The Ocean is a Big Blue Desert
I’m ashamed to say that I was 36 years of age before I first questioned the prevailing party line about the ocean. I was always told it was teeming with life, and it was easy to believe, growing up in a small seaside village. The littoral rocks covered with slime and seaweed and molluscs, the air thick with the smell of salt and fish and the cry of birds. In the rockpools along the beach we found all manner of little things, eels and crabs in borrowed shells. Limpets that we sent flying with glee.
There, or at other stops along the Scottish coast, we would see all the rare and splendid characters that you hope to see– seals, dolphins, puffins and gannets. I saw an orca and its calf once outside Aberdeen harbour from the top of the hill.
And yet, and yet. I think I always knew. The ocean feels more like a void than a jungle. I’m not ashamed to say that I feel something close to true terror when I’m in the open ocean, even if I have just swum out a few meters past the drop where the seabed shelves away. Oh I’ve done my PADI open water, I dived as a child with my tank strapped to my back, out into the depths. But swimming alone, unaided, out over the dark. The abyss seems to open beneath me, bottomless and unfathomable.
Bar-On 2018, The Biomass Distribution of Earth [1], is considered the definitive census of life on this planet. The numbers are mortifying. Forgive me– but the ocean, our so-called mother, is a barren cradle. Here is the base value: 6 gigatonnes of carbon, to the land’s 470. 6 gigatonnes! And this isn’t some particular ocean I’m talking about, the Baltic perhaps or the Adriatic. The entire thing!
Let me give a little more context. When doing a census of the biomass on Earth you don’t want to be distracted by the small print. One animal carries humps filled with watery fat, the other is lean like jerky. Some biomass is living green, some dead wood. Get rid of the details and the finery, strip it right back to the carbon atoms, the structural backbone of life right down to the molecule. Count those. A tonne of carbon might be eight tonnes of living animal, or two tonnes of dry wood. A gigatonne one million of the same.
Isn’t that absolutely humiliating? The entire ocean, all of it, every last cubic meter together contains less life than Papua New Guinea. Not New Guinea, mark you, the island. The nation that only covers half of it!
In terms of raw biomass, the ocean is in fact among the most barren deserts– about 20 grams of carbon per meter squared [2]. Deserts range from 10 to 500 grams / \(\mathrm{m}^2\). A tropical rainforest by comparison has about 20kg. And that’s summing the entire water-column. Imagine how little there is in the average cubic meter– 0.005g C per \(\mathrm{m}^3\)! Of course, the depth of the ocean is exactly the problem, the great distance to its sandy base– but more on that later.
There are other ways to cut it. The biomass of the land is dominated, by a wide margin, by trees or more specifically by wood. Wood is a wonderful material but it isn’t the most exciting example of a living thing– in fact, some have argued that most wood is dead wood, even the wood of a living tree. The living cambium of a tree makes up a small ring around the trunk, and less than 10% of its total mass is properly alive. Instead of counting all that dead weight, we can measure the NPP, net primary production, a measure of the amount of excess carbon (sugars, generally) that the producers produce [3, 4].
This measure doesn’t make things look so bad. The land and the ocean both produce around 50Gt of carbon per year. But the ocean is about two and a half times larger than the land and consequently has less than half the average productivity– 150 g of carbon per square meter per year to lands 375 (Note that I am including the deserts and Antarctica in the land surface area! Exclude those and the picture is even worse.) This is not as bad as a desert, in fact the ocean produces significantly more living matter every year than the average desert– it’s closer to tundra or shrubland on average. But the biomass turns over much quicker– no wood– than it does on land, so the standing stock is much smaller.
Why is this?
Most of the ocean is deep. Terribly, incomprehensibly, shockingly deep1. The simple fact of the matter is that essentially all life on Earth is driven by photosynthesis. Sure, it’s great that there is all that space in the ocean to squirm about under inconceivable pressures, but there just isn’t anything to eat. The only part of the ocean that can even be used for photosynthesis is the first tens of meters at the surface. Everything beneath that, with some notable exceptions around thermal vents, depends on things falling down there from above.
1 Read Starfish by Peter Watts if you want some intense hard sci-fi deep ocean horror from a marine biologist with a profound disquiet about life on Earth.
2 Mangrove and estuarine nurseries shelter a strikingly large share of fish species, especially tropical and commercially important ones– see [5, 6]. The exact “half of all fish species” figure is widely repeated in popular sources but I haven’t found a clean primary citation for it; treat it as illustrative.
3 Naive ceiling: ocean area (361 Mkm²) × mangrove NPP (~1200 g C/m²/yr) ≈ 430 Gt C/yr. The per-area mangrove-to-open-ocean productivity ratio is ~13-16× (mangrove ~1200 vs open ocean ~75 g C/m²/yr– ~135 g/m²/yr averaged over the whole ocean — but only ~75 in the open-ocean gyres that are most of its area). Multiplying current total ocean NPP by that ratio overcounts, since today’s total already includes the productive coastal fringe– hence ~430 rather than ~1500 Gt C/yr.
But what about those first few meters? Shrubland, really? What on earth are they up to? It’s absolutely tragic. Mangrove swamps manage to utilize about 15 times more solar energy than phytoplankton do. And in the process generate an ecosystem so rich that something like half of all fish species sometimes call them home2. The mangroves should be our model for what we expect from the ocean, frankly. In fact it almost looks like the mangroves are trying to reclaim and populate the sea. What’s stopping them? It certainly isn’t light– there is enough solar energy, per the mangroves, for the ocean surface to produce on the order of 430 GtC/Yr3, call it nine times what the whole ocean manages today.
The answer is probably quite mundane, but it’s wonderfully generative. The answer is rocks. Specifically the ocean is lacking the rock-derived nutrients: most often fixed nitrogen and phosphorus, very often iron, sometimes silicon for the diatoms, and the trace metals that enzymes depend on– molybdenum, cobalt, zinc, manganese. Outside of the shallow coastal regions (which, to be fair to them, are almost as lively as a forest) the only way for nutrients to get to the phytoplankton is for it to somehow upwell all the kilometers from the seabed or to fall in as dust from the air. In fact some 0.7 Gt of mineral dust falls into the sea every year4, most of it blown off the Sahara desert– and it’s the sliver of soluble iron in that dust that the phytoplankton are really after. Much of what lands on the open sea falls into the depths before it can be used– as does much of what is used.
4 ~750 Tg ≈ 0.75 Gt of dust reaches the ocean each year, the Sahara supplying roughly half of the global iron input [7]. Only ~0.2% of the iron in that dust is bioavailable.
The open ocean is incontinent. On land, the trees and fungi and bacteria and most everything else (humans notably excepted) collaborate to enrich the soil more and more, thickening the topsoil day by day and year by year. Some take on the slow, hard task of breaking down rocks, the only real source of new nutrients. Others pull them out of the sky. Others break down once-living things and process them into their component parts, ready to be brought up again into the circle of life.
The ocean is a bottomless pit, a void (did I mention that already? it cannot be stressed enough). There are no rocks to provide nutrients, and no rocks to provide the foundations for stable structure. And there is too much damn salt– ask any mangrove tree. If it weren’t for all the salt! You hear them cry. Ask any shipwrecked sailor his opinion about salt. The watery cradle we are shilled by holiday brochures is a giant salty tomb. But perhaps the mangroves could stomach even that if they could find a place to stand, something to hold on to, somewhere to store their detritus and hunt for rare nutrients. Lord, how much the mosses and lichens that first crawled into the harsh glare of the sun must have joyed at the peace, the stillness, the sense of place, the firmness of ground. Carrying your water with you and carefully curating it when it falls, why that’s a small price to pay. The ocean is empty because there is nowhere in the ocean to just be, there is no there there, just a watery soup with not enough sustenance.
I said it was wonderfully generative because– I am filled more and more not only with contempt for the big blue desert and its boasts of genesis5, but moreso with wonder and amazement at the marvellous plans of trees. I always thought they held so tightly to the ground so they weren’t swept away by the harsh Scottish wind. More fool I! The trees are not a canopy of leaves interfacing with the sun tethered to the ground– they are two interfaces, one to nutrients and rocks, the other to the sun, bound to one another by a living thread. The rocks! They are nothing without the rocks! Without the rocks they would be as pathetic and unambitious as the average phytoplankton, a little ball of photosynthetic grue waiting to be swallowed up by the next insect that wriggles in its direction. Oh no. Not our trees. They bind heaven to earth with their dual interfaces as tightly as Janus binds past and future– and they very generously let us, all of us, the entire animal kingdom (excepting, notably, humankind, those treacherous and frankly insane primates) live cradled in this space between, this interzone that they build and curate for our pleasure and entertainment.
5 I don’t actually doubt that the ocean was the first cradle of life– it is easy enough, after all, to float there and hope for the best and sometimes the best, some nutrients and a photon or two, will come along. But land is for the real winners, the real livers and growers.
And I haven’t even thanked them yet for flowers or fruits or nuts! Compare these dazzling treats hanging happily above your head with the sordid offerings of the big blue, seaweed offering sacks of seawater, slippery underfoot. Jellyfish! Tasteless morsels hardly worthy of the name. Trees are the inheritors of those hardy greens that crawled out of the sea and claimed the continents, but the architecture they have developed, the buildup they have orchestrated, the arcologies that they constitute in their proudest densest accumulations. It is the same trick those community organisms pulled, collaborations between fungi algae and yeasts, but on so immense a scale that their symbionts are trillions of times larger than the constituents of a lichen.
The ocean is a big blue desert. Unlike every other desert outside of Antarctica, it is not water that it lacks, but rock. If we filled the ocean with gravel the world would be a better place, I am sure of it. A richer place, a livelier place. The mangroves would do the rest of the work for us, they aren’t the sort that needs much encouragement. They and their cousins would divide the world properly into its proper parts– light above, stone below and life humming merrily in between.