From humpback whales feeding on tiny life to crabs living on plastic islands, there is a lot of life on and near the ocean surface. Today, we’ll talk about the upper layer of the ocean: the Sunlight Zone, with all the life it has to offer.
Chapter 1: Sunlight
We’ve talked a lot about dark depths in the last episodes, working our way from the bottom of the ocean upward. This time, we’ll be talking about the parts of the open ocean where light can reach, especially the Sunlight Zone or Epipelagic.
In this region, photosynthesis plays a major role in the ecosystem. Tiny magicians called phytoplankton take sunlight to produce energy other species can eat. These phytoplankton build the basis of the food web and get eaten by a whole lot of things, most of which are still too small for me to give a crab about.
Don’t get me wrong: phytoplankton is super important for the climate of Earth. Scientists believe that about half of the oxygen production on Earth comes from the ocean–and most of that from plankton.
I promise we’ll get to bigger things in a second but I quickly do want to mention Prochlorococcus, a tiny organism–actually, the smallest photosynthetic organism we know of–that collectively produce about 20% of oxygen in Earth’s biosphere. That’s pretty damned impressive. I guess size isn’t as important as some people claim after all 😉
If you care more about these little things, I made an episode on plankton before and there are also a few on climate cycles and such.
For now, let’s move on to bigger things:
Chapter 2: What lives in the epipelagic?
Temperatures on and near the ocean surface vary widely. Temperatures range from a freezing (well, no, because it’s salt water but still ducking cold) -2 degrees (28 F) to almost body temperature, 36 degrees (97 F) in the Persian Gulf. Quite the range!
Naturally, different animals live in pelagic Arctic waters than near the equator. And what a variety there is!
About 90% of ocean life lives in the epipelagic, so the first 200 or so meters of water column. I’m pretty sure this includes the coastlines, and as such some benthic organisms that should not be included. But it’s hard to find actual statistics.
There is quite a bit of overlap of the kinds of life in the deeper ocean layers and upper ocean layers, though the exact species are different. Just like in the deep pelagic, you’ll find squid and other cnidarians, all kinds of jellies, and a lot of fish. Some species even migrate from the deeper layers to the upper layers–or dive from the Twilight Zone to deeper zones. The ocean isn’t static.
Speaking of migration: a lot of the migratory species in the ocean inhabit the upper layers of the ocean.
Migratory species, well, migrate, so they roam around the ocean. This can be a smaller range or thousands of kilometers. For simplicity, let’s keep the birds for another day and just mention that there are plenty of birds migrating all across the globe every year.
A few years ago, on a trip along the Pacific Coast of California, we were lucky enough to see a few humpback whales. I still remember one whale poking their head out of the waters right next the boat, showing off every single barnacle on their face.
Migrating whales
Humpback whales have seasonal migration routes. They migrate to colder waters with more food for the summer months, then migrate back toward the equator to mate and give birth. Their migration routes are thousands of miles long. That’s quite a bit of swimming.
As baleen whales, so whales with those weird-looking parallel plates of tooth-stuff, they are among the really cool, really interesting large species that almost directly interact with the phytoplankton I care so abstractly about: they feed on zoo plankton, so one rung up from those tiny photosynthesizing magicians.
There are other whales that also migrate like the blue whales. But let’s leave it at that. We will talk a lot more about these pretty giants when we reach the whales on the Swimming the Tree of Life series. Swimming? Yes, swimming. We’ve been in the water long enough 😉
Migrating fish
Bluefin tuna are among the biggest migrators. They just go where life is good and migrate from the Western Pacific to the Easter Pacific or between the Western Pacific and New Zealand frequently. So, instead of the North-South migration we know of many seasonal species, these largest of the tuna migrate sideways. Interesting.
Quite the feat that they still manage to pull all that off with how many of them are getting fished by human predators every year.
In more tropical waters, you’ll also be able to find blue marlins and swordfish, two other migratory fish that inhabit the Sunlight Zone.
But there are so many more migratory fish species that it’s hard to even list the groups. It really goes from A like anchovy all the way to… wait, no, I don’t know a fish with a Z. Let’s just say there are a lot of fish in the pelagic, many of which migrate, too. And remember, fish includes sharks.
Migrating turtles
Turtles, in general, tend to be migratory, roaming hundreds or even thousands of kilometers to get from where they like to live and where they like to duck and give birth.
Leatherback turtles are especially impressive, as they can travel 10,000 miles or more in a year. In a year! But Loggerheads are just a couple thousand miles behind, so it’s all pretty impressive.
And turtles are getting hit pretty hard by humans: light pollution makes turtle babies walk the wrong way and never find the ocean; rising sea temperatures duck with the balance of male to female; and they keep getting run over by ships and boats. A friend just interned at a turtle rehab, and just hearing about them made me angry.
Anyway… Speaking of reptiles: there is even a snake, yes, a ducking snake, that lives in the open ocean. The yellow-bellied sea snake, Hydrophis platurus, seems to only get born (and I assume lay eggs?!) on land but despite this supposedly being the most widely distributed snake species, wikipedia is the only source I have, so I’m highly skeptical. They say, they form groups of up to a thousand, too. Damn, now I wanna see a thousand snakes swimming around the ocean.
The Great Pacific Garbage Patch
People keep talking about the Great Pacific Garbage Patch as if it is an island. It should probably better be called the Great Pacific Garbage Soup. We’ve long been supporters of the Ocean Cleanup Project, one of the few tech solutions I can get behind (because they are working on all ends from figuring out the source of the plastic, preventing river run-off, and collecting what’s already in the ocean). Not sponsored, of course. Advertising sucks. I won’t do that to you.
Where was I? Right, The GPGP. Trash floating around the ocean creates refuge for species that used to rely on other flotsam and the very turtles we talked about earlier. In one episode, we talked about a species of crabs that inhabit the butt crack of turtles. They now also live on plastic flotsam, though it has changed their social structure.
I would have liked to spend a complete episode on this very special ecosystem but there just isn’t enough science yet.
The very trash that allows certain animals to live also kills others. Turtles, dolphins, and fish can get tangled or eat some of the debris. So, don’t worry too much about the cute fish we see floating around the trash. Yes, that individual group of fish might die, but we’ll be saving others. I’m sure the crabs would prefer to return to the butt crack of turtles instead of bickering on plastic floats. Okay, yeah, no, I’m not sure. But you get the point: we need to restore balance.
And that means removing our trash from the ocean and dealing with it.