They are weed fields, not lawn which by definition, has to be uniform and dwarf in height like a golf course. If I neglect mowing my lawn, it will turn into a weed field. In nature, there are terrestrial lawn found in great plains of Africa, Mongolian steppe, Alaskan tundra, and Iceland moss forest. I don’t think aquatic lawns exist in nature and none of the posted pics resemble a sunken lawn.
This then becomes a definition issue. A field of 99% hair grass is a weed field but 100% is a carpet? /shrug
I have a patch in my Walstad that is 2 varieties of hair grass plus microsword thrown in. Is that a carpet?
Then also becomes the issue of whether we are talking about an area of ground the same size as the footprint of the aquarium, or are we talking about an entire landscape we are trying to replicate. I posted pictures where if you were only looking at say a 2 foot by 4 foot area you would only have one plant species growing in that space. But if you looked a wider shot that encompassed the entire horizon then you will end up with hundreds of plants species (just a guess).
If the question though is whether you see plants of the same species growing in close proximity to each other, the same plants that we grow as carpets in an aquarium, then the answer is yes, it does happen in nature.
Here are some more
Unknown could be Callitriche, Micranthemum, or Hemianthus.
Here is a link
to Tom Barr talking about monkey flower growing permanently submerged in CA as a carpet.