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The ocean of my very earth-like planet is covered in its entirety by vegetation, forming a kind of carpet on top of the water's surface. This free-floating halophyte is a superorganism formed by trillions of individual plants. The squishy upper layer always stays on top of the water thanks to pneumatocysts underneath, which help it maximise its exposure to sunlight. Nutrients are absorbed through a forest of low hanging roots which reach just a couple meters under the surface. The plants reproduce not by pollination but by cloning themselves repeatedly.

In the case of ponds, floating plants are problematic and frequently wipe out local populations. They starve the plants underneath from sunlight, thus killing them and get a monopoly on the nutrients. The mat of plants would create a low oxygen environment much closer to the surface. This wouldn't spell doom for all sea life however. Life usually finds a way.

Fish with larger gills would be comfortable in these conditions. Aquatic mammals would be at an advantage as they can breath oxygen and rest on the surface of the mat. The mat itself would also be a food source for grazers, while predators would use the cover to ambush their prey. All in all, a very interesting biome.

However, reality seems to suggest that this isn't possible. At least not at 100%. If this halophyte superorganism were to take over the sea, what various challenges would it need to overcome?

Assume earth conditions (e.g. climate and animals) but no human presence. This plant is naturally evolved.

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    $\begingroup$ "Nutrients are absorbed through a forest of low hanging roots which reach just a couple meters under the surface": This is the problem. There is very little in the way of nutrients there. In particular, there is almost no phosphorus, no nitrogen in a biologically useful form, and precious little iron. (If there were sufficient nutrients in ocean water, don't you think that we would have found a way to use seawater for agriculture?) $\endgroup$
    – AlexP
    Jun 13 at 21:49
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    $\begingroup$ I agree with AlexP that this is (at least part of) the problem. I'm familiar with a plant called "frogbit" (Limnobium laevigatum) that floats on the surface and has roots that hang down. The roots, it seems to me, are mostly for stabilizing the plant so that it floats right-side up. If they were for getting nutrients they would not have to be so long, and there would be more of them. $\endgroup$
    – Wastrel
    Jun 14 at 14:52
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    $\begingroup$ Some existing plants have ways of overcoming problems with nutrient deficiency. Clover for example, has a symbiotic relationship with bacteria that lets it pull nitrogen from the air. Also, carnivorous plants specifically evolved to deal with nutrient deficient soils. $\endgroup$
    – user4574
    Jun 14 at 22:17

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NUTRIENT DEFICIENCY:
Many other answers highlighted the problem of nutrient deficiency. Here are some solutions.

  • Fixing Nitrogen from the Air. Some existing plants like Clovers, can pull Nitrogen from the air.
  • Animal poop. Birds fly over the sea-matt and periodically "deposit nutrients". Also, if there were large herds of grazing animals that wander onto the matt for part of the year they would leave behind nutrients. Obviously for this to work the animals need to periodically move between land and sea-matt.
  • Carnivorous plants are another solution that evolved specifically to deal with nutrient deficient soils. You could put some features on the plants for trapping prey. Perhaps roots like jellyfish tentacles or copy a Venus fly trap or pitcher plant.
  • Absorbing dead land animals. Even if the sea-matt is not actively trapping animals, if its sturdy enough to walk onto, then it's likely that some animals will wander out onto it and die one way or another. The sea-mat could be evolved to absorb nutrients from the corpses.
  • Dead Fish Float. As they decay, dead fish float to the surface, and can be absorbed by the roots.
  • Symbiotic bacteria Let there be a micro-organism that moves between the sea floor and the sea-matt. The bacteria moves to the sea floor to get minerals, and moves to the sea-matt roots to get sugar. As it obtains its sugar, it leaves behind some minerals.
  • Nutrient sharing. If the plants can share nutrients with adjacent plants, then they can be absorbed by those plants near the shore, and the nutrients will slowly diffuse from plant to plant towards the center. Plants, even of different types, can be grafted together. Perhaps adjacent plants can naturally graft themselves to their neighbors if they are in contact.
  • Nutrient conservation. When the plants die, the nutrients are absorbed by the adjacent plants. That way, even if the flow of nutrients to the sea-mat is very slow they can survive because the nutrients don't leave the matt.
  • Slower growth. Plants don't need to grow fast. When put into a low resource environment, many plants will just stop growing until nutrients become available. In the face of nutrient deficiency, the sea-mat can just grow more slowly. As a personal example, I have on multiple occasions sprouted seeds from citrus plants on moist paper-towel in closed Ziplock bags. If I don't open the bag the seeds will from into a small plant and can stay that way for many months without issue.
  • Convection currents. If there were hydrothermal vents under the ocean, they could bring nutrients to the surface via convection currents.
  • Dust. It's likely that dust from the mainland will make its way onto the sea-matt either by wind, volcanic eruption, movement of animals, etc. The rate of nutrient deposition from this method is likely to be low, but combined with nutrient conservation and slower growth it might work.

Some combination of all the above is likely to provide the required nutrients.

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Thermoclines are a reason why on Earth, oceans are really not very productive, basically if water is deep enough, that is most of the oceans, only surface is heated, which makes two separate layers, and most nutrients sink to the heavier, cold water, being unreachable to surface plants (on Earth mostly phytoplankton). But, further south and north, water is colder, and thermoclines vanish, that's why colder seas are more biologically productive, and if your world is cold, then okay. Second thing is, why almost only phytoplankton lives in our oceans, because there isn't really too much nutrients for plants to grow huge, they have to be as economical as they could.

I hope you could do something creative, this vision is really appealing to me!

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  • $\begingroup$ Thermoclines must vanish somewhere, otherwise the heat working it's way downward would never escape. $\endgroup$ Jun 14 at 21:29
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Currents and waves.

mats of floating vegetation exist on earth but they can only exist in places far from shore or places without waves so wave action can't just dump it all on a shoreline. currents, wind, and wave action cam move material way way faster than it can grow. If it floats it will end up getting dumped on shorelines creating huge clear patches. these mats can only exist in gyres or recirculating currents.

this is what happens to floating vegetation if it gets anywhere near a shoreline. currents can move material like this at about one foot per second. so unless it can grow at feet per seconds the currents win and clear out large swaths.

enter image description here

worse if the mats are too solid, wave action will rip them apart just like ships that are too long.

A colleague and I tried to work out how to do something similar. Our mats were based on floating grasses and trees. We ended up having to have a planet with large gyres by having shallower oceans, no ice caps and a different continental arrangement, although I can't for the life of me remember the arrangement. But that is a good follow up question for this site.

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    $\begingroup$ Good image. Where is that? $\endgroup$
    – Willk
    Jun 13 at 23:19
  • $\begingroup$ @Willk somewhere in the Caribbean not sure exactly where, there has been a huge bloom of sargassum there. $\endgroup$
    – John
    Jun 14 at 21:45
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The Riddle of Phosphurus

We usually think of living things as being made from all the minerals and nutrients of the Earth. But in some very important ways, we really represent just one mineral in particular, namely hydroxyapatite. There is some reason to suspect that this mineral first converted formaldehyde gas into ribose, the basis of our sugars and nucleic acids. As the predominant form of apatite, is the major geological source of the phosphorus cycle. It is the mineral found in our bones, the dentin and enamel of our teeth, and even in the plaques of calcified arteries.

It sticks there because it naturally adheres to phospholipids, which we need to make the membrane of each and every one of our cells, each of which contains a phosphate on its surface. But we also need a phosphorus atom between every two nucleotides of DNA (or RNA), and three for every molecule of ATP that stores cellular energy, and two for every NADH and FADH2 that mediates as the reduced fuel of metabolism. Every molecule of sugar our cells take in is decorated with a phosphate group. In short, our sort of life has developed to be almost completely dependent on a special mineral and the element phosphorus within it. And specifically, phosphorus levels are a key factor limiting productivity of the Sargasso Sea, which is what you want to improve on.

Your life forms have somehow become liberated from the tyranny of phosphate. Perhaps they evolved another way in the first place, or received an artificial retrofit of the many thousands of genes that work via phosphate biochemistry. Perhaps they have genomes of peptide nucleic acid (PNA), and use some other amphiphile at the cell membrane.

Alternatively, perhaps they are "intentionally" fertilized at the surface by some symbiotic life form, which swims back and forth to the depths and recovers phosphate from a geology with deeper reserves.

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    $\begingroup$ or it could have shallower oceans. $\endgroup$
    – John
    Jun 14 at 21:59
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Challenges:

  • Currents. Can easily beach plants.
  • Waves. Can rip apart mats.
  • Storms. Can rip apart mats faster then waves alone.
  • Tides can create some harsh land ocean boundaries. Slowing down mats from forming shore-to shore stabilization.
  • Lack of Nutrients, probably the biggest issue, the most stable areas away from shore are the lowest nutrient areas.

Edit:

  • People. Large mats of vegetation interfere with shipping. Expect various 'solutions'.
  • Disease. If these mats are mostly mono-culture, expect bacterial, fungal, viral presence that can cause rapid devastation.

Has occurred in the past.

I recall reading a hypothesis that this happened in an enclosed ocean. Which due to scale, caused a run-away global climate change event severe enough to kill off the plants/biome in question. The enclosed ocean had ok nutrients, low waves, low wind low storms low currents.

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One vision I have is to make a lot of spiky mountains underwater, as a kind of bases for plants to develop from. These spikes would function like coral reefs, being the life hotspot, and life developing around them, main root plants attached to mountains, other plants using haustoria to attach to them.

A thing why I don't like waves arguments, is because wind is required for waves to appear, so what if all of the water was already somehow densly carpeted? The waves wouldn't appear! Storms would be just strong rain. And also, if plants were common through water surface, thermoclines would not appear too, cuz all of the sunlight would be absorbed by the plants anyway.

Arbuscular mycorrhiza fungi makes symbiosis with plant roots, gain sugars and energy, and creating nitrogen and phosphorus out of it. In our conditions. I bet in these conditions some other fungi could do a lot better job, I imagine very long, thin roots with fungi attached to it, just make your ocean shallow and nutricious enough, and I think it'll work

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  • $\begingroup$ Regarding plants absorbing all the sunlight: No they wouldn't, plants are green and translucent, there would be light (mostly green light) filtering into the depths. And even if plants were black and opaque, that just makes them a blackbody, which would radiate the heat back into the environment (albeit at a lower wavelength), both into the air and into the sea $\endgroup$
    – No Name
    Jun 14 at 10:45
  • $\begingroup$ Sure, but not enough to create termoclins, leaves are translucent, but green color is for deflecting light that would otherwise hurt them, as green is the most intense wave in our sun's spectrum. $\endgroup$
    – Arha'eG
    Jun 14 at 11:41
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I'm not entirely confident posting my answer as I have no scientific data, research, or expertise to offer, however, without any of that, while perhaps a bit vague, the biggest challenge and most dangerous and detrimental outside influence to the ocean life you described are the byproducts, industrialism, and waste produced by human beings. Hands down.

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    $\begingroup$ Welcome to the site! Please note that the author of the question assumed "no human presence". $\endgroup$
    – Glorfindel
    Jun 15 at 8:02
  • $\begingroup$ I think the equivalent of this is volcanic activity and natural deposits of radiactive minerals (specially after an earthquake). As a conclusion, natural disasters in author's planet must be rare to let his plant-mat grow. $\endgroup$ Jun 15 at 15:16

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