This is a follow up question to Sky of Earth and Seas of Sky
I have a medieval fantasy setting where people live in a vast expanse of giant caves contained in a sort of giant Stanford torus. This is not quite a true Stanford torus though; it was created from a rapidly spinning disk shaped planetoid about 100,000-200,000 years ago. Rather than building a whole mega structure themselves, the "gods" who built this habitat took this pre-existing planetoid and just encased it in a "firmament" which is an unobtanium clear shell that keeps the oceans and atmosphere from flying away.
The result is that the outermost level of these caves have many lakes and oceans through which you can see a sun, moon, and stars, and the water has a day night cycle. The sun shines through bright enough to provide light for a dense forest of plant life on the ceiling of the cave. The apparent gravity is about 1G and points outward toward the seas due to the planetoid's spin imparting a greater than gravity outward force. Most of the forests are suspended hundreds of meters above the water.
My original thought was to feed this canopy of trees with water from the clouds that form off of the lakes and oceans similar to a Cloud Forest, but as I've read more into this I don't think this will work. Cloud forests have the water vapor condense on the leaves and then fall to the ground as rain where it moistens the soil. But, in this case, the condensation would fall away from the roots. I'm also concerned about soil fertility since dead plants would presumably fall away from the ground both choking up the seas by blocking out the sunlight and depriving the ceiling of nutrients needed to sustain the forest.
This question: How can I water/soil upside-down trees and grass? is similar, but in their scenario, they were not really looking for a particularly fertile or ancient upside down ecosystem; so, they were able to handwave away a lot of my concerns that it was a mediocre ecosystem for plant life.
How would I create a stable ecosystem where this forest gets enough water, nutrients, and sun light to be sustainable?
A best answer will be one that does not require any of the builders' technology/magic to still be doing things to maintain the cycle, but this is not strictly necessary.
The trees themselves are assumed to be evolved or engineered for this environment; so, while I need them to be photosynthetic tree like organisms, they do not necessarily need to be any existing species of tree. A best answer will stick to adaptations that already exist in plants, but this is also not strictly necessary.
I need the seas to remain clear enough to see the sun, moon, and stars; so, a best answer also needs to avoid situations where the water gets all mucked up with plantlife and debris over time; so, if a method does not actively recycle the nutrients back up to the plants, it should explain how the water is kept clear, where the plants get more nutrients from, and why such a method would be sustainable over hundreds of thousands of years.