I dunno if turnover of detritus material contains phosphorus, but sure could. And that could be the return on your meter. Reason I said pvc is because I've seen a bunch of it on Conroe. Very hard to discern. That doesn't really look like pvc concrete bases, but it's certainly not fish. Nor anything else I've seen, other than fish attractors.
Knowing the depth now, and looking at the vertical length of the returns higher in the water column, I think I may have just learned something new about turnover and signatures on electronics.