Zones of the Ocean

To most people, the ocean is a more or less flat-surface, just like the ground beneath our feet. But there is so much more under the waves. From the surface to the deepest depths, from the shore to the open ocean, we’ll take it apart—and even get into some special zones.

What to expect from this episode:

  • Zones from top to bottom
  • Zones from shore to open ocean
  • Special zones:
    • Underwater mountains
    • Underwater canyons
    • Hydrothermal vents
    • Cold seeps

The zones of the ocean from top to bottom

Let’s start simple: top to bottom.
The very simplest division based on depth is into benthic and pelagic. Benthic is near, on, or even in the ground, while pelagic is, well, the rest. But that’s far too simple. So, let’s dive a little deeper.

Light determines a lot when it comes to ocean life—and that light vanishes quickly with depth.

The surface of the ocean, naturally, gets the most light. The sun hits the surface, and because of the water, the organisms, and all the shit—literal and metaphorical—that swims around in the water, less and less filters down.

Light can be perceived up to 1,000 meters deep. Wow!
I’ve been below the waves, but my deepest depth was around the recreational limit of SCUBA, so 40ish meters or 130ish feet. There was still light there, but it’s nothing like the light in the first meters below the surface. It seems to dim fast, so any light reaching multiples of that is hard to fathom.

Of course, there are a lot more particle shit in the near-shore areas, so the light reaches a lot less deep there. The euphotic zone is often only 50 meters deep near the coast due to river run-off, sediment getting stirred up, and such.

As the only open-ocean/blue-water diving I’ve done was at an oil rig, it’s hardly a good measure for untouched, undisturbed open ocean.

But, considering how quickly it gets too dim for, at least, humans to see, it does make sense that the “we can see light” zone was split in two: The euphotic (sunlight) zone is the area with proper lighting. It’s usually down to about 200 meters and where all the super important photosynthesis happens.

Below that, you’ll find the dysphotic (twilight) zone, where light decreases rapidly and photosynthesis is usually not an option. Hard to convert light when there’s so little and so unreliably.

And finally, there is the aphotic zone, the zone without any light. To borrow the poetic wording of NOAA: “This zone is bathed in darkness.”

While summing up everything below a thousand meters might sound totally reasonable to most people—after all, their world is essentially all above even the sunlight zone—this does not make sense for anyone in the ocean sciences.

Graphic by Kate Hildenbrand, based on information from NOAA.

Remember, the average depth of the ocean is 3.7 kilometers, so almost four times that limit. The deepest known point of the ocean is around 11,000 meters, so a couple thousand meters above even the highest mountain on Earth. There are a lot of depths to the ocean, so we can’t just sum all that up into “deep.”

The euphotic or sunlight zone is also considered the epipelagic. The dysphotic or twilight zone is also considered the mesopelagic. Both of these zones have pelagic in the name, so it probably makes sense to exclude the coast from this metric.

Below, so in the aphotic zone, the following zones are subdivided: the bathypelagic (midnight) zone lies between 1,000 and 4,000 meters. Then, the abyssopelagic (or just abyss) follows between 4,000 and 6,000 meters depths. The lowest of the low is the hadopelagic or hadal zone below 6,000 meters. This latest zone is essentially the trenches or canyons in the ocean.

So, in short:
sunlight, twilight, midnight, abyss, hadal

Or in “smart:”
epipelagic, mesopelagic, bathypelagic, abyssopelagic, hadopelagic, the last three of which are all aphotic.

Moving on to the second way to zone the ocean:

The zones of the ocean from shore to open ocean

Instead of going deep, next we’ll go from coast to open ocean. Where to start is a bit hard, as there are about a million definitions when it comes to the near-shore. What’s the difference between a beach, the shore, the coast? The answers to those will come later in this series. For now, we’ll only include areas that are underwater—at least sometimes.

The intertidal zone is the area that’s “beach” at low tide but “ocean” at high tide, so the area where the tides change life dramatically due to the tidal range.

Below that, we’ve got the continental shelf. As the continental shelf is important for jurisdiction, there are a lot of hard-to-understand definitions like “the continental shelf of a coastal State comprises the submerged prolongation of the land territory of“—yeah, no. Not even gonna quote that to you.

In essence, the part of the continent that’s underwater is the continental shelf.

After that, it’s all about angles. The edge of the Continental Shelf is where the slope of the bottom gets a lot steeper, so the coast essentially dips off.

Though, to be fair, there is a lot of legal jargon around this one, as it seems to have to do with territory and ocean law. But we’re not talking legal shit right now, so let’s not get into any of that. Let’s get through one episode without getting into politics or doom and gloom, shall we?

So, we’re following a steep wall down into the abyss—ha!—until the slope flattens out. This flatter section is the Continental Rise which reaches all the way to the bottom.

The bottom isn’t the bottom of the ocean, but the flat plains that make up the bottom of ocean basins. It’s like a very, very, very big meadow: there are things sticking up or going deeper, but it’s still mostly flat surface.

The flat surface at the bottom of every ocean basin is the Abyssal Plain, only disrupted by the ocean trenches, sea mountains, and the like.

So, in short:
Intertidal Zone until we reach the low-tide line, then Continental Shelf until the dip, then Continental Slope until it gets less steep, then Continental Rise until things get flat, where we find the Abyssal Plain.

The zones of the ocean from EEZ to international waters

The next way to divide the ocean is by who the waters belong to, so from exclusive economic zones to international waters, but I’d really like to not get into politics today, so we’ll leave that for another episode. It’s dry stuff, anyway. Instead, let’s talk about special zones.

Special zones

Some of the special zones are made up of the very disruptions to the ocean floor: cracks, canyons, and even mountains. Others are defined by their water chemistry, and yet others by what’s allowed in them.

The least fun special zones are low-oxygen zones, or “dead zones,” where the oxygen levels are so low that few organisms can deal with the conditions. We’ll talk about those in an episode soon.

The most fun special zones are marine protected areas, areas where there are restrictions on human activity. I’m currently writing my bachelor thesis on these zones, so they are a bit of a special interest to me. We’ll talk about them later in this series, but probably also a million more times because I’m a bit obsessed with them.

Underwater mountain ranges

But back to our ocean floor: Did you know the longest mountain range is underwater?

The mid-oceanic ridge system spans like the seams of a baseball across the globe. The total length is about 65,000 kilometers (40,000 miles). More than 90% of that is submerged, so underwater.

But what’s a mid-oceanic ridge? These seafloor mountains are formed by plate tectonics, so the plates that make up our planet moving toward each other, away from each other, or alongside each other.

In the case of mid-oceanic ridges, the plates are pulling apart, so diverging. When they pull apart, the planet’s mantle wells up between the two separating plates as lava. That lava cools and leaves new ocean floor.

So, technically, the mountain range is a volcanic mountain range. Fun!

These mountains are, on average, reaching up to about 2,500 meters depths. Okay, because mountains are usually above water, and we care only about their height, let’s try that again: the average depths of the top of the mountains is 2,500 meters. That’s not the height of the mountains, but how high they reach toward the surface.

What those mountains look like mostly depends on how fast the plates are pulling apart. Slower divergence leads to steeper mountains, while faster divergence creates smoother and gentler slopes.

Oh, and, naturally, mid-oceanic ridges don’t necessarily have to be in the middle of the ocean. At least not literally. The first one seems to have been the Mid-Atlantic Ridge, which happened to be pretty much dead center. Well, the term stuck even after less centered ridges started getting discovered.

Graphic by Kate Hildenbrand.

Underwater canyons

The opposite, essentially, are oceanic trenches. If mid-ocean ridges are sea mountains, then these are the sea canyons.

And, just like with the ridges, they are massive. With about 50,000 kilometers (31,000 miles), though not consecutively, there are quite a few of these canyons under the waves. I could also not find a reliable source for this number, but considering the Pacific Ring of Fire alone is 25,000 kilometers long, it does sound reasonable.

Where the mountains were caused by plate divergence, the trenches form where plates converge. When two plates meet, the denser plate pushes underneath the more buoyant plate.

These oceanic subduction zones usually happen when a continental crust meets an oceanic crust—in which case the continental crust always stays on top. Imagine it didn’t, and the coast would be literally pushed underwater in addition to getting drowned by sea level rise. No doom and gloom. Right. Moving on.

The deepest trench, the Mariana Trench, which most people have probably heard of before, is one example where it’s two oceanic plates meeting. Pressure there is about 11,000 times what it is at the surface. This kinda limits what can live there. Gas-filled lungs are kinda out of the question, so most things down there—even the fish—are gelatinous like jellies.

There will be a deep-sea episode later in this Zones of the Ocean series, so I will definitely tell you more about how organisms survive down there without light, at near-freezing temperatures, and at those pressures.

Hydrothermal vents, cold seeps, and volcanoes

No matter if we’re talking about ridges or trenches, a lot happens at these places where tectonic plates interact. If you’ve ever been to a geothermal park, you’ve probably waited to see a geyser in action, smelled the sulfuric off-gassing of toxic-looking pools, and felt the heat emanate from the hot pools.

Similar things happen at the bottom of the ocean, though conditions are quite different there.

Hydrothermal vents

Hydrothermal vents are essentially cracks in the seabed that let lava-heated water escape. Logically, this tends to place them near volcanic activity. They are usually found at the mid-ocean ridges.

And conditions there are tough. In addition to the pressure we already talked about—even at the shallower ridges—the temperatures in the deep sea are another factor.

So, while the water around the vents is near-freezing, the vents spew out hot and hotter water. When they meet, the minerals crystallize out. The stacks these minerals form can get quite high. Some reach heights of 55 meters.

They come in two flavors: Black smokers with dark sulfides and chimney-like structures, and white smokers which emit less dark minerals. You’ll usually find—well, if you’ve got a submarine—the black smokers closer to the heat source, the white ones further away.

In essence, they are the geysers of the underwater world—just without the tourists waiting for them to finally spew hot liquid again.

Cold seeps

Cold seeps are areas where gases such as methane seep from the ocean floor and enrich the waters there. While some organisms can chemosynthesize these hydrocarbons, the gas is toxic to most oxygen-breathing organisms. Methane fucks with respiration. Despite this, there is a high biomass at these cold seeps. While there might be less biodiversity at these seeps, there definitely is a lot of life.

Cold seeps might be a bit of a misnomer, as they are usually still warmer than the surrounding waters, but compared to hydrothermal vents, they are definitely cold.

As you can tell, there is a lot going on in the ocean. It’s like there is another entire planet down there. I can’t wait to dive deeper into all of these zones. So much to explore; so much to learn.



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