High sea, deep sea? Ocean zones explained

The ocean supplies more than half the oxygen we breathe, and provides food and livelihoods for billions of people worldwide. It is also home to a wondrous array of species, from tiny plankton to blue whales.

But the ocean is in trouble. 

Centuries of overuse and neglect have damaged the ocean. It’s time to change the way we see it, from a place where we take what we want and dump what we don’t, to a shared resource. 

We recently celebrated the agreement of a High Seas Treaty, creating a framework to conserve marine life and restrain harmful activities in two-thirds of the ocean.  But what exactly are the high seas?

Let's take a look some of the terms used to describe different areas of the ocean and the particular threats they face. 

So what are the High Seas?

The United Nations Convention on the Law of the Sea (UNCLOS) divides the ocean up into areas.

• The territorial sea or waters of a country extends 12 nautical miles (22 km) from the coast.
• The exclusive economic zone (EEZ), the area of the ocean extending up to 200 nautical miles (370 km) immediately offshore.
• The high seas, also called international waters, the ocean outside of all countries’ EEZ. The area that belongs to everyone, and no-one. 

Because they are beyond national jurisdiction, protecting the high seas requires international agreement. The new Treaty finally establishes a way for us to do that. 

What do we mean by deep sea? 

It’s not just the law that divides the ocean into different areas. Geographically and biologically the sea has zones and subzones.

The deep sea starts at the depth at which sunlight starts to dim, typically around 200 metres (656 feet) and goes right to the bottom. It makes up about 95% of Earth’s living space. Because it is cold, dark, and under extreme pressure it is largely unexplored by humans. Which hasn't stopped us from trying to exploit it. Deep-sea mining would involve using heavy machinery to suck rocks or nodules containing minerals off the sea floor, causing unknown damage. We need to stop it from happening.  

So what do we call the bottom of the ocean?

A number of things. It can be referred to as the benthic zone, the sea floor or the sea bed. Just like the land surface, the sea floor varies considerably in height and ecosystems. From coastal kelp forests to deep mid-oceanic trenches. While many benthic environments need protecting, we particularly need to protect the seamounts (undersea mountains) that are home to dense corals from destructive fishing methods like bottom trawling. 

What does pelagic mean?

That’s the open sea, basically all of the ocean that’s not the shore or seabed. It’s where fish like tuna and most species of whales swim freely. The pelagic zone is the largest habitat on earth but it is less densely populated than the coastal areas or the seabed. 

As you can see these terms, because they have different purposes, can overlap.  An area can be part of the high sea, open sea, and deep sea all at once. And in fact most of the ocean falls into that category - open waters beyond national borders and below 200 metres. Which is why protecting these regions is so crucial. Currently only 1% of the high seas are protected, and we will not achieve the aim of protecting 30% of the ocean by 2030 without including these regions.