Howdy folks, let's get this post on the road.
Today's marine science fact is: ocean food webs and the biological pump!
Technically, the origin of this particular spiel begins in 2022, after seeing this whalefall comic by grubloved on tumblr. (https://grubloved.tumblr.com/post/629810577831837696/thinking-about-whale-falls-inspired-by-the-work)
The short version is, some people looked at this evocative-as-hell comic, and went "damn, imagine how dark it would be to realize that manna from the heavens is actually dead bodies". And unfortunately for me and everyone involved, the biggest baddest "WELL ACTUALLY-" came tumbling out of my mouth.
Because the thing is, to people who live on land and aren't generally scavengers, this is gonna seem pretty grim! Whalefalls are an object of fascination, because yeah, the death of such a charismatic and giant animal is a tragic thing, and there's this general melancholic throughline of a tragic death going on to breed subaltern pockets of life way way down where nobody involved has ever seen the light of a sun. Circle of life stuff. But the thing is, because of how the ocean works, the majority of life in the ocean relies on dead things falling from the surface.
The thing you need to know about the ocean is that it's deep as hell. Truly baffling depth scales. If you have about 15-20 min to kill, I'd recommend going to https://neal.fun/deep-sea/ and just scrolling for a while if you really want a sense of what we're working with. All this to say, there are 10,000m worth of environment, and only 100-200m of it has access to sunlight (i.e. the photic zone). And that's before you think about things like water clarity that could make that light slice even smaller. In the same way that all land-based food webs rely on plant photosynthesis to exist, the entire oceanic food web has to rely on whatever photosynthesis can happen in that light layer.1
Thankfully, phytoplankton are some of the most efficient motherfuckers on the planet. Their size is precisely what makes them efficient - because they're a single cell, they can suck water and nutrients directly from their environment, and their entire body is devoted to photosynthesis, unlike trees which only photosynthesize with their leaves. If your lawn photosynthesized at the rate that plankton did, you'd have to mow it every day or every few hours. They are, as I put it in my original tumblr post, "shitting out organic matter all the time".
Just like phytoplankton are efficient, so too are the zooplankton that eat them. They're the microscopic animals to these microscopic plants. They can graze through a phytoplankton bloom almost as quickly as it grows. And here's where the transition to the bigger guys down the food chain begins. Some of this stuff gets eaten by animals who feed at the surface - the typical "plant -> herbivore -> carnivore" chain.2 But some of it leaves the surface. As phytoplankton die, or get eaten by zooplankton, they will get transported from the photic zone at the surface, to deeper and darker zones. Either because the phytoplankton have started sinking, or because the zooplankton have digested the phytoplankton and turned them into a nice heavy fecal pellet. And much like raindrops in a cloud, these little pieces of dead organic matter can bump into each other and start aggregating into bigger clumps.
This particulate matter, also known as marine snow, makes up most of what deeper organisms eat. On an annual scale, about 10.2 gigatons of carbon are transferred from the surface to the bottom of the ocean through this "dead things falling" pipeline. Yeah, sometimes you will get "large particles" like a whale, or a fish, or some other animal that sinks and falls to the bottom. (It is also extremely funny to think of such a large being getting referred to as a particle.) But this rain of surface matter supports most of the ocean by volume - not the living phytoplankton, but their dead and digested matter. This is what feeds fish, worms, crabs, all the middle-of-the-chain things that go on to feed the bigger fish. This is what draws carbon from the surface to be decomposed into the seawater by bacteria or get buried into sediment, where it stays for hundreds and hundreds of years. This is the bedrock upon which the deep ocean food web runs. All that life in the dark, fed by the dead of the light.
And that's something I just think is beautiful.
Thank you all for joining me on this personal infodump fave. I'll sea you all next week!
Sources:
https://en.wikipedia.org/wiki/Biological_pump
https://oceanservice.noaa.gov/facts/marinesnow.html
https://en.wikipedia.org/wiki/Marine_snow
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There are technically other forms of organic matter synthesis that aren't reliant on light (i.e. chemosynthesis). That's how deep sea vents work. But that's a whole different
kettle of fishvolcanic bundle of worms. -
There's some real interesting trophic level math about how whales can support themselves entirely on krill because they're so efficient at taking up what the phytoplankton are putting out.