so this is a thought that i've read recently:

most photosynthetically active organisms (PAOs, gosh, we really need an abbreviation for this) on land are complex plant life (mostly trees which are 10 meters high on human-unmodified land and bushes and grasses, flowers, cereals etc elsewhere); but in the ocean it's mostly single-celled bacterial life that's just freely floating around (plankton). so this got me thinking, what could be the reasons behind this?

and i came up with two explanations: the ocean currents and the lack of solid ground

• the lack of solid ground means that trees can't make roots on the water, so they would tip over if the wind blows even at medium strength. that, and the waves which shake the ground mean that a tree wouldn't stand tall at all. this could be alleviated with a swimming platform (also made of wood)
• but more importantly, the ocean currents. on land, if a tree grows on spot A, it's still gonna be there much later in life. but in the ocean, the currents (e.g. gulf stream) would carry it away somewhere else. you might start growing in the carribean sea, and later end up in the arctic, where you're stuck. the water flows back to the carribean below-sea-level, i.e. warm water rises in the carribean, moves north, then cools down and sinks, moves south. but if a tree is floating on the surface, it wouldn't be able to move back with the sub-surface current streams. but small single-cells can do that, because they don't actually have enough mass / buoyant force to lift themselves up to the surface reliably.

picture of gulf stream:

This makes me think of Mangroves, the only trees I’m familiar with that grow along the ocean and which had to evolve crazy root systems and special salt filters to do so.

(Anyone who has the chance should float a mangrove forest, they’re beautiful and thriving with fish life)