I can not answer the how deep question, but you can sample your pond and find out for sure. Take a length of hose and get out on the dock or in a boat. While the hose is out of the water, put your thumb over one end then drop the other end straight down until you get to the test depth, let your thumb off which allows water from the test depth to fill the hose up to the surface. Put your thumb back over the end and pull your hose up and then put the water in a container by releasing your thumb while the other end is in the container. If the water is green at that test depth then go another foot deeper for the next sample.

I guess this would be a lot easier with a pump set-up, but I'm thinking quick, cheap, down and dirty.

Fish on!,
Noel