Question

Phosphorus in rivers can lakes sinks to the bottom and over thousands of years becomes ______ again.

Answer 1

Your question isn't the most clear, but I think your answer is rocks, or more specifically phosphorous rich rocks because the phosphorous will mix with the sediments on the river's/lake's ground forming phosphorous rich rocks.

That means that It'll become a phosphate mineral and later become phosphorous again when the minerals weather.

Hope it helped,

BioTeacher101