Miracle Happens Everyday: The Marine Migrations
By: Arslan Mirza
Tags: Migration, Miracles, Underwater, Marine, Ocean Life

They come from the cold deep sea and are extremely greedy for Plankton. Moreover, their schedule is very tight, because when the sun shines on the sea again, all these creatures must go back to the darkness again to avoid being swallowed by predators roaming the sea during the day.

This “Dance” between light and darkness, warmth and cold, Predator and Prey is called “diel vertical migration” (DVM). This is considered to be the largest animal movement on the planet. Whether it’s the Wildebeest on the Serengeti grassland, the monarch Butterfly migrating on the North American continent, or the Penguin that traverses the Antarctic Continent of Vietnam, none of them can be compared with the simultaneous global migration of marine life every day.

Of course, most people still know very little about this migration, although DVM plays a huge role in absorbing carbon from the deep sea and fattening the fish we eat.

Rebecca Helm, a marine biologist at the University of North Carolina at Asheville in the United States, said: “When you think of an animal living in an open ocean, it is likely to experience a certain Vertical migration of form.”

Although the details of the model of the day and night vertical migration of marine organisms may be complicated and will vary depending on the species and location, a Copepod may only move a few meters, while a Gelatinous Sea Squirt may move nearly 1,000 Meter. However, the basis of this migration seems fairly simple-mainly about sunlight and food.

Throughout the day, the sunlight has been providing the energy base for the tiny Algae — Phytoplankton — at the top of the ocean. Although these organisms are small, they are numerous and form the basis of the food chain.

They nourish countless life forms, from Crab Larvae as small as a few millimeters to Car-Sized Whale Sharks. The problem is that to maintain life-sustaining photosynthesis, Phytoplankton must stay on the surface of the ocean within 200 meters of water because that is the longest distance that sunlight can penetrate.

Rebecca Helm said. “You can think about the entire depth of the ocean, and you can find that this layer of photosynthesis is like the skin of an apple — it’s too thin”, “So, to make the most of this In rich areas, the creatures in the ocean must migrate here to collect all the food”.

It is difficult for us to intuitively understand how many animals participate in this daily migration, but we can think about it this way. During the Second World War, when scientists first tried underwater sonar to detect German U-shaped submarines, the navy’s echo sounder consistently showed a “solid layer” between about 120 and 180 meters below the sea surface.

At first, scientists inferred that sonar had detected the seafloor

But here comes the problem. This “sea bed” has been moving, becoming shallower at night and deeper during the day. Over time, the scientists discovered that what they saw was a “fake seabed”. This dense “solid layer” is actually composed of densely packed large groups of Shrimp, Squid, and Pipe Jellyfish.

Echo detection image was taken in Saanich Bay, British Columbia, Canada. Krill is a small shrimp-like crustacean whose population forms a thick layer that looks almost like a solid seabed

More importantly

Scientists have found that the gathering of these lives is so regular and predictable. In the end, Sonar researchers named this “deep-sea scattering layer”, which is a dense horizontal area of ​​organisms that can scatter or reflect sound waves. This scattering layer is so thick that there is still some debate about whether the enemy submarine can hide it.

Rebecca Helm said,

“A lot of scientific research is promoted by the military personnel, and their thinking is, We must figure out what is going on!, “If it weren’t for these underwater During the war, we may not know what this strange mysterious layer is”.

Since the Second World War, scientists have mastered a lot of knowledge about the ocean, and each discovery paints a more complex picture of marine life.

For example, although day-night vertical migration is mainly a biological phenomenon, it is also influenced by physics.

Philip Hosegood, a physical oceanographer at the University of Plymouth in the United Kingdom, said that the maximum depth of photosynthesis that Phytoplankton can carry out is determined by factors such as water clarity.

The clarity of the water is determined by a series of other variables, such as water temperature, wind speed, tides, ocean currents, salinity, and light.

Hosgood said,

“The ocean is a three-dimensional space,” “Of course, in fact, it is four-dimensional because it not only changes horizontally and vertically but also changes over time.”

If you have some understanding of Fluid Dynamics, you can have a deeper understanding of the distance that micro-organisms migrate. Otters, Octopuses, and even humans can slide in the water with relatively low resistance, but the smaller the size of the organism, the more difficult it is to overcome the natural viscosity of water (this is why ants are trapped on the water, and we can be free in the pool, the reason for swimming, all this is due to the difference in Reynolds number).

For the particle-sized Zooplankton, swimming in the water is like a person paddling in molasses-and these creatures have to trek back and forth in the sea more than 100 meters deep every day!

Although there is some form of Diurnal vertical migration where Plankton grows, there are still many mysteries that have not been solved.

For example, many animals move vertically in the reverse direction of day and night, they migrate to places with sunlight during the day and leave the water when they leave at night.

Even within a species, migration behavior changes due to different locations

Sammy Andrzejczak, a shark researcher at Stanford University in the United States, co-published a review of the vertical movement of large fish in 2019. “Coral reef ghost manta rays can carry out normal day and night vertical migration.

For example, in places like the Chagos Islands, they swim deep during the day and shallow at night,” He said. “But in some areas, they behave oppositely. For example in the Red Sea and Seychelles.”

Day-night vertical migration is not only important for filter-feeding animals such as Manta rays, but also animals at all levels of the food web.

Big fish eat small fish, small fish eat dried shrimps, dried shrimps eat zooplankton

The larger the animal, the larger the Predators that will Prey on it. The team of Anjay Zach mainly studies how some shark species use lateral migration to Prey on fish that migrate vertically day and night, especially tuna.

David Curnick, a marine biologist at the Zoological Society of London and a colleague of Anjay Zach, said: “The sickle shark seems to stay near the seamount during the day and go out to feed at night.”

This means that marine animals will not only migrate up and down but also laterally, from relatively safe areas to areas with higher productivity.

Perhaps the most interesting point is that what happens in this thin layer of the ocean surface has an impact on the world above and below.

Rebecca Helm said that when all these tiny Phytoplankton photosynthesizes on the surface, they consume a lot of carbon dioxide, and when they are eaten, the carbon will be taken to the deep ocean.

In fact, scientists created a model in 2019 to determine how much carbon is deposited into the deep ocean by vertical migration day and night

They found that those Squids, Juveniles, and Shrimp Larvae can carry 1 pg (Pg, 1 pg = 10¹⁵ grams, or 1 billion tons) of carbon to the deep seabed every year.

Kevin Archibald, a biological oceanographer at the Woods Hole Oceanographic Institution in the United States and the lead author of this study said,

“All vehicles on the roads in the United States generate approximately 1.5 Pg of carbon per year.” In other words, the day and night vertical migration of marine life offset two-thirds of the total U.S. car emissions.

Photo by David Clode on Unsplash

Archibald said that day and night vertical migration accounts for only 16% of the total carbon captured by the ocean

Other contributing factors include natural water movement, sinking Phytoplankton Cells, and marine animal Feces-indeed a massive phenomenon on the ocean scale.

The sinking of Feces will also be greatly affected by vertical migration day and night. Vertical migration day and night also accelerates the rate of nutrient digestion.

Some studies have shown that day and night vertical migration transports nutrients to the deep sea faster than the sinking speed of these substances themselves. It is precise because of the vertical migration of marine organisms day and night that these substances can reach the deep sea faster instead of endlessly drifting.

The vertical migration of day and night also means that many underwater creatures that have never been exposed to sunlight also rely on the sun upon the final analysis.

In fact, we are not much different from them. Although we are basically unable to witness these activities in our lifetime, the vertical migration of marine life not only provides us with food but also plays an important role in the rapidly changing Earth’s climate.

Changing climate will also affect the vertical migration of day and night, although the exact method of impact is still unknown.

Higher water temperatures may reduce the day and night vertical migration activities in tropical seas, which are less active in these areas, while in cold regions such as the Poles, day and night vertical migration activities will be larger and more active.

There are still many uncertain issues, and scientists still need to further study the mechanism of day-night vertical migration. Although most people just know the existence of this phenomenon, we have to say that we should still be grateful for the marine creatures who trek back and forth every day.

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