Ocean as an ecosystem

Stella is spending a sunny afternoon at the beach. She loves the ocean. Sometimes she imagines what it would be like to live in the sea. To be a mermaid, and swim among all the ocean creatures… There would be plenty of space for her to swim! Ocean covers more than 70% of the Earth’s surface and is on average three and a half kilometres deep.

The entire volume of the ocean makes up more than 90% of living space on the planet. As a mermaid, Stella would probably spend most of her time in the shallow and surface water. Here, there is the most sunlight. It’s bright and warm. This is the sunlight zone of the ocean.

This zone is colourful and so full of life! Light in the sunlight zone allows plants and algae to produce their own food and oxygen, through a process called photosynthesis. Tiny free floating algae, called phytoplankton, are especially abundant here. Phytoplankton is food for tiny animals called zooplankton, small fish, and crustaceans. All these, in turn, feed bigger fish, small sharks, sea turtles, corals, and baleen whales.

The final link in this ocean food chain is predators such as large sharks, dolphins, or seals. In clear water, the sunlight can reach down as far as 200 meters below the surface. But the deeper you dive, the darker it gets. This is where the twilight zone starts. There is not enough light here for plants and phytoplankton to be able to photosynthesize.

So plants and phytoplankton here are scarce. This means not much food for other organisms. Many organisms living in the twilight zone depend on dead plankton, animal remains, animal waste products, and other organic material that sinks from the upper layers of the ocean. This so-called “marine snow” is often the main source of food in the twilight zone. Animals that live here include small crustaceans, some types of fish, jellyfish, and octopus.

Some of these animals, for example lantern fish, can produce their own light to help them find food! The twilight zone can stretch 1 kilometre deep down into the ocean. And what’s below it? It gets really dark and cold here. This deep, there is no light at all - it’s never-ending night.

This is the midnight zone. The only food here is dead organic matter that sinks down from above, and minerals from the Earth’s crust on the seabed. Only very few types of organisms have adapted to live in these extreme conditions, with the lack of food, very low temperatures, and the enormous water pressure at these depths. These include: bacteria, worms, and animals, such as sea cucumbers, blobfish, and vampire squid. The darkness, extreme conditions, and the sheer size of this midnight zone make it very difficult to study.

That’s why the midnight zone remains largely unexplored. The ocean depths are still a mystery, and no place for a mermaid! Back at the beach, Stella is thinking about all the living and non-living things in the different zones of the ocean. The algae, animals, temperature, water pressure, amount of sunlight - they all affect each other, and create the largest ecosystem on Earth!