is to move to a place with sunlight during the day, and then leave the sea to sink to the maximum depth at night. What’s interesting is that the same species will also undergo changes in migration patterns depending on the location. Taking coral reef ghost manta rays as an example, they carry out normal “night migration” in the Chagos Islands and other places located in the center of the Indian Ocean. However, in places such as the Red Sea between northeastern Africa and the Arabian Peninsula and Seychelles in Africa, they adopted “reverse migration.” This marine creature has a flat body and a huge body. The protruding head fins on both sides of the head look like small hands, and a long and thin tail is dragged behind it. The most striking thing is that their pectoral fins are specialized as a pair of “big wings”, which swim like flying bats, hence the name “Manta Rays”.

Lidar detection law
During World War II, when scientists first tried underwater sonar to detect German U-shaped submarines, the echo detector consistently showed a “solid layer” between 120 and 180 meters below the sea surface. At first, scientists inferred that the sonar detected the bottom of the sea, but what is puzzling is that it has been moving, and it is in a state of becoming darker during the day and shallower at night. With the deepening of research, scientists discovered that this is a “fake seabed”, which is actually a “solid layer” assembled by a large group of shrimps, squids and pipe jellyfish. According to echo-detection images taken in Saanich Bay, British Columbia, Canada, the thick layer of krill looks almost like a solid seabed. Krill is a small crustacean similar to shrimp. Sonar researchers named this dense biological area that can scatter or reflect sound waves as the “deep sea scattering layer.” Such a thick scattering layer even triggered the debate about whether the submarine can hide.

With the development of modern technology, observation methods are becoming more advanced. On April 28, 2006, the research team of Oregon State University and other institutions used the launched “Cloud-Aerosol Lidar and Infrared Explorer Satellite Observation” satellite to conduct the first global study of tracking large-scale marine animal migration through Lidar. . This satellite is a joint venture project of NASA and the Space Research Center of the French Space Agency. It detects twice a day and can perform new three-dimensional observations of the earth’s clouds and aerosols to understand how global clouds affect the earth’s atmosphere and global warming. , To obtain new knowledge about the global distribution and evolution of clouds, to answer questions about how clouds and aerosols form and develop, and how they affect water supply, climate, weather, and air quality. The main payloads are orthogonally polarized clouds- Aerosol lidar, wide-field camera and infrared imaging infrared radiometer. The target area of ​​observation is wider than previous observation methods and lasts longer, which will help researchers to better conduct global marine biology and geochemistry research. The day and night vertical migration of marine organisms is one of the important contents of this research field. Although the orthogonally polarized cloud-aerosol lidar carried on the satellite is used to measure cloud layers and atmospheric aerosols, it can penetrate the surface of the ocean 20 meters and find migrating marine animals that reach this layer.

Copepods are just one of many different animals involved in vertical migration day and night

The data provided by the satellite during the 10-year study period (2008-2017) shows that the vertical migration of marine animals during the day and night in spring is not significant, and most of the time is dense in the upper and middle water layer (0-10 meters), only at 8:30. And at 20:30, the dense layer moves to the lower layer (10-15 meters). Summer generally shows a rhythm of declining during the day and rising at night. During the day, marine animals are mainly concentrated in the lower layer. Among them, they occupy a larger proportion in the lower layer before evening, and the decline is the most significant. After dusk, some individuals move to the upper layer, and the distribution at night tends to be scattered. The number of layers is relatively uniform; it concentrates on the upper layer again at dawn, and drops rapidly to the lower layer after sunrise, and the vertical migration of day and night is more significant than that in spring. In autumn, marine animals are more numerous in the middle and lower layers during the day, and move to the upper layers in the dense layers in the evening. The night is similar to summer, the distribution tends to be scattered, and the number of each water layer is relatively uniform; the dense layer drops to the lower layer at dawn. The vertical movement of day and night also generally showed a trend of declining during the day and rising at night, but the number of different water layers is not very different, indicating that the vertical movement of marine animals during the day and night is not very significant. In winter, the marine fauna stays in the middle level during the day, rises to the upper level in the evening, and then quickly descends to the lower level after sunrise. The vertical migration of day and night also shows a trend of declining during the day and rising at night, but the range of movement is relatively small, and is mostly limited to the movement between the upper and middle strata. Only a large number of marine animals appear in the lower strata at individual moments.

At the same time, satellite data also showed long-term changes in the migrating populations of marine animals. These satellite data allow researchers to easily combine satellite observations with models to better quantify the impact of this marine animal migration on the Earth’s carbon cycle. Satellite data is also relevant to global fisheries. Small plankton marine animals that migrate vertically day and night are an important food source for large fish in the deep ocean, and these large fishes are the target of commercial fisheries. The stronger the signal of day and night vertical migration reflected by satellite data, the deeper the ocean is attracted The greater the number of fish.

The latest research of the research team confirmed the distribution of light signals for the day and night vertical migration of marine animals in the global seas, provided a detailed view of the global day and night vertical migration of marine animals, and released new insights on the earth’s climate. On November 27, 2020, the international academic journal Nature published the research results of the research team online.

Subsea storage carbon sink
In terms of total number, the day and night vertical migration of marine animals is the largest migration in the Earth’s biosphere and an important part of the Earth’s climate system. Marine animals that move vertically every day and night have an obvious impact on the cumulative changes of the earth’s climate. Marine phytoplankton photosynthesize during the day and absorb a large amount of carbon dioxide in the process, which helps the ocean absorb greenhouse gases in the atmosphere. Marine animals upstream to the ocean surface to feed on phytoplankton, and then swim back to the depths of the ocean with the carbon dioxide absorbed by the phytoplankton. Most of this carbon dioxide is then released into the deep ocean, effectively trapped deep in the ocean, preventing it from being released back into the atmosphere.

In 2019, a research team led by Kevin Archibald, a biological oceanographer at the Woods Hole Oceanographic Institution in the United States, created a model to determine how much carbon dioxide is deposited into the deep ocean through the vertical migration of marine animals day and night. As a result, they found that squid and shrimp larvae can carry 1 billion tons of carbon dioxide to the depths of the ocean floor every year. According to statistics, all vehicles on American highways produce approximately 1.5 billion tons of carbon dioxide each year. In other words, the day and night vertical migration of marine life offsets two-thirds of the total U.S. vehicle emissions. Kevin Archibald stated that the day and night vertical migration of marine animals only accounts for 16% of the total carbon captured by the ocean. Other ways to capture carbon in the ocean include natural water movement, sinking phytoplankton cells and marine animal Stool etc. Among them, the sinking of marine animal feces will also be greatly affected by the vertical migration of day and night, which accelerates the rate of digestion of nutrients. Some studies have shown that day and night vertical migration transports nutrients to the deep sea 10 times faster than the sinking speed of these substances themselves. It is precisely because of the vertical migration of marine life day and night that these substances can reach the deep sea faster instead of endlessly drifting.

Use satellites to track the vertical migration of large-scale marine life day and night through space lasers.

Excessive emission of carbon dioxide, the main greenhouse gas, has caused a surge in global temperature and caused serious consequences such as melting ice and snow, rising sea levels, and extinction of species. Therefore, controlling the concentration of carbon dioxide in the atmosphere has become a thorny issue facing all mankind. There are usually ways to solve this problem: One is to reduce carbon emissions. Carbon emission reduction includes improving energy efficiency and shifting to a low-carbon energy structure. However, at least until the middle of the 21st century, the primary energy supply will still be dominated by fossil energy. The second is to increase carbon sinks. Carbon sink generally refers to the process, activity or mechanism of removing carbon dioxide from the air, which has the advantages of low cost and good effect. Taking forestry carbon sinks as an example, carbon dioxide is absorbed by forests and collected on trees and land to achieve the purpose of reducing carbon dioxide on the earth. Correspondingly, there are grassland carbon sinks, ocean carbon sinks and biological carbon sinks. In essence, as long as it can produce carbon fixation and reduce carbon dioxide in the atmosphere, it can be called a “carbon sink”.

At present, about 10 billion tons of carbon are transported from the surface of the ocean to the depths each year, which is equivalent to the total amount of carbon emitted by fossil fuels each year. With global warming, how the ocean, as the largest carbon reservoir on the planet, captures and stores carbon becomes more and more important. Studies have confirmed that the vertical migration of marine animals day and night has become one of the ocean carbon storage mechanisms. Marine phytoplankton absorbs carbon dioxide in the earth’s atmosphere and releases oxygen through photosynthesis, and become organic food sources for other levels of organisms in the marine food chain. At the same time, it produces various calcareous biological skeletons or shells. bottom. This is like a water pump, the carbon dioxide in the upper seawater is finally “extracted” and transported to the ocean bottom sediments.

Day and night vertical migration is common in marine animal groups and is a regular and periodic movement that adapts to external living conditions. Many marine biologists have put forward many explanations and hypotheses for this very complicated ecological phenomenon, but so far there is no unified opinion. But one thing is certain: there are many “benefits” in the vertical migration of marine animals day and night, the most important of which is to save energy and breed populations. When shallow water has a higher temperature than deep water, vertical migration is beneficial to marine animals. Marine animals eat in the warm surface water at night, and “rest” in the deep cold water during the day, which reduces their metabolic consumption, so that more energy is used for growth and reproduction, which can effectively increase the population of individuals.