The ocean is the biggest habitat on Earth. In total, 361 million square kilometers of water cover the surface of the planet, which totals up to 71% of Earth’s surface area. That means there’s more than twice the area of ocean than dry earth on this planet that we call “Earth.” And that’s a good thing, because in a very real way, all life here depends on the ocean. For marine life, this dependency is obvious. But even terrestrial life needs the ocean for things like the moisture it gives off constantly. Without this, rains would not come, plants could not grow, animals would have nothing to eat, and all terrestrial life would cease. So it might come as a surprise to know that, well, the ocean is actually a desert. I can explain. Don’t worry. But, to do this, first, we need to take a look at the breakdown of Earth’s different biomes. Green is terrestrial; blue is marine. We can see that the open ocean is the clear winner here, making up 65 percent of all the surface on Earth. To make everything easier to see, I’m going to let the open ocean go off-screen, but just remember, it’s much larger than the rest of these. The next biggest piece of Earth’s surface is submerged continental shelf, which is basically just less-deep ocean. Below this, we have all of the terrestrial ecosystems. The most common environment on land is– –and this is how the research text described it– EXTREME. DESERT. This includes bare, exposed rock (like on mountains), sand dunes (or any other place sand completely covers the ground), and ice. You know, like glaciers and Antarctica. Basically, this category is anywhere that nothing whatsoever can grow. And it’s the biggest terrestrial one we have. Then, right after EXTREME DESERT, the second most common terrestrial ecosystem is desert and semidesert. So, pretty similar to the last category. But in these, you’ll find some occasional shrubs and grasses. Together, these two desert regions make up over a quarter (28%, actually) of all the land on Earth. Below these are the rest of Earth’s terrestrial ecosystems: your forest, grassland, farmlands, you know, places where things can actually grow. And then, finally, below these, we have the much smaller marine ecosystems like swamps, lakes, estuaries, and so on. I would just like to point out real quick that algal beds and coral reefs make up only about 0.1% of the Earth’s surface. Just keep that in mind for later. It’ll come back, I promise. But, to figure out what I meant when I said “the ocean is a desert,” we now need to take a look at a different graph. This one shows the “average net primary production,” which basically means the rate at which the primary producers (like plants and anything else that go through photosynthesis) produce useful chemicals (you know, like glucose or something) in an ecosystem. This is measured in “grams per square meter per year,” so “how much mass of biologically useful materials is created over a given area over a given amount of time.” This is the most important figure to look at when assessing an ecosystem’s productivity, because it’s the primary producers that everything else feeds on– –either directly or indirectly through a complex food web. All energy in the ecosystem is managed by these primary producers, and so *they* determine how productive the rest of the ecosystem is. What we can see from this graph, then, is that surface area definitely does not equal productivity. This becomes very clear when looking at the open ocean–remember, the largest biome on Earth by far– –which has a productivity that’s barely visible on-screen. In total, the open ocean only produces around 125 grams of organic matter per square meter per year. The most comparable terrestrial ecosystem is the regular desert, which only produces around 90 g/m^2/yr — –which means one square meter of the ocean produces a similar amount of organic matter as a desert. In sharp contrast to this, we can see the most productive ecosystem on Earth is our algal beds and reefs, with an average net primary productivity of 2500 g/m^2/yr. That means the open ocean has only about 5% the productivity compared to reefs. In fact, it’s here, in the algal beds and coral reefs, that 25% of all marine species call home– –in, if you’ll remember, was remember only 0.1% of the Earth’s surface. What this also means, is that at 2500 g/m^2/yr, these minuscule marine ecosystems are more productive than tropical rainforests– –which are the second most productive in the world, producing 2200 g of organic matter per m^2 per year. But, back to the open ocean. Besides desert, tundra is just slightly more productive, meaning, yes, this^ is more productive than the open ocean. If you wanted to take a look at this graph in map form, it would look something like this. The darker blue and purple areas are the lowest productivity, while the closer to red we get means the more productive the area is. There’re some things I want to point out on this map, so we’ll return to this later. But, if we wanted to put the two graphs we just loooked at together, they’d look like this– which shows the percentage of Earth’s primary productivity each biome accounts for. Remember, these are organized by the percent of land area they occupy on Earth. So, yes, while the ocean is on top, with 24.4% of the world’s primary production, that’s spread over 65% of the surface area. Meanwhile, the tropical rainforests account for almost the same exact amount of primary production–22%–while occupying only 3.3% of its surface. If this doesn’t convince you that the rainforests are worth protecting, I don’t think anything will. Then, let’s look at the EXTREME DESERT and regular desert. Well, we can’t really look at them, because they’re so small. Despite being 28% of the Earth’s dry surface, together, they don’t even account for 1% of the Earth’s productivity. Now, just looking at desert and semidesert, we can see it accounts for 0.94% of this total productivity– which just so happens to be the same amount as algal beds and coral reefs– which is crazy, because deserts make up 3.5% of the Earth’s surface, while reefs and algal beds only take up 0.1%. And if that doesn’t convince you just how important coral reefs are, well, then you must be watching this on mute.
[and with no captions] I’m going to show all three graphs now, and I highly encourage you to pause the video to just take a look at all these together and find some more crazy comparisons on your own. …OK, I’m going to assume you paused it and had your fun, so let me know in the comments if you found anything surprising. But, back to the video, the next thing you must be wondering is: “WHY?” “Why is the ocean so unproductive?” “I thought these were super-important sanctuaries that life on Earth couldn’t exist without.” “I thought I trusted you, ocean.” Well, OK, don’t get me wrong; the ocean is still tremendously important, but I get what you’re asking. To explain why there’s a huge discrepancy between the ocean’s size and its relative productivity, we’re going to need to take another look at this map. To me, there are five interesting things happening here. First, you’ll notice the least productive areas are these huge cells of open ocean roughly between 30°N and 30°S. But then, at higher latitudes, we enter the temperate regions, and the waters actually become more productive–especially in the northern hemisphere. In addition to this, we can see the west coasts of many continents harbor small areas of high productivity, while we don’t really see the same thing on the east coasts, and sometimes even the opposite. There’s also an interesting phenomenon happening at the equator, where the waters become slightly more productive than the surroundings, despite being completely surrounded by water. And then, we have a few seemingly random patches of higher productivity throughout the world. So, what’s causing all of this? To understand this, first you need to know what [the] primary producers of the ocean actually are. You see, here on land, plants occupy this incredibly important role of turning sunlight into biologically available energy. But, plants need to grow roots. And to do this, they need soil. Without it, they aren’t capable of taking in nutrients from their surroundings. This isn’t a problem on land, well, for obvious reasons: there’s soil everywhere. That’s kinda what makes it “land.” And this isn’t really even a problem on the coast, because the water here isn’t too deep, and so plants can grow in the submerged soil as long as they still have access to light in order to photosynthesize. This area, where the sunlight penetrates the water, is what we call the “euphotic zone,” and, in clear water, it can extend down to over 200 meters deep. But, the average depth of the ocean is around 3700 meters deep, which means, for the majority of ocean water, no light shines upon it. At this depth, the conditions are what’s called “aphotic,” meaning “without light.” So, plants are only able to grow roots for nutrients here^ [green zone], but are only able to make use of those nutrients here^ [yellow zone] So, yeah, plants like what we’re used to don’t really grow in the majority of the ocean. What’s needed instead is an organism that can remain in the euphotic zone of the ocean, right by the surface, and must also be able to access the nutrients just floating around in the water. What we have in this role is phytoplankton, which isn’t actually any single type of organism, but rather, a collection of many different microscopic bacteria, algae, and yes, even some very interesting plants. There are too many varieties of these to even begin to count, and they all live together, floating around in the water, completely unseen by us humans. But if you took a drop of ocean water and put it under a microscope, it would look something like this^, absolutely brimming with microscopic life. What all this means, is that, as the primary producers in the ocean, phytoplankton control the distribution and density of all other organisms. So then, what controls where phytoplankton grow? Well, because phytoplankton are still photosynthetic organisms, they can only occupy the top layer of the water column: the euphotic zone. Growing here, they’re almost never limited by the sunlight that’s available to them. The thing that is limiting, then, is access to nutrients. Yeah, not a lot of nitrogen and phosphorus float around in this water. So, while this map shows the primary productivity in the ocean, it can basically be read as a map showing where nutrients are distributed on the surface of the ocean. Where it’s dark blue / purple, it’s nutrient-poor. Brighter red? It’s nutrient rich. Understanding this relationship between phytoplankton, light, and nutrients, we can begin to learn about the unique features on this map. First, the large, dark, unproductive deserts covering most of the planet. Since there’s little productivity, we have to assume, too, that there’s little nutrients in these areas. So the question now becomes, why aren’t there nutrients here? In short, this is due to what’s called a thermocline. These things are kind of complex and deserve their own video, but I’ll try to explain them here really fast. Basically, because light and infrared don’t really penetrate deep into water, only the top layer of the water column receives the warmth from the sun. We already talked about this; it’s called the euphotic zone. Naturally, this water that’s exposed to light heats up, while all the other water below it remains cold, no matter where in the world it is. Because warm water is less dense than cold water, this creates two layers of differing densities in the ocean: the cold, dense, deep part, and the warm, less dense, shallow, upper layer. These layers behave completely separate from one another, with different systems of circulation, and can even have completely different currents. What ends up happening in this stratified water column is that all the heavier nutrients sink to the deeper, colder water, and because these two layers barely interact with one another, the nutrients never get taken back up to where there’s light, so they can’t be used. And that’s what’s happening in all these huge areas of low productivity throughout most of the world’s ocean. It’s this thermocline that separates the primary producers from the nutrients they need to flourish, and therefore, no highly productive ecosystem can develop. Just like how “no rain” causes deserts to be barren, “no nutrients in the euphotic zone” makes the ocean a desert. This also explains why we have a greater amount of productivity in the higher latitudes. As we all should know, the higher we move up in latitude, the colder it gets, on average. This leads to a breakdown of the thermocline every year in this area, allowing nutrients to cycle in from deeper waters. With this increased circulation of nutrients into the euphotic zone, more phytoplankton can grow, jumpstarting the entire food web. But, as we continue further towards the poles, it gets too cold for a thermocline to ever develop, so nutrients cycle throughout the waters year-round here. Because of this, you would think these would be the most productive areas, and, well, yeah, they *are* very productive. If you think about it, all the places that harbor massive fishing operations are in these colder climates, but it’s at this point that light begins to become a limiting factor. Above 60° latitude– –well, in the south, that’s where Antarctica sits, so we aren’t getting any fish there anyway– –but in the north, the sun won’t rise above the horizon for several months out of the year, and this decreases the amount of time phytoplankton have to grow and reproduce, because they still use photosynthesis. So, productivity in the open ocean reaches a maximum somewhere between the temperate and polar zones. Now, the only places I haven’t really talked about is the coastal waters, which we can see do have a fairly high productivity. OK, so first, why are the west coasts more productive than east coasts? So we know that productivity is determined by the abundance of nutrients in the water, so something must be causing an upwelling of nutrients on the western shores of the continents– –and not the east. As it turns out, this is due to the Coriolis effect, which I already made a whole video about, so I’m going to go ahead and skip the explanation here. All you really need to know is that from 30°N to 30°S, winds tend to blow east-to-west. This means on the west coast of land, the warm, nutrient-poor layer is pushed away from the shore, and this allows the colder, nutrient-rich water to come up. This leads to the waters here being more productive, while on the east coast of landmasses, the opposite happens, and warm, nutrient-poor water builds up, and we get these areas of lower-than-expected productivity. The next mystery we have is this nearly perfect line across the equator. What causes that? Well, this time, we can thank ocean currents, which, well, are also sorta caused by the Coriolis effect. All you need to know for this one is that in the northern hemisphere, the water circulates in a clockwise rotation, while below the equator, in the southern hemisphere, water circulates counterclockwise. What all this means, is that the ocean has two opposing circuits colliding with one another exactly at the equator, where the effects of Coriolis flip. These colliding circuits force the cold waters from beneath upwards, and yeah, anytime we have an upwelling of water, this brings nutrients to the euphotic zone, and we get higher productivity. OK, and lastly, we have these “anomaly areas,” where nothing we’ve learned so far really explains them. For these, it’s not the ocean that we need to look at, actually, but the land. I’m going to point out some land features, and how about *you* tell *me* if you recognize a pattern? OK, so here’s the Amazon River, and here’s where the Paraná River dumps out into the Río de la Plata. Right here is the Mississipi River delta, and over here, we have the Yellow and Yangtze Rivers. Oh, and here’s the Brahmaputra River, part of the largest river delta in the world, actually. OK, I think you get the point. All of these spots receive their nutrients from the discharge of rivers into the ocean. This has only gotten more noticeable in recent years, as rivers have begun to collect more nutrient-rich runoff from farms using fertilizers, and in some cases, this can actually cause eutrophication and massive fish die-offs. Outside of these coastal areas, however, the vast majority of the ocean remains virtually desolate, producing very little life. That’s not to say the ocean isn’t worth protecting; these areas still provide immense services to our planet that I can’t begin to explain here. But hopefully, perhaps in a future video, if people end up liking this one, I can. This week, I’d like to give a special thanks to my patrons over on Patreon for helping make this video happen. I finally got my first Patreon payment the other day, and immediately used it to buy groceries, because, well… it’s a long story, but I’m still not getting paid for these videos. So your Patreon donations literally helped me stay alive. If you’d like to help in this cause of “keeping me alive,” there’s a link somewhere on-screen to my Patreon, and any amount helps. So, thank you guys. Other than that, thanks for watching, and if you’d like to see more videos about the ocean, it never hurts to give a video a like. And, if you want to catch any other videos I make, well, YouTube has subscriptions for a reason. I also have a Twitter, yadda yadda, I’m tired of talking, OK, I’ll be back next week with another video. Thanks! 🙂 [theme music]