How much does animal life itself influence the physical and chemical functioning of the sea?
We have already described how a whale, dying after a lifetime of travelling the ocean, falls, its great body becoming a moving, changing ecosystem. As it drifts downward, it is a source of food, energy and nutrients for an astonishing succession of sea organisms until it lands and creates its own ecosystem in the deep sea. A whale fall can sustain communities of animals and microorganisms for years. More importantly, it carries material from the productive surface ocean into the depths, where some of the carbon in the whale's body may remain isolated from the atmosphere for a very long time.
The whale fall is therefore more than a story about death. It is a story about the way animals move matter through the ocean.
Living whales are important contributors to the ocean pump as well. A whale feeding in the depths and defecating near the surface is moving nutrients in the opposite direction: upwards instead of down. And whales are not the only animals. Many animals feed at depth and release nutrients near the surface. Others transport carbon downward, while some move horizontally between ecosystems.
We tend to imagine the ocean as a planet-wide physical system governed by currents, temperature, salinity, tides and winds. But that picture leaves something out. For oceanic life not only responds to the conditions created by physics and chemistry—it has great influence upon them.
In fact, the ocean is crossed by invisible highways—not only currents of water, but highways of living bodies carrying carbon, nitrogen, phosphorus and iron in every direction. Fish consume organisms in one place and excrete nutrients in another. Jellyfish bloom, die and sink. Zooplankton graze on microscopic plants and transform them into faecal pellets, dissolved substances, and living tissue.
The twilight zone
Phytoplankton that flourish in the sunlit surface layers, take carbon from CO₂ through photosynthesis (like trees) and it becomes part of them. Then, with the fall of darkness, billions of animals rise from the twilight zone—myriads of microscopic creatures, invertebrates, and countless species of fish—to feed near the surface. Plankton consume enormous quantities of phytoplankton and bacteria, and carry it into the depths as dawn comes.
For centuries, humans had no idea this migration existed—that billions of animals rise hundreds of metres to eat those capable of capturing the sun’s energy, then carry the material into the deep ocean. Their feeding alters the chemistry and the movement of the sea.
The diel vertical migration
This migration is called the diel vertical migration, and it takes place throughout the world's oceans. In terms of the numbers and biomass involved, it has been described as the largest migration of animals on Earth. But these creatures are not simply commuting between two levels of the ocean. They are recycling the material they carry with them.
They are actively transporting carbon and nutrients downward in a process known as active transport: an immense living conveyor belt that moves material from the surface into the ocean’s dark interior.
Thus, the ocean’s animals influence the amount of carbon removed from the atmosphere and sequestered below. They function as a component of the ocean's biological carbon pump. Recent research emphasises just how difficult it is to measure this process directly: we can calculate and model the quantities involved, but much of the actual flux remains poorly understood.
A branching network
Not all biological activity sends carbon downward. Microorganisms and animals also break down sinking material as it falls, dissolving carbon and ultimately producing carbon dioxide again. Tiny grazers and microbes recycle the elements on which the entire food web depends. The biological carbon pump is not a simple one-way elevator. It is a vast, branching network of creation, consumption, respiration, recycling and sinking.
Even swimming fish contribute by creating turbulence, and in huge aggregations, their collective movements can provide fine-scale mixing. This is called biogenic mixing, but researchers think that it has importance only locally because the ocean is primarily mixed by far greater forces—the planet's rotation for example.
A planetary recycling system
Animals not only redistribute carbon. They continually scatter nitrogen, phosphorus, iron, and other elements needed by living organisms. A molecule of phosphorus or nitrogen doesn't simply move once through the ocean. It can pass through an extraordinary succession of organisms. A single celled phytoplankton takes it up, a microscopic grazer eats the phytoplankton, a larger zooplankter eats the grazer, and a fish eats the zooplankter. Then a tuna eats the fish and a shark eats the tuna. The nutrient is eventually excreted, dissolved, incorporated into another organism, or sinks.
Large areas of the ocean contain plenty of sunlight and macronutrients but surprisingly little iron, which phytoplankton need. Animals help redistribute iron. Whales, for example, can acquire iron from their prey and redistribute nutrients when they feed and excrete. Fish and other animals transport trace elements through their bodies that would otherwise be very unevenly distributed. Thus, some of the most important ocean currents aren’t made of water at all.
Animals are constantly passing essential elements from one to another. So the ocean is not merely a food web; it is a planetary recycling system. Indeed, on the molecular level, this is how ecosystems work.
Animals as oceanographers
In places where nutrients are scarce, these movements can be particularly important; the animals are, in effect, transporting pieces of the ecosystem with them.
We usually ask how ocean conditions determine where animals can live. But perhaps we should also ask how the animals themselves alter those conditions. The influence of an individual animal may be tiny compared with the enormous physical forces of the ocean. A whale does not control an ocean current, and a copepod cannot change the temperature of the Pacific. But the oceans host countless local interactions.
Billions of animals feeding, swimming, excreting, respiring, reproducing, dying and sinking, together are part of the machinery that moves elements through the sea. The ocean is not just a physical environment containing life. Life is one of the processes that makes the ocean what it is.
And what does this mean for climate?
This brings us to an even larger mystery. The ocean absorbs enormous quantities of carbon dioxide from the atmosphere. If it is transported deep enough, it can remain isolated for decades, centuries, or even thousands of years.
This means that the animals of the sea are participating, in ways we are only beginning to understand, in the regulation of Earth's climate. A whale moving through the ocean, a school of fish, a swarm of tiny crustaceans rising from the darkness each night, or a microscopic animal feeding on a microscopic plant—none of them appears capable of changing the planet. Yet together, the inhabitants of the ocean are constantly moving carbon and nutrients between the atmosphere, the surface waters, and the deep sea.
The ocean may look like an immense physical machine driven by currents, winds and tides. But hidden inside that machine is another system—alive, mobile and endlessly active. The ocean is not simply home to its animals. It is the animals that help make it work.
Ethologist Ila France Porcher, author of Yes, Fish Feel Pain, The True Nature of Sharks, and six other books on wildlife behaviour, spent fifteen years closely observing fish and shark behaviour in Tahiti, resulting in several scientific papers. Her writings are based on decades of first-hand observations of wildlife and focus on the individuality and intelligence of individuals, challenging traditional views of animal minds. Her work has been featured on Shark Week, in scientific discussions, conservation debates, and international media for its unique blend of field observation, art, and science.
