**My ocean biome is almost entirely covered by a mat of linked free-floating plants.** You can read further details on my original question "[What challenges must oceanic plant life overcome in order to create a mat on top of the ocean?](https://worldbuilding.stackexchange.com/questions/231314/what-challenges-must-oceanic-plant-life-overcome-in-order-to-create-a-mat-on-top/231376#231376)". In a nutshell, the main issue was how to deliver sufficient nutrients (mainly phosphorus) to the plants.

The solution lies in a mutualistic relationship between animal and plant. Just have an animal dig up soil from the shore or seabed and deposit it on the roots of the floating plants. The purpose of the behaviour would probably be for nest building. I don't believe that building a nest out of sand on suspended roots would be very practical tough. It seems more logical to build a nest out of the roots themselves, which doesn't help the plant.

Alternatively, it could be the plants that started this partnership. The roots of free-floating plants serve no purpose other than keeping the plant upright. **A curved concave root would allow the plant to collect a small amount of particles, which would then evolve to become the substrate on which animals deposit sediment.**

So what kind of creature would be best suited for this task?